Muscle strain injury
Etiologie
– Elongation injury
• Strain
• Partial rupture
• Complete rupture
– Avulsion fracture
– D.O.M.S.
Complications:
– Myositis ossificans
– Compartment Syndrome
Diagnosis
– Sonographic evaluation
– Examinator and equipment dependent
Backowski 2006
A new look into kicking a football: an investigation of muscle activity using MRI
The kicking action predominantly used in Australian Rules football is considered to be responsible for many
lower limb injuries. The aim of this study was to describe a non-invasive method of identifying the thigh muscles
involved in kicking an Australian Rules football, using MRI. Both upper thighs of 10 recreational footballers
were examined using a 1.5-T General Electric MRI scanner before and immediately after carrying out a set
kicking exercise protocol. The signal intensity (SI) changes in 14 individual muscles were investigated using a
standardized region of interest to determine the levels of muscle activity. Significant SI changes were observed
in several muscles of the kicking and stance legs among all participants. In the kicking leg, the greatest SI
changes were observed in the adductor longus and tensor fascia latae muscles (49.38% (+/-8.95) and 45.47% (+/7.91), respectively; P < 0.05), whereas in the stance leg, the muscles displaying the highest changes were the
semitendinosus and tensor fascia latae muscles (46.48% (+/-9.97) and 33.68% (+/-8.36), respectively; P < 0.05).
This study has shown that MRI can be useful for observing the activity of individual muscles in the upper thigh
during the kicking motion. This non-invasive approach provides a detailed analysis of anatomy and emphasizes
the muscles at high risk of injury.
 Recurrent muscle injury or persistent pain after muscle strain injury
Chance of recurrence after return from injury (1992-1998 Australian Football League)
Taylor Am J Sports Med 1990
•
Cumulative risk of recurrence for the remainder of the season (AFL)
– Hamstring strain
30%
– Thigh contusion
12%
–
–
–
•
Concussion
MCL knee strain
Ankle sprain
5%
11%
15%
Previous injury = risk factor n° 1
Intrinsic and extrinsic risk factors for muscle strain in Australian football
Orchard Am J Sports Med 2001
Intrinsic and extrinsic risk factors for anterior cruciate ligament injury in Australian footballers
The aim of this study was to examine the interaction between intrinsic (player-related) and extrinsic
(environment-related) variables as risk factors for anterior cruciate ligament injury in Australian
football. Between 1992 and 1999, 100,820 player-match exposures were analyzed for risk of anterior
cruciate ligament injury using logistic regression analysis. There were 63 surgically proven noncontact
anterior cruciate ligament injuries. The strongest risk factors were a player history of anterior cruciate
ligament reconstruction either in the previous 12 months (relative risk [RR], 11.33; 95% confidence
interval [CI], 4.02 to 31.91) or before the previous 12 months (RR, 4.44; 95% CI, 2.46 to 8.01).
Weather conditions that were associated with dry field conditions--high water evaporation in the month
before the match (RR, 2.55; 95% CI, 1.44 to 4.52) and low rainfall in the year before the match (RR,
2.87; 95% CI, 1.30 to 6.32)--were also significantly associated with these injuries. The increased risk of
injury in the first 12 months after reconstruction was associated with the reconstructed knee, whereas
after 12 months there was an even distribution of new injuries to the reconstructed knee and
contralateral knee. A history of anterior cruciate ligament reconstruction is a risk factor for further
injury. Weather conditions of high evaporation and low rainfall before matches are associated with
noncontact anterior cruciate ligament injury
AFL – 1992-1999:
672 hamstring, 163 quadriceps, 140 calf muscle
1. recent history of same injury
2. past history of same injury
3. history of other muscle injury
4. age (hamstring+calf), short player (quadriceps), less rain (quadriceps),
dominant leg (quadriceps)
Muscle strain therapy
• RICE
• Relative rest – anti-inflammatory medication
• Restore ROM – muscle stretching
• Muscle strengthening – progress to eccentric program
• Correction of intrinsic/extrinsic risk factors
• Functional and sport specific rehabilitation
Adverse effect of used therapeutics ?
Acute treatment:
 External compression
o Almost complete reduction of blood flow
 Cooling
o Reduces the intramuscular temperature by 3-7 degrees
o Reduces the blood circulation by 50% but a major reduction was not seen until
10-15min
Contusion:
 2-4days immobilization as it improves the amount of muscle regeneration

Thereafter early remobilisation… this result in good orientation of the collagen fibers
and good penetration of the gap.
48-72hours post-injury: QUESTIONS
 Has the hematoma spread out distally subcutaneously?
 Has swelling decreased?
 Has ability to contract returned?
o  YES: injury is an intermuscular hematoma:
o  NO: injury is an intramuscular hematoma:
good prognosis
often delayed healing
NSAIDs
• Inflammatory response to muscle injury
– Initiate normal repair process
– Excessive reponse can cause pain inhibition
– Neutrophils release oxygen free radicals and lysosomal proteases and elastases
 further muscle damage
Toumi and Best in BJSM 2003
The inflammatory response: friend or enemy for muscle injury?
Limiting certain aspects of inflammation may be a useful new treatment for sport related muscle injury
– empirically advised
– Delayed muscle regeneration ?
• Obremsky AJSM 1994
• Mishra JBJS 1995
– Rabbit muscle injury treated with NSAID
– Early protective effect
– Late (28 days) loss of function
» Force generation
» Late embryonic myosin expression
• Thorsson AJSM 1998
Obremsky AJSM 1994
Biomechanical and histologic assessment of a controlled muscle strain injury treated with piroxicam
This study was designed to observe the effect of the nonsteroidal antiinflammatory piroxicam on a controlled
muscle strain injury in the rabbit model. The tibialis anterior tendons of 90 New Zealand White rabbits were
detached at their distal insertions, and the right tendon was stretched to the yield point of the deformation curve.
One group of 50 rabbits received piroxicam treatment and the others received no treatment. At 1, 2, 4, and 7 days
the parameters of muscle function, tensile strength, and histology were examined. Muscle contractile force was
significantly greater in the piroxicam-treated group at Day 1, but no difference was noted at any other time
period. Tensile strength was not significantly different at any time period in either group. Histology revealed
delayed degradation of damaged tissue and slowed regeneration of muscle tissue at the injury site in the
piroxicam-treated group. Piroxicam and other anti-inflammatories are frequently given to athletes being treated
for muscle strain injuries to control pain through their effect on the inflammatory process. This study
demonstrates that piroxicam does not adversely influence the recovery of contractile and tensile strength in a
followup period of 1 week. Therefore, antiinflammatory treatment may be beneficial early in the course of
muscle injury
Mishra JBJS 1995
Anti-inflammatory medication after muscle injury. A treatment resulting in short-term improvement but
subsequent loss of muscle function.
We studied the effect of flurbiprofen, a non-steroidal anti-inflammatory drug, on muscles that had been subjected
to exercise-induced injury. The muscles of the anterior compartment in the limbs of rabbits were cyclically
activated as the ankle was simultaneously moved through passive plantar flexion every two seconds for thirty
minutes. This treatment imposed acute passive lengthening (eccentric contractions) of the maximally contracted
muscles of the anterior compartment. After the eccentric contraction-induced muscle injury, one group of rabbits
was treated with oral administration of flurbiprofen, two times a day for six days, while the other group of
rabbits served as untreated controls. The contractile, histological, and ultrastructural properties of the muscles
were measured before the initial exercise and at three, seven, and twenty-eight days afterward. The group that
was treated with flurbiprofen demonstrated a more complete functional recovery than the untreated controls at
three and seven days but had a deficit in torque and force generation at twenty-eight days. The administration of
flurbiprofen also resulted in a dramatic preservation of the intermediate filament protein desmin. After three
days, the proportion of fibers of the extensor digitorum longus that lost desmin-staining was significantly greater
in the untreated controls than in the treated animals (34 +/- 4.1 compared with 2.9 +/- 1.7 per cent) (p < 0.001), a
finding that supports the concept of a short-term protective effect. However, the muscles in the treated animals
still mounted a dramatic regenerative response, as indicated by the expression of embryonic myosin. Early in the
recovery period (at three days), significantly fewer fibers of the extensor digitorum longus (2.2 +/- 1.4 per cent)
expressed embryonic myosin in the treated animals than in the untreated controls (11.8 +/- 1.9 per cent) (p <
0.001). However, at seven days, the expression of embryonic myosin by the muscles from the treated animals
(19.5 +/- 11.9 per cent) actually exceeded that of the muscles from the untreated controls (16.2 +/- 4.1 per cent).
This finding suggests either a delayed or an ineffectual regenerative response by the muscles in the treated
animals.
Thorsson AJSM 1998
Effects of nonsteroidal antiinflammatory medication on satellite cell proliferation during muscle
regeneration
Previous experimental studies have indicated delayed muscle regeneration after nonsteroidal antiinflammatory
drug therapy. Successful regeneration of muscle after injury requires activation of normally dormant satellite
cells that share the basal laminae with adjacent muscle cells. In the presence of adequate capillary ingrowth,
satellite cells proliferate into myotubes and eventually form new muscle cells. In this study, the onset and extent
of satellite cell and fibroblast proliferation as well as the production of myotubes and capillaries were analyzed
with immunohistochemical methods after contusion injuries to rats' gastrocnemius muscles. Two groups of
animals received daily doses of an intramuscular nonsteroidal antiinflammatory drug (naproxen) starting 6 hours
and 3 days after injury, respectively. Treated animals were compared with similarly injured untreated animals.
Satellite cell and fibroblast proliferation were unaffected by the treatment, and there were no significant
differences in myotube or capillary production between treated and control animals. We conclude that naproxen
treatment does not compromise the basic process of myofiber regeneration after injury
–
–
–
–
Possible late negative effect
No major advantages
Considering costs
Considering potential adverse effects
to be used only in early post-injury period and discontinued after inflammatory peak
Almekinders in Sports Med 1999
Anti-inflammatory treatment of muscular injuries in sport. An update of recent studies
Stretch-induced muscle injuries or strains, muscle contusions and delayed-onset muscle soreness (DOMS) are
common muscle problems in athletes. Anti-inflammatory treatment is often used for the pain and disability
associated with these injuries. The most recent studies on nonsteroidal anti-inflammatory drugs (NSAIDs) in
strains and contusions suggest that the use of NSAIDs can result in a modest inhibition of the initial
inflammatory response and its symptoms. However, this may be associated with some small negative effects
later in the healing phase. Corticosteroids have generally been shown to adversely affect the healing of these
acute injuries. Animal studies have suggested that anabolic steroids may actually aid in the healing process, but
clinical studies are not yet available and the exact role of these drugs has yet to be determined. Studies on antiinflammatory treatment of DOMS have yielded conflicting results. However, the effect of NSAIDs on DOMS
appears small at best. Future research may have to focus on different aspects of these injuries as the emphasis on
anti-inflammatory treatment has yielded somewhat disappointing results.
Tscholl 2009
The use and abuse of painkillers in international soccer: data from 6 FIFA tournaments for female and
youth players
BACKGROUND: It is known that in professional men's soccer the consumption of prescription medication is
high. PURPOSE: The intake of medication in female and adolescent male soccer players has not yet been
investigated. STUDY DESIGN: Descriptive epidemiology study. MATERIAL: Team physicians reported
10,456 uses of medication 72 hours before each match in 2488 soccer players participating in 6 international
soccer tournaments. RESULTS: The use of a total of 6577 medical substances was reported, leading to an
average intake of 0.63 substances per player per match (under-17s, 0.51; under-20s, 0.51; women, 1.0; P < or =
.001 [without contraceptive medication, 0.85; P < .001]). Nonsteroidal anti-inflammatory drugs were the most
commonly prescribed type of medication in all tournaments. Women's soccer had the highest percentage of
players using nonsteroidal anti-inflammatory drugs per match (under-17s, 17.3%; under-20s, 21.4%; women,
30.7%; P < or = .001). Relatively few players were taking beta(2)-agonists for the treatment of asthma (under17s, 1.3%; under-20s, 1.3%; women, 4.3%; P < or = .001). CONCLUSION: These findings highlight the
existing problem of excessive medication use in international top-level women's and male youth soccer nearly to
the same extent as in men's soccer. Further steps need to be taken to understand the rationale underlying the
sports physicians' practice and to plan educational programs to avoid the abuse of prescription medication.
CLINICAL RELEVANCE: Continued abuse of medication may otherwise not only negatively influence the
quality of the game but also the health status of the players.
Corticosteroids
• Retrospective analysis
– 431 hamstring injuries
– 58 localised severe injury treated with CS
– No complications
Levine et al in AJSM 2000
Intramuscular corticosteroid injection for hamstring injuries. A 13-year experience in the National
Football League
The purpose of this study was to assess the safety of intramuscular corticosteroid injection in selected, severe
hamstring injuries in professional football players. Clinicians have been reluctant to use corticosteroid injections
in or around muscle-tendon units because of concern of incomplete healing or rupture. We retrospectively
reviewed the computer database of one National Football League team for all hamstring injuries requiring
treatment between January 1985 and January 1998. We found that 431 players had suffered such injury. We
developed a clinical grading system to identify hamstring injury severity and to stratify players for treatment.
Fifty-eight players (13%) sustained severe, discrete injuries with a palpable defect within the substance of the
muscle and were treated with intramuscular injection of corticosteroid and anesthetic. There were no
complications related to the injection of corticosteroid. Only nine players (16%) missed any games as a result of
their injury. Final examination revealed no strength deficits, normal muscle bulk and tone, and the ability to
generate normal power. We believe that the grading system we developed can assist in selection of injury type
for injection. Although lack of a control group limits statements of efficacy of injection, our impression is that
intramuscular corticosteroid injection hastens players' return to full play and lessens the game and practice time
they miss.
– Late effect?
Smidt in Lancet 2002
Corticosteroid injections, physiotherapy, or a wait-and-see policy for lateral epicondylitis: a randomised
controlled trial
BACKGROUND: Lateral epicondylitis is generally treated with corticosteroid injections or physiotherapy.
Dutch clinical guidelines recommend a wait-and-see policy. We compared the efficacy of these approaches.
METHODS: Patients with lateral epicondylitis of at least 6 weeks' duration were recruited by family doctors. We
randomly allocated eligible patients to 6 weeks of treatment with corticosteroid injections, physiotherapy, or a
wait-and-see policy. Outcome measures included general improvement, severity of the main complaint, pain,
elbow disability, and patient satisfaction. Severity of elbow complaints, grip strength, and pressure pain
threshold were assessed by a research physiotherapist who was unaware of treatment allocation. We assessed all
outcomes at 3, 6, 12, 26, and 52 weeks. The principal analysis was done on an intention-to-treat basis.
FINDINGS: We randomly assigned 185 patients. At 6 weeks, corticosteroid injections were significantly better
than all other therapy options for all outcome measures. Success rates were 92% (57) compared with 47% (30)
for physiotherapy and 32% (19) for wait-and-see policy. However, recurrence rate in the injection group was
high. Long-term differences between injections and physiotherapy were significantly in favour of physiotherapy.
Success rates at 52 weeks were 69% (43) for injections, 91% (58) for physiotherapy, and 83% (49) for a waitand-see policy. Physiotherapy had better results than a wait-and-see policy, but differences were not significant.
INTERPRETATION: Patients should be properly informed about the advantages and disadvantages of the
treatment options for lateral epicondylitis. The decision to treat with physiotherapy or to adopt a wait-and-see
policy might depend on available resources, since the relative gain of physiotherapy is small.
Local anaesthetics
• Use of local anaesthetics for pain relief to allow early return to play in professional
football
• Complications
Orchard BJSM 2002
Rib injuries
33
Iliac crest hematomas
32
AC joint injuries
27
Finger injuries
25
Thumb injuries
17
Ankle injuries
21
Metacarpal injuries
7
Sternum injuries
6
Toe phalangeal injuries
5
Prepatellar bursitis
4
Other injuries
44
– Local pain relief while muscle is still healing and remodelling
• Tensile forces above pain-induced threshold
• Primary factor in re-injury
– Role of local anaesthetic in muscle without structural damage
• Chronic pain possibly due to neural branches stretch
• Empirical use – evidence lacking
Immobilization
• Immediate mobilization of injured muscles may
– increase scar formation
– interfere with orderly regeneration of myofibers
•
Motion after a short period of immobilization
– more rapid disappearance of the hematoma and inflammatory cells
– more extensive, rapid, and organized myofiber regeneration
– more rapid increase in tensile strength and stiffness
•
Prolonged immobilization after injury
– muscle atrophy and poor organization of the regenerating myofibers
– disuse atrophy with decreased tensile strength
Buckwalter in JAOSS 1999
Loading of healing bone, fibrous tissue, and muscle: implications for orthopaedic practice
One of the most important concepts in orthopaedics in this century is the understanding that loading accelerates
healing of bone, fibrous tissue, and skeletal muscle. Basic scientific and clinical investigations have shown that
these tissues respond to certain patterns of loading by increasing matrix synthesis and in many instances by
changing the composition, organization, and mechanical properties of their matrices. Although new approaches
to facilitate bone and fibrous tissue healing have shown promise (e.g., the use of cytokines, cell transplants, and
gene therapy), none has been proved to offer beneficial effects comparable to those produced by loading of
healing tissues. For these reasons, patients with musculoskeletal injuries and those who have recently undergone
surgery are now being treated with controlled physical activity that loads their healing tissues. Evaluation of new
approaches to the promotion of healing of bone, fibrous tissue, and muscle should include consideration of the
effects of loading on tissue repair and remodeling
Timing of return to sport ?
• No gold standard
– Full range of motion
– Strength recovery !!!
– Functional activities tested
Muscle tissue regeneration
• Pathophysiology
– Necrosis of myofibres
– Inflammatory response
– Regeneration of myofibres
– Formation of connective scar tissue
– Neovascularization
– Adhesion of myofibres to extracellular matrix
•
•
•
•
Muscle degeneration and inflammation
– First few days
Muscle regeneration
– After 7-10 days, peaks at 2 weeks, decreases at 3-4 weeks
Scar formation
– After 2 weeks, increases over time
Mobilization versus immobilization (rat model)
– Immobilization immediately after injury
• limits the size of the connective tissue
• muscle fibers orientation is complex
• immobilization for longer than 1 week resulted in marked atrophy
– Mobilization immediately after injury
• dense scar formation
• interference with muscle regeneration
– best results when mobilization was started after 3 to 5 days of immobilization
Jarvinen and Lehto
The effects of early mobilisation and immobilisation on the healing process following muscle injuries.
The biological processes following muscle injury include 2 competitive events; regeneration of muscle fibres
and the simultaneous production of granulation tissue. We have studied the effects of early mobilisation and
immobilisation on the healing of rat gastrocnemius muscle following partial rupture by a controlled contusion
mechanism. Muscle fibre regeneration is inhibited by the formation of dense connective tissue scar.
Immobilisation following injury limits the size of the connective tissue area formed within the site of injury; the
penetration of muscle fibres through the connective tissue is prominent but their orientation is complex and not
parallel with the uninjured muscle fibres. Immobilisation for longer than 1 week is followed by marked atrophy
of the injured gastrocnemius muscle. Mobilisation started immediately after injury is followed by a dense scar
formation in the injury area prohibiting muscle regeneration. When mobilisation is started after a short period of
immobilisation a better penetration of muscle fibre through the connective tissue is found and the orientation of
regenerated muscle fibres is aligned with the uninjured muscle fibres. Although a little delay in healing processes
in muscles mobilised after short immobilisation was found morphologically, the gain in strength and energy
absorption capacity was quite similar and as good as that of muscles treated by early mobilisation alone.
formation of scar tissue
• new mini-musculotendinous junction formation
• new tendon-muscle-tendon unit
•
•
Muscle regeneration and scar formation > 3 weeks
Type III collagen mRNA expression 
• Collagen prior to myofiber regeneration
Fibrose na letsel m. gastrocnemius
Adverse neural tension
– Sciatic nerve branches vs scar tissue
–
e.g. slump test + in > 50% hamstring injury
Turl et al in JOSPT 1998
Adverse neural tension: a factor in repetitive hamstring strain?
The etiology and nature of repetitive hamstring strain is complex and not fully understood. The purpose of this
study was to investigate the presence of adverse neural tension in 14 male Rugby Union players with a history of
grade 1 repetitive hamstring strain. Comparison was made to an injury-free matched control group. Adverse
neural tension was assessed using the slump test. Hamstring flexibility was measured using the active knee
extension in lying test. Results indicated that 57% of the test group had positive slump tests, suggesting the
presence of adverse neural tension. None of the control group had a positive slump test. Analysis of variance
revealed no differences in flexibility between groups or between those demonstrating a positive or negative
slump test. Results suggest that adverse neural tension may result from or be a contributing factor in the etiology
of repetitive hamstring strain. Residual decreased flexibility is not apparent in this subject group.
Huard 2003
Gene therapy and tissue engineering for sports medicine
Sports injuries usually involve tissues that display a limited capacity for healing. The treatment of sports injuries
has improved over the past 10 to 20 years through sophisticated rehabilitation programs, novel operative
techniques, and advances in the field of biomechanical research. Despite this considerable progress, no optimal
solution has been found for treatment of various sports-related injuries, including muscle injuries, ligament and
tendon ruptures, central meniscal tears, cartilage lesions, and delayed bone fracture healing. New biological
approaches focus on the treatment of these injuries with growth factors to stimulate and hasten the healing
process. Gene therapy using the transfer of defined genes encoding therapeutic proteins represents a promising
way to efficiently deliver suitable growth factors into the injured tissue. Tissue engineering, which may
eventually be combined with gene therapy, may potentially result in the creation of tissues or scaffolds for
regeneration of tissue defects following trauma. In this article we will discuss why gene therapy and tissue
engineering are becoming increasingly important in modern orthopaedic sports medicine practice. We then will
review recent research achievements in the area of gene therapy and tissue engineering for sports-related
injuries, and highlight the potential clinical applications of this technology in the treatment of patients with
musculoskeletal problems following sports-related injuries.
Growth factors and myoblast proliferation and fusion
Menetrey et al in JBJS 2000
Growth factors improve muscle healing in vivo.
Injury to muscles is very common. We have previously observed that basic fibroblast growth factor (b-FGF),
insulin growth factor type 1 (IGF-1) and nerve growth factor (NGF) are potent stimulators of the proliferation
and fusion of myoblasts in vitro. We therefore injected these growth factors into mice with lacerations of the
gastrocnemius muscle. The muscle regeneration was evaluated at one week by histological staining and
quantitative histology. Muscle healing was assessed histologically and the contractile properties were measured
one month after injury. Our findings showed that b-FGF, IGF and to a less extent NGF enhanced muscle
regeneration in vivo compared with control muscle. At one month, muscles treated with IGF-1 and b-FGF
showed improved healing and significantly increased fast-twitch and tetanus strengths. Our results suggest that
b-FGF and IGF-1 stimulated muscle healing and may have a considerable effect on the treatment of muscle
injuries
Sato et al in Muscle & Nerve 2003
• Mice – surgical gastrocnemius laceration
• Injection of IGF-1 and Decorin (=anti-TGF-beta1)
– Group 1: IGF-1 and control
– Group 2: Decorin and control
– Group 3: IGF-1 + Decorin and control
Improvement of muscle healing through enhancement of muscle regeneration and prevention of fibrosis.
Skeletal muscle is able to repair itself through regeneration. However, an injured muscle often does not fully
recover its strength because complete muscle regeneration is hindered by the development of fibrosis. Biological
approaches to improve muscle healing by enhancing muscle regeneration and reducing the formation of fibrosis
are being investigated. Previously, we have determined that insulin-like growth factor-1 (IGF-1) can improve
muscle regeneration in injured muscle. We also have investigated the use of an antifibrotic agent, decorin, to
reduce muscle fibrosis following injury. The aim of this study was to combine these two therapeutic methods in
an attempt to develop a new biological approach to promote efficient healing and recovery of strength after
muscle injuries. Our findings indicate that further improvement in the healing of muscle lacerations is attained
histologically by the combined administration of IGF-1 to enhance muscle regeneration and decorin to reduce
the formation of fibrosis. This improvement was not associated with improved responses to physiological
testing, at least at the time-points tested in this study
Medical improvement of muscle healing
– Stimulation of muscle regeneration
– Reduction of fibrosis
– Optimal balance histological regeneration and functional recovery
– Clinical relevance ?
muscle strength
Correlation between biochemical and structural changes during the regeneration of skeletal
muscle after laceration injury
•
mechanical muscle failure after laceration injury in rats
•
•
< 10 days: within scar tissue
> 10 days: within myofibers (muscle atrophy)
– Recurrence on different location
Kaariainen J Orthop Res 1998
Correlation between biomechanical and structural changes during the regeneration of skeletal muscle
after laceration injury
A standardized and reliable model for muscle laceration injuries was developed. The biomechanical and
morphological changes during the process of muscle repair after injury were analysed, and the reproducibility of
the methods was evaluated. The soleus muscles of Sprague-Dawley rats were completely transected and were
allowed to heal for 5, 7, 10, 14, 21, 28, or 56 days, when the muscles either were pulled to failure to measure
different parameters of tensile strength or were removed for morphological analysis. During the repair process,
the regenerating myofibers penetrated into the connective-tissue scar and formed new myotendinous junctions,
thus restoring the functional continuity across the muscle stumps. The muscle atrophied significantly during the
recovery period. Mechanical failure occurred in the scar until day 10, and thereafter it occurred within
myofibers. Until day 10, the failure load, strain, and specific energy increased to as much as 46, 59, and 36% of
the control level, respectively; thereafter, there were only minor changes. Stress (tensile strength per crosssectional area) reached 86% of the control level by day 21 and further increased to as much as 96% of the control
level until day 56. These results indicate that the scar becomes stronger than muscle within 14 days; thereafter,
the weakest point is the atrophic muscle. The fact that the stress value was most rapidly normalized suggests
that, qualitatively, the regenerated muscle had virtually regained its pretrauma strength by day 56 and that the
low values of failure load, strain, and specific energy were mainly due to atrophy of the muscle. Thus, further
increase in the tensile strength of the regenerated muscle-tendon unit may require active exercise to reverse
muscle atrophy.