Brief Review Downloaded from http://journals.lww.com/nsca-jscr by BhDMf5ePHKav1zEoum1tQfN4a+kJLhEZgbsIHo4XMi0hCyw CX1AWnYQp/IlQrHD3i3D0OdRyi7TvSFl4Cf3VC1y0abggQZXdtwnfKZBYtws= on 08/29/2024 The Use of Free Weight Squats in Sports: A Narrative Review—Squatting Movements, Adaptation, and Sports Performance: Physiological Michael H. Stone,1 Guy Hornsby,2 Satoshi Mizuguchi,1 Kimitake Sato,3 Daniel Gahreman,1 Marco Duca,1 Kevin Carroll,1 Michael W. Ramsey,1 Margaret E. Stone,1 and G. Gregory Haff4 1 Center of Excellence for Sport Science and Coach Education, Department of Sport, Exercise, Recreation and Kinesiology, East Tennessee State University, Johnson City, Tennessee; 2School of Sport Sciences, College of Applied Human Sciences, West Virginia University, Morgantown, West Virginia; 3Peak Force International, Taichung, Taiwan; and 4School of Medical and Health Sciences, Edith Cowan University. Joondalup, Western Australia Abstract Stone, MH, Hornsby, G, Mizuguchi, S, Sato, K, Gahreman, D, Duca, M, Carroll, K, Ramsey, MW, Stone, ME, and Haff, GG. The use of free weight squats in sports: a narrative review—squatting movements, adaptation, and sports performance: physiological. J Strength Cond Res 38(8): 1494–1508, 2024—The squat and its variants can provide numerous benefits including positively affecting sports performance and injury prevention, injury severity reduction, and rehabilitation. The positive benefits of squat are likely the result of training-induced neural alterations and mechanical and morphological adaptations in tendons, skeletal muscles, and bones, resulting in increased tissue stiffness and cross-sectional area (CSA). Although direct evidence is lacking, structural adaptations can also be expected to occur in ligaments. These adaptations are thought to beneficially increase force transmission and mechanical resistance (e.g., resistance to mechanical strain) and reduce the likelihood and severity of injuries. Adaptations such as these, also likely play an important role in rehabilitation, particularly for injuries that require restricted use or immobilization of body parts and thus lead to a consequential reduction in the CSA and alterations in the mechanical properties of tendons, skeletal muscles, and ligaments. Both volume and particularly intensity (e.g., levels of loading used) of training seem to be important for the mechanical and morphological adaptations for at least skeletal muscles, tendons, and bones. Therefore, the training intensity and volume used for the squat and its variations should progressively become greater while adhering to the concept of periodization and recognized training principles. Key Words: strength, specificity, resistance training, lower-body strength Introduction The importance of muscular strength should not be underestimated. Strength is the ability to produce force against an external resistance (181,183); its dynamic application provides a mechanism for movement (F 5 ma). Because of its relationship to movement, strength is an integral aspect underpinning sport performance capability. Furthermore, strength should be thought of as a “vehicle” that carries with it numerous characteristics important for sport performance (1,39,50,52,64,66,71,176,179). The basic characteristics of maximum strength enhancement promotes several characteristics of importance to sport; these characteristics are shown in Table 1. Note, among weaker athletes, likely including most athletes (189), evidence indicates that strength training alone produces equal or superior results in enhancing rate of force development (RFD), power, etc. compared with power or speed training alone (1,39,41,102). This is especially evident when performed in an integrated fashion within a holistic sport training program (189,213). Numerous authors indicate that the squat is one of the most important exercises for increasing the strength of the athlete and the overall development for a variety of sports, particularly the Address correspondence to Michael H. Stone, Stonem@etsu.edu. Journal of Strength and Conditioning Research 38(8)/1494–1508 ª 2024 National Strength and Conditioning Association lower body (12,29,154,207). Among relatively untrained, welltrained and various athlete groups, improvements in squatting ability and capacity have been shown to result in improvements in lower-body strength, jumping ability (19,88), change of direction ability (19,88), and sprinting performances (34), all of which can be critical components of sporting success. Numerous examinations of squat training have shown gains in maximum strength both dynamically (1RM) and using isometric test (19,46,82,190). In addition, it is widely reported that the use of squatting movements enhances speed-strength characteristics, associated concentric muscle actions (e.g., static vertical jump) (19,82,84,85,167,210), and jumping motions using a stretch shortening cycle (e.g., countermovement jump) (6,9,10,19,38,39,82,84,89,182,185,191). The vertical jump has also been related to sprinting performance. Because of the widely reported interrelationships between strength and vertical jump performance and the fact that vertical jump capacity relates to sprinting performance, it is logical that strength characteristics would be generally positively correlated to both sprinting and jumping. Indeed, there are several studies that have noted a positive relationship between squat strength and sprinting performances (33,34,39,129,171). Based on this line of reasoning, for this review, the intention of the authors is to highlight and to provide an exploration of the potential benefits of using squats as a primary resistance training movement for athletes. 1494 Copyright © 2024 National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited. Squats in Sport: Terminology and Biomechanics (2024) 38:8 | www.nsca.com Table 1 Basic characteristics associated with increased maximum strength. Downloaded from http://journals.lww.com/nsca-jscr by BhDMf5ePHKav1zEoum1tQfN4a+kJLhEZgbsIHo4XMi0hCyw CX1AWnYQp/IlQrHD3i3D0OdRyi7TvSFl4Cf3VC1y0abggQZXdtwnfKZBYtws= on 08/29/2024 Magnitude of force production—greater peak and average forces can be produced, allowing for higher velocities and power outputs to be achieved with absolute submaximal loads Greater peak and average power output—work can be accomplished at a higher rate Greater ability to develop and respond to stretch shortening cycles (SSC) Increased absolute endurance, especially high-intensity exercise endurance (HIEE) through an enhanced mechanical efficiency and exercise economy and altered metabolism and a reduced central nervous system fatigue that can allow more total work to be accomplished Greater rate of force development (RFD)—faster muscle activation, greater force during critical periods—important for acceleration Greater and more efficient motor unit synchronization, which is important for ballistic movements Greater postural/positional strength that can underpin superior balance and enhance the ability to hold static and dynamic positions and perform better during static and dynamic performances Some evidence suggests that force sensitivity and sensation can be superior (may be a result of strength training partially independent of maximum strength)—modulate force production ability may be superior Literature was gathered from Google Scholar and PubMed. With few exceptions, literature cited was confined primarily to those studies using free weights. The literature cited was limited to the back squat, and front squat, and their derivatives (e.g., partial, speed squats). Free weights and squat type (back or front) were chosen as these are commonly used in the training of athletes worldwide. Although, other forms of squats, such as overhead and Zercher squats, are used in training to a greater or lesser extent, there is limited research available exploring their efficacy. Key words and phrases (English) used in the search included, “squats,” “front Squats,” “strength,” “strength-endurance,” “power endurance,” and “high intensity exercise.” Squat Training and Strength Development Examining the scientific literature, it is apparent that squatting movements used as a primary part of sport-specific strength training programs result in substantial increases in overall lowerbody dynamic and isometric maximum strength in a variety of populations (19,34,46,82,89). Generally, training with full or parallel squats for 10–12 weeks results in statistically significant increases of ;12–40% in the 1RM (19,89). As would be expected, over a similar short training period, better trained subjects would realize smaller improvements in squat performance (200). Based on the examination of the training response to ½ back squats, there is typically a 17–40% increase in maximum strength after only 7–10 weeks of training in relatively untrained individuals (39,89,214). This type of training can also increase the relative squat (per kg body mass) strength. For example, Hartmann et al. (89) found short-term training (i.e., 10 weeks) in previously untrained male athletes with a relative squat strength of 1.05 3 body mass (initial value) increased the parallel to approximately 1.3 3 body mass. In athletic populations, 7–8 weeks of full and ½ back squat training have resulted in a 12–25% increase in maximal squat strength (34,155,167,168). Although peak strength gains are commonly measured using 1RM values, gains can also be measured using “nonspecific” measurements, such as isometric testing (110,192). However, the gains in maximum strength are generally smaller when strength is quantified using these types of nonspecific measurements (195). For example, 12 weeks of full squats (dynamic multi-joint closed kinetic chain) resulted in small but statistically significant increased knee extension MVC (single joint) at 75° (6 6 2%, p , 0.05) and 105° (8 6 1%, p , 0.05), whereas parallel squats resulted in a nonstatistically significant increase in knee extension MVC at 40° (6 6 3%, p . 0.05) and 75° (2 6 2%, p . 0.05) (19). As an alternative to single-joint isometric testing, isometric (multijoint isometric) measures that allow somewhat greater positional or mechanical specificity can also be used to track strength alterations (27,82,155,214). When considering isometric force-time curve assessments, it is clear that there can be an angle-specific adaptation resulting from the characteristics of the training exercise (19,65,89,113,214). However, based on the current body of scientific evidence, squatting through a full range of motion results in an increased MVC at a variety of joint angles (19,81,83,89). It becomes apparent that regardless of whether dynamic (i.e., 1RM) or isometric (i.e., MVC) assessments are made, squats are effective training exercises for increasing lower-body strength. However, it is difficult to make clear comparisons between the various training studies presented in the literature because there are differences in the training methods, duration of training, and Table 2 Relationship between absolute back squat strength and jumping performance.* Back squat Author Blackburn and Morrissey (18) Carlock et al. (26) Helgerud et al. (91) Kirkpatrick et al. (112) Nuzzo et al. (152) Peterson (156) Requena et al. (163) Requena et al. (164) Wisløff et al. (207) Subjects/athletes Physiotherapy students Weightlifters Soccer players Rugby league forwards Rugby league backs Male athletes Collegiate athletes Sprinters Soccer players Soccer players Type Half Full Half Half Parallel Half Half Half Half Correlation (r) 1RM (kg) Countermovement vertical jump height 74.8 6 28.7 163.6 6 51.2 176 6 16.2 140.2 6 26.2 132.7 6 9.4 170.8 6 22.6 110.4 6 38.8 171.7 6 21.2 119.5 6 26.2 171.7 6 21.2 0.72† 0.52† 0.38 0.57‡ nr Static vertical jump height 0.22 0.86† 0.18 0.50† 0.78† nr nr nr 0.58† nr nr 0.50† nr *nr 5 not reported. †p , 0.05. ‡p , 0.01. 1495 Copyright © 2024 National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited. Squats in Sport: Terminology and Biomechanics (2024) 38:8 Table 3 Relationship between relative back squat strength and jumping performance.* Back squat Downloaded from http://journals.lww.com/nsca-jscr by BhDMf5ePHKav1zEoum1tQfN4a+kJLhEZgbsIHo4XMi0hCyw CX1AWnYQp/IlQrHD3i3D0OdRyi7TvSFl4Cf3VC1y0abggQZXdtwnfKZBYtws= on 08/29/2024 Author Subjects/athletes Carlock et al. (26) Kirkpatrick et al. (112) Weightlifters Rugby league forwards Rugby league backs Soccer players Male athletes Collegiate athletes Sprinters College students Helgrud et al. (91) Nuzzo et al. (152) Peterson (56) Requena et al. (64) Young and Bilby (214) Full Half Half Parallel Half Half Half squat (machine) Correlation (r) 1RM (kg.kg21) Type Countermovement vertical jump height nr 1.56 6 0.20 1.61 6 0.13 1.48 6 0.26 1.93 6 0.21 1.47 6 0.37 1.92 6 0.17 2.79 6 0.30 Static vertical jump height 0.69† 0.42† nr 0.72† 0.23 0.69† 0.85† 0.32 0.47† nr nr nr nr nr *nr 5 not reported. †p , 0.05. . methods used to assess strength gains. Therefore, further research that clearly outlines the variables used to create a well-structured training program is warranted. Squat and Jumping Performance. Several authors have suggested that sporting success is largely predicated by the development of strength and power, which both play a large role in vertical jump performance (122,138,162,178,183). Of note, is the observation that the ability to jump is considered one of the fundamental skills for success in numerous sports (22,25,54,69,70,89,100,122,156). Based on previous research, the vertical jump is a reliable predictor of sporting success in American football (140), soccer (72,144,207), ice hockey (15), sprint cycling (184), and numerous other sports (50). Successful vertical jump performance is therefore predicated by the ability to generate maximal forces and express those forces rapidly to maximize jumping height (1,10,80,122,137). Because strength underpins vertical jumping ability, it is widely believed that increases in maximum leg strength are related to improvements in vertical jumping performance (6,10,18,19,38,39,82,84,85,167,182,185,190,191). Although not all research show strong correlations (91,152,163), strong relationships between leg strength and vertical jump performance have been reported between the absolute 1RM in the half and full back squat and countermovement vertical jump height in untrained students (18), weightlifters (26), soccer players (164,207), rugby league athletes (112), and various collegiate athletes (156) (Table 2). In addition, support for this relationship can be noted as a result of the strong relationships that have been reported between the absolute 1RM and the static vertical jumps of weightlifters (26), soccer players (91), sprinters, and various male athletes (152). Based on the current body of research, it seems that approximately 25–75% of the variance in the countermovement vertical jump performance can be explained by the athlete’s absolute back squat strength (181). Although absolute strength seems to be an important contributor to an athlete’s ability to jump, it may be more important to consider the athlete’s relative strength levels. Bobbert (20) have reported that the size of the jumper and the amount of force that is generated relative to the size of the jumper can dictate acceleration before takeoff and the overall vertical displacement achieved. Therefore, Table 4 Relationship between absolute back squat strength and sprinting performance.* Correlation coefficient (r) Absolute strength and sprint time Back squat Author Subjects/athletes Type Baker and Nance (8) Rugby league Full Brechue et al. (21) Chaouachi et al. (30) Comfort et al. (35) Cronin and Hansen (42) Cunningham et al. (43) Helgerud et al. (91) Kirkpatrick et al. (112) Football Basketball Soccer players Rugby league Parallel 170.5 6 28.2 Half Half 142.3 6 25.3 Parallel Rugby union Deep 186.2 6 22.6 Soccer players Rugby league forwards Rugby league backs Sprinters Soccer players Soccer players Half Half 176 6 16.2 140.2 6 26.2 132.7 6 9.4 Half Half Half 149.0 6 12.0 119.5 6 26.2 171.7 6 21.2 Requena et al. (163) Requena et al. (164) Wisløff et al. (207) 1RM (kg) 3RM (kg) 4.5 m 5m 157.9 6 18.8 9.1 m 10 m 15 m 20 m 30 m 36.6 m 20.06 40 m 20.19 0.11 143 6 13.4 nr 20.63† 20.60‡ 20.05 20.68† 20.65‡ 20.01 20.65 20.29 0.17 20.25† 20.51† 20.28† 20.38 20.94† 20.57‡ 20.47† 20.26 20.20† 20.54† 20.49† 20.71† *nr 5 not reported. †p , 0.05. ‡p , 0.01. 1496 Copyright © 2024 National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited. Squats in Sport: Terminology and Biomechanics (2024) 38:8 | www.nsca.com Table 5 Relationship between relative back squat strength and sprinting performance.* Correlations (r) Relative back squat Author Downloaded from http://journals.lww.com/nsca-jscr by BhDMf5ePHKav1zEoum1tQfN4a+kJLhEZgbsIHo4XMi0hCyw CX1AWnYQp/IlQrHD3i3D0OdRyi7TvSFl4Cf3VC1y0abggQZXdtwnfKZBYtws= on 08/29/2024 Baker and Nance (8) Brechue et al. (21) Comfort et al. (33,34) Comfort et al. (35) Cunningham et al. (43) Helgrud et al. (91) Kirkpatrick et al. (112) Subjects/ athletes Type 1RM (kg.kg21) Rugby league Full Football Rugby league Recreational Soccer players Rugby union Parallel 1.69 6 0.33 Half 1.78 6 0.27 1.77 6 0.33 Half 1.97 6 0.34 Deep Soccer players Rugby league forwards Rugby league backs Football players Half Half McBride et al. (136) Meckel et al. (142) Sprinters Requena et al. Sprinters (164) 3RM (kg.kg21) 4.5 m 5m 9.1 m 10 m 20 m 30 m 36.6 m 20.39 nr 20.34 20.62‡ 20.52‡ 20.55† nr 1.48 6 0.26 1.56 6 0.20 1.61 6 0.13 Parallel 1.94 6 0.33 Half Half Relative strength and sprint time 20.45† 1.92 6 0.20 1.92 6 0.17 100 m 20.53† 20.67‡ 20.30 20.32 20.46‡ 20.45‡ 40 m 20.66† 20.55† 20.44‡ 20.60† 20.53‡ 20.59† 20.56‡ 20.50‡ 20.89‡ *nr 5 not reported. †p , 0.05. ‡p , 0.01. relative strength levels may be more important when examining the relationship between squat and jumping performances (Table 3). Considering the relative ½ and parallel back squat 1RM, a moderate-to-large relationship has been reported with countermovement vertical jumps (112,152,212). Strong correlations have been found between the relative ½ squat maximum strength and countermovement vertical jump in collegiate athletes (156). In addition, the relative full back squat 1RM has been reported to be strongly associated with countermovement vertical and static vertical jump height (26). In summary, it seems that lower-body strength, as indicated by the absolute and particularly the relative 1RM back squat, is substantially related to vertical jump performance. Therefore, it seems that the use of loaded squatting movements in the preparation of athletes who need to maximize vertical jump performance is warranted. Squat Training and Jumping Performance. In the scientific literature, there are numerous studies ranging from 5 to 24 weeks in duration, which demonstrate that increases in deep or parallel back squat strength (1RM) translate into improvements in countermovement jump and static jump performance (9,19,39,41,85,87,88,94,102,105,109,143,192,197,201,209). Although, the percent gain and magnitude of effect were moderate (as calculated by the authors of this article), studies using short-duration training (,5 weeks), basketball players (93), and American football players (94) have not shown statistically significant improvements in jumping performance. Such short studies may be of insufficient duration, thus not allowing time for the strength gains achieved in the back squat to be translated into countermovement jumping performances (91). Although not all (74,93,121,130,168,199), most studies indicate that 7.5–10 weeks of training targeting strength gains, with the use of ½ squats, produced substantial static (6,19,31,37,40,89,194,195) and countermovement (19,40,89,194,195,206,214) vertical jump performance gains among relatively untrained subjects with a low initial performance capability. Interestingly, the parallel squat has demonstrated the greatest gains in strength and the greatest transfer of training effects to jumping movements based on the method of Zatsiorsky (216) and Zatsiorsky et al. (218). This method (coefficient of transfer) estimates the degree of trained exercise transfer to a nontrained performance measure using the following equations: Result gain ¼ Gain of performance Standard deviation of performance Coefficient of transfer ¼ Result gain in nontrained performance : Result gain in trained performance The higher the calculated training scores, the greater the transfer of training effect. Based on these calculations, the parallel back squat produces the greatest positive transfer of training effects to jumping movements (199). Collectively, it seems that based on the majority of the findings in the scientific literature increases in 1RM squat strength, especially relative squat strength (absolute per kg body mass), can lead to increases in vertical jump performance. One aspect that is of particular importance in assessing the improvement of squat strength and its influence on vertical jump performance deals with force impulse. Often among study participants or athletes gaining weight, the vertical jump does not show concomitant alterations over time (187). However, the impulse does increase (i.e., greater body mass same jump height). It is of note that in many of the studies indicating that squat training did not alter jump height, body mass did increase, indicating an increased impulse and thus a positive training effect (187). In addition, the depth of the squat may play an integral role in mitigating the transfer of training effects from the squat to jumping tasks. Even though there is compelling evidence that improvements in squat strength correspond to improvements in vertical jump performance, more research is needed to fully 1497 Copyright © 2024 National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited. Squats in Sport: Terminology and Biomechanics (2024) 38:8 understand the improvements and how these alterations are stimulated by increasing squatting strength. Downloaded from http://journals.lww.com/nsca-jscr by BhDMf5ePHKav1zEoum1tQfN4a+kJLhEZgbsIHo4XMi0hCyw CX1AWnYQp/IlQrHD3i3D0OdRyi7TvSFl4Cf3VC1y0abggQZXdtwnfKZBYtws= on 08/29/2024 Squat and Sprint Performance. Running speed and more importantly the ability to accelerate during sprinting based activities are critical components for success in performance-based improvements (132). This ability is a critical attribute that encompasses performance in numerous individual and team sports (25,30,55,58,69). For example, when examining Rugby Union players (69), the ability to accelerate is considered one of the critical attributes because the average sprint duration during a match is only approximately 3 seconds in duration (47,48,55,56). Taken collectively, it is very apparent that the ability to accelerate and sprint at high velocities should be a key target for most strength and conditioning professionals. Careful inspection of acceleration and sprinting characteristics reveals a clearly defined relationship between the ability to exert a given ground reaction force (GRF) and subsequent running velocity (202,203,208). Strong correlations have been reported between GRF magnitude, impulse, and the ability to accelerate and achieve higher velocities during sprinting actions (99,199). During the acceleration phase of sprinting, it is evident that athletes must accelerate their center of mass (CoM), through an explosive concentric force production, generated by both the knee and the hip extensor muscles (46,59,61,132,205). Thus, expression of high concentric forces and power is required to generate high velocities during this time frame (143). On achieving maximal running speed, forward propulsion is accomplished by the actions of the hip and ankle extensors (143,205). During this phase of sprinting, each foot strike can occur in less than 0.1 seconds and can result in vertical GRF in excess of 5 times body weight (143). These findings highlight the critical importance of developing the athlete’s overall lower-body strength and ability to express high power outputs. Therefore, it is universally accepted that to develop acceleration and sprinting speed, athletes should use lower-body strength training methods, including squats, to enhance these capacities (8,45,46). When examining the relationship between lower-body strength and sprinting ability, studies that have used single-joint exercises such as the leg extension performed in either an isometric or an isokinetic condition have found no (164), small (17,53,212), or moderate (149) relationships between strength and sprinting performance. Conversely, when multi-joint exercises, such as the squat, have been examined, stronger relationships have been found between maximum strength and sprinting performance (175). Indeed, when ½, parallel, or deep back squat have been correlated with various sprint performance distances, moderate-to-nearly-perfect correlations have been found (8,30,42,43,91,112,163,207) (Table 4). Calculation of the relationship between absolute back squat strength and sprinting performances between 5 and 40 m resulted in minor-to-nearly-perfect correlations (30,36,91,112,136,163,207). For example, Comfort et al. (35) indicated that among youth soccer players, the absolute 1RM ½ back squat is strongly correlated with 5 m (r 5 20.60, p , 0.01) and 20 m (r 5 20.65, p , 0.01) sprint times. Although there are several studies that demonstrated large-tonearly-perfect correlations between the absolute ½ back squat and sprint times, there are several studies that report trivial-to-small correlations (17,42,43,112). For example, among Rugby League athletes, Cronin and Hansen (42) and Kirkpatrick and Comfort (112) reported trivial-to-small correlations between sprint times at 5, 10, 30, and 40 m and the absolute 1RM in the parallel and ½ back squat. Interestingly, it seems that these data indicate that weaker correlations exist between absolute back squat performance and sprint times at various distances in sports such as rugby and football, where the athletes tend to have a large overall body mass. Conceptually, this would make sense as Weyland and Davis (201) indicated that intermediate levels of massiveness are conducive for optimizing human speed. Thus, it may be more appropriate to examine the relationships between relative back squat strength and sprinting performance to better understand the impact that squatting exercises and training have on performance (21). The most common method for determining relative strength is to simply divide the weight lifted by the body mass of the athlete (101). When squats are expressed relative to body mass, moderate-to-very-large relationships to sprinting performance have been noted (8,21,33,35,43,112,136,142,163) (Table 5). This increased relationship has been illustrated by Baker and Nance (8), who reported large correlations between the relative 3RM full back squat and 40-m sprint time (r 5 20.66, p , 0.01) in Rugby League athletes. When the absolute 3RM values are used to examine this relationship with the same athletes, the correlation is substantially reduced (r 5 20.19, p . 0.05). Similarly, among American football players, the relationship between the 1RM relative parallel back squat and the 36.6-m sprint resulted in a strong relationship (21,136). In addition, among sprint athletes, the half back squat is correlated with the 10 m (r 5 20.53, p , 0.01), 30 m (r 5 20.56, p , 0.01), 40 m (r 5 20.50, p , 0.01), and 100 m (r 5 20.89, p , 0.1) (21,133). In addition, Keiner et al. (108) reported that relative parallel back squat of youth soccer players explains 36% of the variance (r 5 20.60) in their 30-m sprint performance. Collectively, when relative back squat maximum strength is examined, between 19 and 79% of the variance in sprinting performance between distances of 5–100 m can be explained. Although dividing the back squat RM values by the athlete’s body mass is a common practice, it is well documented that strength does not increase in direct proportion to body mass gains in trained individuals (91,101). Jaric (101) suggested that the athletes’ body size may affect the strength testing results and that the principle of “geometric similarity” should be considered. Based on this principle, muscle force-generating capacity should be proportional to either body height squared (H2) or body mass raised to the two-thirds power (m2/3). In addition, Jaric (101) recommended that to truly understand relative strength, the load lifted should be allometrically scaled by dividing the load lifted by body length squared (H2) or a body-volume-related index raised to the twothirds power (m2/3). Ultimately, there are very few studies in the scientific literature that have been investigations of the association of allometrically scaled squat performance and sprinting performance. Helgerud et al. (91) examined ½ squat 1RM scaled relative to body mass and scaled allometrically and their relationship to sprinting performance. When the 20-m sprint time was correlated with relative ½ back squat 1RM (kg.kg21), there was a nonsignificant moderate correlation (r 5 20.33, p 5 0.14). However, an allometrically scaled ½ back squat 1RM (kg.kg20.67) correlated to sprint time was substantially larger (r 5 20.41, p 5 0.07), agreeing with the findings of Swinton et al. (190). These data suggest that allometrically scaled metrics may reveal stronger relationships than previously noted between back squat strength and sprinting performance. Therefore, future research should investigate allometrically scaling the squat 1RM by dividing its value by the mass of the subject raised to the two-thirds power. In summary, it seems that both absolute and relative strength levels as expressed during various types of back squats are related to acceleration and sprinting performance (8,30,33,43,91,112,136,156,163,207). Thus, scaled results likely 1498 Copyright © 2024 National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited. Squats in Sport: Terminology and Biomechanics (2024) 38:8 | www.nsca.com produce stronger relationships, but more research is needed to confirm this contention. However, the common belief that squatting abilities are an important contributor to sprinting speed and acceleration seems to be justified by the scientific literature. Downloaded from http://journals.lww.com/nsca-jscr by BhDMf5ePHKav1zEoum1tQfN4a+kJLhEZgbsIHo4XMi0hCyw CX1AWnYQp/IlQrHD3i3D0OdRyi7TvSFl4Cf3VC1y0abggQZXdtwnfKZBYtws= on 08/29/2024 Squat Training and Sprint Performance. Examination of the relationship between squat training and sprint performance gains demonstrated that increases in absolute or relative half or deep back squat maximum strength results in improvements in sprinting performance (34,39,92,104,108,109,121,133,170). Relatively untrained or weaker subjects seem to experience improvements in sprinting performance as a result of increasing absolute and relative squat strength (39). For example, in relatively weak individuals, Cormie et al. (39) reported that after 10 weeks of back squat training, there was a statistically significant increase in relative ½ back squat performance (31.2 6 2.3%) that corresponded to a statistically significant improvement in 40 m (2.2 6 1.9%) sprint time. However, as might be expected, Kiener et al. (109) indicated that as squat strength improves, it explains less of the variance in sprinting performance. Considering athlete groups, including juniors, evidence indicates that increasing absolute and relative lower-body maximum strength (1/2 and full squats) has substantial impact on running speeds, particularly over the initial first 5–10 m of a sprint (34,91,108,109,168,175). The greater improvements in the first 5–10 m were expected because the greatest forces are required during this time frame, and it contains the initial acceleration phase of sprinting. Not all training studies have reported improvements in sprinting performance that corresponded to increases in squatting performance (171). However, combined training programs, using combinations of the full squat, ½ squat, ¼ squat (Figure 1A–C), jump squat, and plyometrics, have resulted in a small improvement in sprinting performance (79,183,186). This finding is in line with a number of authors who recommend that a mixed method of training that targets maximal strength (i.e., back squat) and power development (e.g., plyometrics, weightlifting, jump squats) in a logical order may be needed when attempting to maximize athletic development (79,102,179,183,186). Collectively, the contemporary body of scientific evidence indicates that increased back squat strength, particularly relative back squat strength can result in improvements in sprinting performance. Although this contention seems to be based on sound science, again, more research is warranted to truly understand the role squatting has in the development of sprinting performance. Squatting Depth and Performance. One common question in the applied literature is what squat depth offers the best transfer of training to actual sports performance. Zatsiorsky (216) suggested that various depths of squats should be performed depending on the phase of the annual training plan. For example, during offseason periods, he recommended a 70–30% split between deep and partial squatting movements. More recently, Zatsiorsky and Kraemer (217) suggested that if the sporting movement requires high force to be applied in a short movement segment, there is no reason to perform full squats. Thus, it promotes a form of anglespecificity–based transfer of training effect. Conversely, Hartmann et al. (89) questioned this line of thought and offered compelling evidence that this may not be an accurate or appropriate recommendation. To achieve a maximal vertical jump height, the applied impulse can be modified through the acceleration sequence. Specifically, one could modify the eccentric phase by increasing descent velocity, minimize transition time, maximize RFD, and optimize the impulse with targeted training methods to optimize jumping height (37,40,89). Ultimately, the training goal when targeting jumping improvements is to increase the size of the overall impulse, which is applied during a specified time frame (89). Hartmann et al. (89) suggested that the recommendation to limit the range of motion (ROM) to better match the competition movement presented by Zatsiorsky et al. (216) does not allow for the optimization of the overall impulse during jumping movements. In support of this contention based on recent research into the squats depth, full squatting movements produce superior improvements in vertical jump performance when compared with parallel (19) or partial squats (89). in addition, Hartmann et al. (89) suggest that the full or parallel squat exercise allows for an elevated force development that is available across a full range of knee joint angles. Overall, the full squat allows for the development of lower-body strength capacities throughout a full ROM serving as the foundation from which sport-specific technique training, such as those observed in competitive movements such as sprinting and jumping, can be developed. Specifically, the deep joint positions achieved in the full squat supply the required neural and morphological stimuli necessary to develop the knee and hip extensors ability to positively influence acceleration. To maximize the transfer of training effects to reactive speed-strength sports, a mixed-methods approach in which heavily loaded and ballistic strength exercises, that can include partial squat movements, is required (7,79,89,183). Based on this line of thought, Hartmann et al. (89) suggested that the full back squat should rarely be removed from a periodized training program even during competitive periods because it is essential for maintaining lower-body strength. Indeed, full squats (including full front squats) can develop and maintain muscle volume better than partial squats (124). By maintaining strength over a full ROM (i.e., full squats), the athlete is then better able to safely handle the high relative and absolute loads associated with speed strength methods such as plyometrics and speed training. From a specificity standpoint, examination of strength gains indicates training with the full squat resulted in statistically greater gains in full back squat strength, whereas training with the ¼ back squats resulted in superior gains in the ¼ back squat 1RM (19) (Figure 2). Importantly, scientific literature provides substantial evidence indicating that the depth of the squat exhibits a training specificity effect and seems to affect the overall performance capacity (19,89). Collectively, data suggest that full back squats produce a superior transfer of training effect to jumping movements. For example, Bloomquist et al. (19) reported that 12 weeks of full back squat training resulted in a statistically greater increase in the countermovement vertical jump performance (113 6 2%) compared with a ¼ back squat (17 6 4%). Similar alterations were noted in the static jump (19). Hartmann et al. (89) examined the effects of training with various depths of squats on jumping performance. After 10 weeks of training, the full back squat group displayed a statistically greater increase in countermovement and static jumps (CMJ 5 17.79 6 5.43%; SJ 5 15.83 6 6.06%) compared with the ¼ back squat group (CMJ 5 20.01 6 6.77%; SJ 5 12.68 6 7.75%). Interestingly, when the ¼ back squat was compared with the deep front squat, it was noted that the full front squat also resulted in a statistically greater increase in countermovement jump (18.29 6 6.15%) and static jump (17.19 6 7.33%) when compared with the ¼ back squat. In addition, there seemed to be a conformity of motion quality existing between full front and full back squats (Figure 3). Specifically, the full front squat 1RM 1499 Copyright © 2024 National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited. Squats in Sport: Terminology and Biomechanics (2024) 38:8 Downloaded from http://journals.lww.com/nsca-jscr by BhDMf5ePHKav1zEoum1tQfN4a+kJLhEZgbsIHo4XMi0hCyw CX1AWnYQp/IlQrHD3i3D0OdRyi7TvSFl4Cf3VC1y0abggQZXdtwnfKZBYtws= on 08/29/2024 Figure 1. A) Full squat, (B) ½ squat, and (C) ¼ squat. statistically increased (percentage) after training only with the full back squat, which was similar to the gain achieved by training only with the full front squat. Conversely, training with the ¼ back squat resulted in essentially no alteration in 1RM front squat strength (Figure 3). Considering the percent change in the full back squat 1RM, training with either the full front or the full back squat resulted in a statistical increase. Training with only the ¼ back squat resulted in a statistically significant percent reduction in the full back squat 1RM (Figure 3). Finally, the greatest percent gains in ¼ back squat strength came from training with the ¼ back squat. Training with both the full front and the full back squat resulted in increases in the ¼ back squat 1RM but the greater percent change occurred in response to the full back squat (Figure 3). Ultimately, based on the current data, training with full front and back squatting movements have the ability to transfer strength gains between one another (77,89). Collectively, the data suggest that full squatting movements have the potential to better “translate” strength gains, allowing for the development of strength capacities necessary to induce large increases in vertical jump performance. Currently, there is an increased interest in researching the effects of squatting depth and its effects on strength gains, jumping performance, and the translation of strength gains to athletic performance. Indeed, although research indicates that the full squat is a foundational exercise, some relatively recent research indicates that partial ROM squats in conjunction with full squats may be an effective training method for improving maximal strength as measured by a full squat 1RM and an isometric squat peak force (13). In addition, enhancement of early isometric force-time curve characteristics was noted in male athelets with previous strength training experience (13). Bazyler et al. (13) also noted that partial squats may be beneficial for strength-power Figure 2. 1RM changes in the ¼ squat and full back squat after 12 weeks (19). 1500 Copyright © 2024 National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited. Squats in Sport: Terminology and Biomechanics (2024) 38:8 | www.nsca.com Downloaded from http://journals.lww.com/nsca-jscr by BhDMf5ePHKav1zEoum1tQfN4a+kJLhEZgbsIHo4XMi0hCyw CX1AWnYQp/IlQrHD3i3D0OdRyi7TvSFl4Cf3VC1y0abggQZXdtwnfKZBYtws= on 08/29/2024 Figure 3. Comparison of 1 RM for the front squat, back squat, and ¼ squat: transfer effects (89). athletes during a strength-speed block or while peaking for competition (13). It should also be noted that the addition of ballistic movements, such as jumps in combination with squats, may provide additional benefits for explosiveness and power development (27,196). Ultimately, when attempting to increase the strength capacity of the lower body, it is important to note that more research should be conducted to verify all these findings. Aspects that are not well studied include the differential training level effects, sequential ordering of exercises both acutely and over a training period, combination training (mixed partial and full squats), the use of speed squats, and the effect of partial movements among athletes that may already be well-trained in the full squat. Squats and Muscle Hypertrophy Alterations in CSA and architecture of muscle, along with the nervous system alteration, offers primary mechanisms for the enhancement of force production and therefore performance (11,96,173). Briefly, based on the available literature, the squat can be an effective exercise for inducing hypertrophy of the gluteus maximus and the entire quadriceps, primarily the vastus muscles with lesser effects noted for the rectus femoris (124,150,165). Substantial evidence indicates that relative activation and subsequent hypertrophy from the back squat is quite low for the hamstrings (134,165). Evidence suggests deeper squats, particularly full back squats, may be more hypertrophic for the gluteus maximus, although squat depth deeper 90° of knee flexion may not provide additional hypertrophy of the knee extensors (134,165). Inhomogeneous hypertrophy can occur both within and between muscles as a result of resistance training (215). Indeed, although limited, studies of the squat have also demonstrated intramuscle inhomogeneous hypertrophy (57,165). It is apparent that squat variations (57) and weightlifting training including squats and pulling movements (106) and speed of movement alterations (78) can induce different degrees of inhomogeneous hypertrophy. Importantly, inhomogeneous alterations in muscle and connective tissue, CSA, and architecture, as a result of squat training, could alter the quadriceps moment of inertia and the moment arms of muscle ‘‘compartments,’’ associated limb moments, and influence force transmission and the muscular force expression (57,102). Considering the performance results of selective hypertrophy, these alterations could be biomechanically beneficial to high-force or high-power movements used in training and performance. However, it is also theoretically possible that the selection of inappropriate exercises for a particular sports group could negatively affect performance. For example, compared with back squats, some evidence indicates that front squats better activate the gluteus and quadriceps muscle, particularly the vastus medialis (2013) and may provide a greater overall intramuscle hypertrophy adaptation. Interestingly, faster sprinters (running) display considerable proximal quadriceps hypertrophy and smaller overall pennation angles compared with lesser sprinters (128). So, introducing a squatting exercise(s) (i.e., front squats) that would create considerable distal quadriceps hypertrophy or markedly alter pennation angle could result in decreased sprint performance. Implications for Prehabilitation and Rehabilitation Injuries commonly affect at least one of the following tissues: skeletal muscle, tendon, ligament, cartilage, and bone. The probability and severity of tissue injury depends, in part, on their functions, mechanical, and structural characteristics (204). For example, many noncontact injuries can be regarded as a tear of 1501 Copyright © 2024 National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited. Squats in Sport: Terminology and Biomechanics (2024) 38:8 Downloaded from http://journals.lww.com/nsca-jscr by BhDMf5ePHKav1zEoum1tQfN4a+kJLhEZgbsIHo4XMi0hCyw CX1AWnYQp/IlQrHD3i3D0OdRyi7TvSFl4Cf3VC1y0abggQZXdtwnfKZBYtws= on 08/29/2024 different levels in a tissue (e.g., microtear to complete tear in Achilles tendon). Theoretically, the severity of a tissue tear can be reduced by increased stiffness and stress capacity (material strength). Besides stiffness and mechanical tension, other mechanical properties are often considered important for injury prevention and severity reduction. These are different types of force (e.g., compression, shear force, and torsion), anisotropic property, deformation, mechanical stress and strain, modulus of elasticity, and viscoelasticity (204). Furthermore, tissue content (e.g., collagen, elastin), density, and CSA associate with mechanical properties. Literature suggests that physical activities can alter tissue function, mechanical, and structural characteristics (204). A properly designed resistance training programs provides mechanical overloading, resulting in these adaptations (2,66,103,153,180), likely reducing the probability and the severity of injuries. Skeletal Muscle and Intramuscular Extracellular Matrix. Longterm resistance training increases the CSA of a skeletal muscle (2,66). This is an important characteristic of resistance training for both performance and rehabilitation, particularly when skeletal muscle atrophy is apparent after an injury requiring immobilization or reduced use. Furthermore, the intramuscular extracellular matrix (IEM), which includes the endomysium, perimysium, and epimysium (160), responds to mechanical loading and increases collagen synthesis similarly to increased protein synthesis in skeletal muscle (115,135). Although more research is needed, increased collagen synthesis can be regarded as important evidence that the IEM adapts to mechanical loading (105,135,174). This adaptation should increase its resistance to mechanical loading, reducing skeletal muscle injuries such as muscle strains. Moreover, when a muscle strain occurs, it is usually in or near a myotendinous junction (193). Although, research is scarce, the effects of resistance training, running (44,117), and ladder-based resistance training (158) have been reported to cause the morphology of the myotendinous junction to change in animals, which can be related to an increased support and stabilization of force transmission (157). Furthermore, structural alterations at the myotendinous junction should increase mechanical resistance (e.g., resistance to strain) and decrease injury potential. Regarding reduced skeletal muscle injury risk and rehabilitation, the squats and variations allow exercise with moderate to heavy loads safely (refer to the section on safety). Range of motion has been suggested to be one of the key stimuli for skeletal muscle hypertrophy (68,95). Although all muscles may not hypertrophy to the same extent (165), movements performed with a greater versus lesser ROM seem to be more efficient and effective when performed for the same sets, repetitions, and percentage of 1RM (19). Clear relationships between intensity, the magnitude of mechanical stress, and collagen synthesis have yet to be established. If mechanical loading from exercise increases collagen synthesis, consequently altering morphology and mechanical properties of the IEM and myotendinous junctions, it seems reasonable to speculate that higher intensity with sufficient volume-load can induce a greater magnitude of changes. Furthermore, Andersen et al. (3) reported that conventional rehabilitation exercises are limited in their abilities to provide higher intensity compared with typical resistance training exercises such as the squat. This finding suggests that as the process of rehabilitation progresses, it is important to incorporate typical resistance training exercises such as the squat to provide sufficient overload. Tendon and Ligament. Compared with skeletal muscle adaptations, there is less research examining tendon and ligament adaptations to resistance training. Research on tendon and ligament alteration can be important, particularly to understand the effectiveness of resistance training on prevention and rehabilitation of injuries such as anterior cruciate ligament tear. Tendons. Human tendons are metabolically active tissues (114) readily adapting to physical activities (24,90,115). A bout of physical activity can result in elevated levels of autocrine and paracrine factors such as transforming growth factor beta 1 and insulin-like growth factor 1 and an increase in collagen synthesis (90). It seems that human tendons do hypertrophy (90); however, the hypertrophy seems to be largely confined to the proximal and distal ends of tendons (120,177). The exact training induced architectural alterations likely depend on the specific training method (62,123,141). Increases in content and changes in mechanical properties of human tendons in response to training have been reported (28,62). Increases in tendon stiffness and estimates of tendon content (e.g., the amount of collagen) occurred as early as 2 months postisometric training (123,125). However, it seems that the level of mechanical loading must exceed a certain threshold for a tendon CSA to increase (4,5,120) and that the degree of tissue remodeling and increased tendon CSA seems to depend on the magnitude of mechanical strain (198,219). Thus, resistance training seems to be an effective method to induce tissue adaptations in tendons (157,158). Although direct evidence seems to be lacking, increased tendon CSA and content and changes in tendon mechanical properties are speculated to reduce tendon injury risk, such as Achilles tendon rupture. In addition, resistance training performed in particular manners (e.g., eccentric or heavy-slow resistance training) has been reported to be an effective treatment method for chronic tendinopathy (107,119,131,166), particularly in male athletes (116). Although not optimum for strength gains, one heavy slow resistance training session has also been reported to be effective in tendinopathy rehabilitation (146) and may be more effective than a corticosteroid injection in reducing pain over a 12-week period (119). Scant research exists on squatting adaptations, such as the patellar tendon. Early study reported that there were no changes in patellar tendon CSA after 12 weeks of training using parallel squat or 120° internal knee angle partial squats (19). However, a variation of the squat, eccentric declined squats, have been reported to be effective in reducing pain in chronic patellar tendinopathy, indicating that the patellar tendon undergoes a level of mechanical stress sufficient to elicit physiological changes (119,211). Importantly, evidence also indicates that tendon hypertrophy in concert with muscle CSA alterations (177). The exact mechanisms of how eccentric declined squat can aid in healing chronic patellar tendinopathy are not clear. However, the loaded eccentric component seems to be a key factor in treating chronic tendinopathy (147). Furthermore, although declining (squat depth: to an 85° knee angle, with 180° being the full extension and to parallel) seems to increase mechanical loading on the patella tendon (67,118), this does not mean that the squat and variations do not have any degree of mechanical loading on the patellar tendon. In fact, Frohm et al. (67) reported tension greater than 6,000 N in the patellar tendon during a Bromsman parallel squat (a bilateral eccentric overloaded squat performed with a loaded barbell, to which steel wire cables were attached to move the barbell up and down at a preset velocity) on a flat 1502 Copyright © 2024 National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited. Squats in Sport: Terminology and Biomechanics (2024) 38:8 | www.nsca.com Downloaded from http://journals.lww.com/nsca-jscr by BhDMf5ePHKav1zEoum1tQfN4a+kJLhEZgbsIHo4XMi0hCyw CX1AWnYQp/IlQrHD3i3D0OdRyi7TvSFl4Cf3VC1y0abggQZXdtwnfKZBYtws= on 08/29/2024 surface. A Bromsman squat may not produce exactly the same stimulus as the typically performed squat. However, a similar level of tension in the patellar tendon is expected during the squat. In addition, it is rather unreasonable to speculate that the morphology, mechanical properties, and/or content of the patellar tendon do not adapt to the squat when the knee extension exercise and isometric sled squat at a 90° knee angle showed increases in the patella tendon CSA and changes in mechanical properties such as stiffness (120,126). Besides the patella tendon, the Achilles tendon is speculated to undergo large mechanical loading during the squat as the plantar flexors are reported to be highly activated at knee angles ranging from 105° to 119° (180° 5 full extension) compared with smaller knee angles (23). More research on these parameters is encouraged. Ligaments. Although there seems to be a paucity of research on adaptations of ligaments to any type of training in humans, including resistance training, evidence indicates that weightlifters, training at least for 10 years, have greater CSAs of the anterior and posterior cruciate ligaments than a control group (75). Evidence also indicates that high-level spring board divers and figure skaters have hypertrophied knee ligaments, especially on the “drive leg” for divers and the landing leg of skaters (14). There is also an accumulation of biochemical research showing increased ligamentous cellular activities (e.g., stimulated proliferation of fibroblasts) and collagen synthesis (directly measured or inferred from mRNA expression) in response to mechanical strain of ligaments (86,97,111,144,148,197). Without more direct evidence from training studies in humans, the degree of resistance training (including squats)-induced adaptation in ligaments cannot be ascertained. However, the literature suggests that resistance training has substantial potential to alter ligament tissue leading to reduced risks, severity of injuries, and can aid in injury prevention and a rehabilitation process back to play. Many studies have been conducted to examine mechanical stress on anterior cruciate ligament (ACL) during different types and depths of squat. Most of these studies reported that bilateral squats (performed to 90–110° knee angles and parallel) do not place an alarming level of mechanical stress on the ACL, particularly compared with other modalities such as an isokinetic knee extension (16,62,159,172,186,196,209). In addition, the PCL is reported to undergo a greater level of mechanical tension during a squat (to a 95° knee angle) than the ACL, although the level of mechanical stress still seems to be much smaller than its maximum tensile strength (60,161). These findings have 2 implications. First, a squat can place mechanical stress on the anterior and posterior cruciate ligaments without a high risk of injury. Whether the level of mechanical stress provided by a squat is sufficient to result in beneficial adaptations is still not clear because of the lack of research. However, the observed greater CSA of the anterior and posterior cruciate ligaments in competitive weightlifters, speed skaters, and divers supports the idea that the level of mechanical stress can be sufficient to induce adaptations of the morphology and potentially mechanical properties and content particularly as weightlifters perform many squats (full depth) on a regular basis (75). Second, the squat can be used safely, particularly for rehabilitation at some point after an ACL surgery. Previous studies have reported minimal levels of mechanical stress on the ACL during a squat (to 90 and 85° knee angles) (16,60). However, for rehabilitation after a PCL surgery, more caution should be exercised as a peak tensile force of approximately 1,500 to 2,200 N has been reported during a squat (to a 85° knee angle) with a 12RM load (60). Although stance width may play a role, these values seem to be below the reported maximal tensile strength of the PCL of 4,000 N (60). Regarding different variations of the squat, Kulas et al. (127) reported that mechanical stress on the ACL is reduced when individuals are instructed to moderately lean forward (75° internal knee angle) during a unilateral squat. However, an upright unilateral squat (75° internal knee angle) was still estimated to induce a peak strain value of only 3.50% as compared with 2.94% of maximum tensile strength with single-leg squat with forward trunk lean in the ACL. The peak strain value of an upright unilateral squat is comparable with strain values reported during the bilateral squat (to a 90° internal knee angle) and is still small (16). At the same time, this finding should be taken with caution as the study was conducted without loading, and thus, the finding might not be entirely applicable to loaded unilateral squats. Bone. Bone tissues are also responsive to physical activities, especially closed kinetic chain exercises and are particularly responsive to mechanical stimuli of a large magnitude or of a high loading rate (76). Of different types of physical activities, resistance training seems to be able to provide a potent stimulus for bone (76,134). Resistance training increases bone mineral density, content, and mass over a wide range of age for both sexes (76). Although, impact exercises such as hopping also induce bone tissue adaptations; these adaptations are mainly observed only in the proximal end of the femur (e.g., the femoral neck) (134). Resistance training also induces bone tissue adaptations in the L-spine and the femoral neck (134). Differences between exercises are likely because of the heavy load placed on the axial skeleton using resistance exercises, such as the squat. Moreover, certain types of resistance training exercises are capable of providing a high rate of mechanical loading (e.g., explosive exercises such as weightlifting movements and speed squats). Indeed, weightlifters have been reported to have greater bone mineral density both in the L-spine and femoral neck than age-matched controls (36). Advantages of resistance training in providing stimuli to bone seem to be the ability to provide a large magnitude of loading progressively, and safely overloading specific areas that are common sites of fracture such as L- and T-spines, greater trochanter, and intertrochanteric and femoral neck regions, particularly under conditions of reduced bone mineral density, content, and, or mass (73,76). The squat is effective in providing a mechanical stimulus to bone tissues. Studies and reviews showed that weight-bearing exercises are effective in inducing bone adaptations in the lowerextremity (e.g., femur trochanter and femoral neck) (76,98,151). Furthermore, the squat places a high magnitude of mechanical loading on the spine. This, in turn, results in bone tissue adaptations in the lumbar spine, which might be difficult with other modes of exercise such as weight-supportive exercises (e.g., swimming) or typical impact exercises (e.g., running) (134,169). Mosti et al. (145) reported that heavy ½ squat training using a machine down to a 90° knee angle induced 2.9 and 4.9% increase in bone mineral content of the L-spine and femoral neck, respectively, over the course of 12 weeks. Moreover, Dickerman et al. (49) reported that a male powerlifter holding the squat world record exhibited L-spine bone mineral density of ; 2 3 average men of the same age. In addition, the estimated spinal loading during the world record squat was more than double the previously estimated upper limit of compressive force for the human spine. However, the powerlifter had a normal physical examination, with no abnormalities in the spine (e.g., no 1503 Copyright © 2024 National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited. Squats in Sport: Terminology and Biomechanics (2024) 38:8 neurological deficits, spinal pain, signs of compressive disc diseases, and neural foraminal canal stenosis). Synthesis of these reports suggests that the squat may be one of the most effective exercises to induce bone tissue adaptations at least in the spine and the proximal end of the femur. and reduce the likelihood and severity of injuries. (b) These same adaptations are also expected to play an important role in rehabilitation particularly in injuries that require restricted use or immobilization of body parts and thus lead to consequential reduction in the CSAs and alterations in mechanical properties of tendons, skeletal muscles, and ligaments. (c) Intensity (e.g., levels of loading used) seems to be important for the mechanical and morphological adaptations for at least skeletal muscles, tendons, and bones. Therefore, intensity for the squat and its variations should progressively become greater while adhering to the concept of periodization and recognized training principles. Squats Mitigate the Risk of Lower Limb Injuries Downloaded from http://journals.lww.com/nsca-jscr by BhDMf5ePHKav1zEoum1tQfN4a+kJLhEZgbsIHo4XMi0hCyw CX1AWnYQp/IlQrHD3i3D0OdRyi7TvSFl4Cf3VC1y0abggQZXdtwnfKZBYtws= on 08/29/2024 It is well-established that resistance training attenuates the risk of sports injuries by increasing the structural strength of tendons, ligaments, cartilage and connective tissues (63). Performing regular resistance training also increases joint stability through greater muscle recruitment and enhanced rate of muscle activation that increases musculotendinous stiffness, resulting in more support for a joint (188). Heavy squats improve the overall and lower extremity strength, which could reduce the risk of injury among athletes. To evaluate this theory, Case and his colleagues investigated the efficacy of relative strength level in 1RM barbell back squats as a predictor of seasonal lower-extremity injuries. Their data were collected from Division 1 male football players and female volleyball and softball athletes. They reported a significant difference between the relative back squat strength among injured and uninjured athletes. The authors concluded that male athletes with relative squat strength below 2.2 and female athletes below 1.6 in sports mentioned above could be at a greater risk of lower-extremity injuries throughout a sporting season. Researchers in other sports also reported a significant difference in control of the lower extremities between injured and noninjured athletes. For example, Dix et al. (51) reported a larger hip abduction angle in female soccer players who suffered from ACL injuries. A larger hip abduction angle (valgus knees) occurs during deceleration and cutting in soccer, which is the result of suboptimal base strength and poor lower extremities control. The authors concluded that programs that address proximal control and strength of lower extremities can prevent ACL injuries in female soccer players. These findings were further supported by a study conducted on senior and junior female Australian Rules Football and soccer players. The researchers developed a multivariate prediction model consisting of countermovement jump peak take-off force, dynamic knee valgus, and ACL injury history. Their prediction model classified ACL injured from uninjured football players with a total accuracy of 78% (32). Collectively, it seems that resistance training programs that include heavy squats are not only beneficial to overall athletic performance but also can reduce the risk of injuries in both sexes by enhancing the structural strength of soft tissues, activation of involved muscles, and increased joint stability. References Practical Applications The squat and its variants potentially provide several benefits affecting performance, injury prevention and severity reduction and rehabilitation. (a) Squats, along with appropriate programming, are likely to effectively induce mechanical and morphological adaptations in tendons, skeletal muscles, and bones such as increased stiffness and CSAs. Although direct evidence is lacking, structural adaptations can also be expected in ligaments. These adaptations are thought to increase mechanical resistance (e.g., resistance to mechanical strain) 1. Aagaard P, Simonsen EB, Andersen JL, Magnusson P, Dyhre-Poulsen P. 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