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The effects of varying time under tension and volume load on acute
neuromuscular responses
Article in European Journal of Applied Physiology · November 2006
DOI: 10.1007/s00421-006-0297-3 · Source: PubMed
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Eur J Appl Physiol (2006) 98:402–410
DOI 10.1007/s00421-006-0297-3
O RI G I NAL ART I C LE
The eVects of varying time under tension and volume load
on acute neuromuscular responses
Quan T. Tran · David Docherty · David Behm
Accepted: 22 August 2006 / Published online: 13 September 2006
© Springer-Verlag 2006
Abstract The purpose of this study was to examine
the eVects of diVerent methods of measuring training
volume, controlled in diVerent ways, on selected variables that reXect acute neuromuscular responses.
Eighteen resistance-trained males performed three
fatiguing protocols of dynamic constant external
resistance exercise, involving elbow Xexors, that
manipulated either time-under-tension (TUT) or volume load (VL), deWned as the product of training
load and repetitions. Protocol A provided a standard
for TUT and VL. Protocol B involved the same VL as
Protocol A but only 40% concentric TUT; Protocol C
was equated to Protocol A for TUT but only involved
50% VL. Fatigue was assessed by changes in maximum voluntary isometric contraction (MVIC), interpolated doublet (ID), muscle twitch characteristics
(peak twitch, time to peak twitch, 0.5 relaxation time,
and mean rates of force development and twitch relaxation). All protocols produced signiWcant changes
(P · 0.05) in the measures considered to reXect neuromuscular fatigue, with the exception of ID. Fatigue
was related to an increase in either TUT or VL with
greater fatigue, as reXected by MVIC and peripheral
measures, being associated with diVerences in TUT.
The lack of change in ID suggests that fatigue was
Q. T. Tran
University of Queensland, Brisbane, QLD, Australia
D. Docherty (&)
School of Physical Education, University of Victoria,
Victoria, BC, Canada V8W 3P1
e-mail: docherty@uvic.ca
D. Behm
Memorial University, St Johns’, NF, Canada
123
more related to peripheral than central mechanisms.
It was concluded that the load and contraction velocities of the repetitions have diVerent eVects on acute
neuromuscular responses and should, therefore, be
clearly calculated when describing training volume
for dynamic constant external resistance exercise
training.
Keywords Contractile properties · Contraction
velocity · Central · Peripheral · Fatigue
Introduction
Resistance training is an eVective method for developing muscular strength and hypertrophy. However, the
optimal resistance training program and the relationship between the training variables and neuromuscular
adaptation remain elusive. The diYculty in optimizing
a resistance training program may be due to its complex nature, which can be attributed to the many training variables that can be manipulated. The training
variables include the load that is lifted, the volume and
frequency of training, speed of contraction, work to
rest ratios, fatigue, and the order in which the exercises
are performed, as well as the interaction that may exist
between these variables. Volume is considered to be
one of the most important training variables, especially
in regard to hypertrophic adaptations (Byrd et al.
2005) as well as improvements in force generation
(Munn et al. 2005; Sclumberger et al. 2001). However,
not all studies have found a signiWcant relationship
between training volume and subsequent neuromuscular adaptations (Hass et al. 2000; Carpinelli and Otto
1998).
Eur J Appl Physiol (2006) 98:402–410
One explanation for the equivocal Wndings may be
related to diVerences in the way training volume has
been calculated. One common method for calculating
training volume is to count the total number of repetitions completed during a speciWed training period. An
alternative method is to describe volume in regard to
the amount of work performed. Mechanical work is
calculated by multiplying the force required to move
the load, or resistance, by the distance traveled by the
load. With the assumption that the force involved is
equal to the load being lifted and that all repetitions
are performed with the same range of motion, work
may be approximated by multiplying the load by the
number of repetitions, referred to as “volume load”
(Stone et al. 1999). Measuring volume load may be a
more precise way of calculating training volume
because it recognizes that the load is a contributing factor to volume. However, this method does not fully
deWne the load and repetitions because similar volume
loads may be obtained from diVerent combinations of
load and number of repetitions.
Potential discrepancies may arise also in describing
training volume if the time the muscle is under tension
(TUT) is diVerent or not controlled. To control for
such discrepancies, volume may also be calculated as
the cumulative time that the muscles are under tension
during a training period. In order to quantify volume
by TUT during dynamic training protocols, load and
contraction velocities of the concentric and eccentric
phases, as well as the duration of any isometric phase,
of the repetitions should be included. Few studies have
directly manipulated TUT as a training variable or
compared the separate inXuences of TUT and volume
load on neuromuscular adaptations. Therefore, the
speciWc eVects of TUT in regard to neuromuscular
adaptations are not well understood at this time.
Training studies that have varied TUT by manipulating contraction velocities during resistance training
programs have produced equivocal results. Wescott
et al. (2001) found that subjects performing slow tempo
(ST) training, resulting in 2.5 times greater concentric
TUT, experienced a signiWcant 50% improvement in
mean strength compared to subjects training with regular tempo (RT). However, Keeler et al. (2001), using a
similar experimental design, found RT produced
greater gains in strength compared to ST (39 and 15%,
respectively). However, it should be noted that there
was a variation in the training loads between groups,
which would impact the results. In both studies, greater
strength improvements were associated with the group
that trained with a greater load Thus, interpretation of
TUT, when confounded by variance in load, must be
made with some caution.
403
Fatigue has been implicated in long-term neuromuscular adaptations with greater fatigue being associated with increased strength and hypertrophy
(Rooney et al. 1994; Schott et al. 1995). Insight into
the eVects of diVerent methods of describing training
volume on neuromuscular adaptation may be initially
gained by monitoring the acute neuromuscular
fatigue responses. Muscle fatigue is multi-factorial
and consequently a number of deWnitions have been
proposed. Within the context of the present study
fatigue has been deWned as an exercised-induced
reduction in force generating capabilities, which may
be central or peripheral in origin. Central fatigue
refers to a reduction in voluntary activation of muscle
during exercise. Fatigue as a result of impairments in
force generating capacities at or distal to the neuromuscular junction has been termed peripheral fatigue
(Gandevia 2001). Fatigue has also been deWned as a
response that is less than the expected or anticipated
contractile response for a given stimulation (MacIntosh and Rassier 2002).
Presently, the majority of studies and training
programs describe volume using the volume load
method. However, without inclusion of TUT, it is
uncertain whether these studies or programs can conWdently state that volume is equated. Few studies
have directly examined the eVects of TUT on dynamic
constant external resistance (DCER) training and
currently no study has directly compared the eVects of
TUT and volume load on acute fatigue. If long-term
enhancement of muscular strength and hypertrophy is
related to both fatigue and volume of training, resistance-training programs should clearly describe the
volume of training using TUT or volume load. Therefore, the purpose of this study was to identify the speciWc eVects of TUT and volume load on acute
neuromuscular markers of fatigue following singlearm elbow Xexions.
Methods
Participants
Eighteen university-aged males participated in the
study (age = 25.1 § 3.5 years; mass = 85.2 § 13.2 kg).
All participants were strength trained and had a minimum of 1 year of upper body resistance training. Prior
to participation, written consent was obtained and all
participants were briefed on the purpose of the study
and potential risks involved in participation. Approval
of the study was granted by the University Human
Research Ethics Committee.
123
404
Eur J Appl Physiol (2006) 98:402–410
Experimental design
Participants performed three fatiguing protocols that
manipulated either TUT or volume load while maintaining the same resistance or load that was lifted.
After satisfactory completion of two familiarization
sessions, participants performed each protocol, in random order, on separate days with approximately 48–
72 h between testing sessions. All sessions were supervised and participants were asked to refrain from performing any resistance training targeting the elbow
Xexors for the duration of the study.
Familiarization session
Following the initial rest period (5 min) participants
performed a warm-up consisting of three sets of ten
repetitions of DCER elbow Xexion, separated by 3 min
rest periods, at a load of 50% of the estimated 10RM.
All warm-ups during the familiarization sessions were
performed using the training regimen of protocol A
(see Table 1). Participants were then tested for their
maximal muscle stimulus (MMS). The interpolated
doublet technique (ITT) was performed to familiarize
the subjects with the protocol.
Testing of the 10RM was conducted 5 min after the
ITT. Participants performed single-arm standing
dumbbell curls of the dominant arm. Participants had
their backs to the wall to maintain form. One complete
repetition consisted of moving the arm through the full
range of elbow motion. The participants were
instructed to maintain a supinated grip, to avoid any
extraneous body movement, and keep in time with a
pre-set metronome throughout the test. All 10RM testing was performed using the repetition scheme of protocol A. Participants started at an initial load of 75% of
the estimated 1RM. The load was adjusted accordingly
by 100 g to 2 kg increments to ensure a 10RM was
obtained. Five-minute rest periods between 10RM
attempts were provided to minimize fatigue. No participants required more than three attempts.
Following the 10RM test, the three diVerent training
protocols were performed in random order at 50% of
the 10RM. This was necessary to familiarize the participants with the various cadences associated with each
protocol (Table 1). Five-minute rest periods were provided between fatiguing protocols.
Testing session
Following an initial rest period of 5 min, the participants
performed an identical warm-up, as in the familiarization
session, but utilized the repetition scheme of the fatigue
protocol being tested to provide participants with additional practice with the timing of lifts. Maximal voluntary
isometric contraction (MVIC), twitch contractile properties, and interpolated doublet (ID), were measured
before and after the fatiguing protocols. Twitch contractile properties were measured immediately after each
fatiguing protocol, whereas the ID was administered
1 min post-fatiguing protocol (Behm et al. 2002).
Fatiguing protocols
The various protocols were designed to manipulate
either concentric TUT or volume load with respect to
protocol A. Participants were instructed to keep time
with a metronome set at the speciWc cadence for the
protocol. In protocol B participants performed the
same volume load but with only 40% of the concentric
TUT compared to protocol A, whereas in protocol C
participants performed 50% of the volume load with
equal TUT compared to protocol A (see Table 1).
Manipulation of the concentric phase was chosen to be
consistent with other dynamic training TUT studies
(Keeler et al. 2001; Wescott et al. 2001).
Ninety percent of the 10RM load was used as the
load for all fatiguing protocols to ensure the volume
load was consistent between trials (Benson et al. 2006).
All participants were able to complete the prescribed
repetitions.
Setup on the modiWed preacher curl
The maximal muscle stimulus, ID, twitch contractile
properties, and MVIC tests were performed on the
Table 1 Repetition scheme for three fatiguing protocols
Protocol
Sets
Repetitions
Concentric
phase (s)
Eccentric
phase (s)
Volume
load*
Total
concentric
TUT (s)
Total
eccentric
TUT (s)
A
B
C
3
3
3
10
10
5
5
2
10
2
2
4
27
27
13.5
150
60
150
60
60
60
Note: Asterisk (*) denotes volume load was calculated by multiplying number of repetitions by 90% (of 10RM). Example calculation
of volume load for protocol A = 3 £ 10 £ 0.9 = 27
123
Eur J Appl Physiol (2006) 98:402–410
modiWed preacher curl apparatus. The apparatus was
adjusted so that the thighs of the participant were parallel with the Xoor with a 90° angle at the knee. The
chest was placed Xush against the arm rest pad. The
arm was placed Wrmly against the pad of the preacher
curl bench with the elbow placed at a 90° angle and the
forearm fully supinated. The joint angles were measured with a goniometer. To minimize extraneous body
movement, metal clamps were lowered until they
pressed Wrmly against the upper arm (Fig. 1). The
height of each clamp was measured and recorded for
each individual. The wrists of the participants were
inserted into a wrist strap attached to the strain gauge.
When subject positioning was satisfactory a standard
force of 10N (resting tension) was set to eliminate slack
in the wire connecting the strain gauge to the wrist
straps.
Maximal muscle stimulus (MMS)
The MMS test was conducted to determine the electrical stimulus that was used during the measurement of
twitch contractile properties and ID tests. Placement of
the electrodes was designed to target the musculocutaneous nerve. The cathode was lowered over the biceps
brachii (midbelly) midway between the anterior edge
of the deltoid and the proximal elbow crease with the
elbow Xexed at 90°. The anode was placed over the distal tendon in the elbow groove (Allen et al. 1998).
Large electrodes (10 £ 5 cm2) were used to fully cover
the belly of the biceps brachii (Behm et al. 2002). Stimulation was administered using a constant high voltage
stimulator (Digitimer Ltd. Model DS7A).
Participants were instructed to remain relaxed and
close their eyes to prevent anticipation of the stimulus.
405
Voltage was set at 100-V rectangular pulse with pulse
duration of 50 s and amperage was progressively
increased (10 mA to 1 A) on consecutive trials until no
further increase in twitch amplitude was detected.
Force values, sampled at 2,000 Hz, were detected by a
strain gauge (Omegadyne Ltd. Model 101–500, range
0–500 lbs), ampliWed (Biopac Systems Inc. MP100),
and analyzed (Acknowledge 3.7, Biopac Systems Inc.).
The minimum electrical stimulus that elicited the
greatest muscle contractile force was considered the
MMS.
Interpolated doublet technique (ITT)
The ITT was used to measure the extent of muscle activation. Pre-ID consisted of two maximal contractions
separated by 3 min rest periods and three submaximal
contractions (75, 50, and 25% of MVIC), in random
order, interspaced by 1 min rest periods. Post-ITT consisted of one MVIC followed by 1 min rest and three
submaximal contractions (75, 50, and 25% of MVIC),
in the same order as pre-ITT, interspaced by 30 s rest
periods. The shorter post-ITT was used to minimize
the eVects of recovery (Behm et al. 2002). Each contraction was 3 s in duration, two doublets were delivered at 1.5 and 3 s of the contraction. Participants were
instructed to cease muscle contraction after the second
doublet. A third doublet was delivered at rest following each contraction. A doublet was used (2 stimuli
interspaced by 10 ms) to increase the signal to noise
ratio (McKenzie and Gandevia 1991).
All maximal and submaximal forces were plotted
with their respective ID ratios. The ID ratio was
expressed as the ID divided by the control doublet
(doublet at rest). A second order polynomial equa-
Fig. 1 Body position on the
modiWed preacher curl apparatus from the side (a) and
front (b)
123
406
tion was constructed (ax2 + bx + c) for each subject to
determine the extent of muscle activation because it is
considered by some authors to best represent the curvilinear relationship of voluntary force and muscle
activation (Behm et al. 1996). The mean R2-values for
the 2° polynomial equation for all pre- and postfatiguing protocol measures were 0.97 and 0.95,
respectively.
Maximal voluntary isometric contraction
Participants performed two MVICs, separated by
3 min rest periods, prior to the testing protocol and one
MVIC following the testing protocol. The average of
the peak pre-protocol MVIC forces was measured.
Maximal voluntary isometric contractions were measured 1 min after completion of the fatigue protocol.
All MVIC attempts were 3 s in duration.
Eur J Appl Physiol (2006) 98:402–410
Results
Fatigue
A signiWcant interaction was detected for isometric
force development from the pre- to post-protocol values (F = 4.73, P < 0.05). All protocols resulted in signiWcant decreases in peak isometric force output from
pre- to post-protocol values (P < 0.05). Force production following protocol A, which involved high volume
load and high TUT, decreased by 19.2 § 1.9%
(mean § SEM), which was signiWcantly greater
(P < 0.05) than force decrements observed in protocol
B (decreased TUT) (¡12.8 § 1.6%). Protocol C, with a
decreased volume load, resulted in a 15.0 § 2.8%
reduction in force but was not signiWcantly diVerent
from protocol A or B (Fig. 2).
Neural measures
Twitch contractile properties
Statistics
Data were analyzed using SPSS 11.5. A two-way analysis of variance (ANOVA) with repeated measures was
conducted (3 £ 2). The two ANOVA levels included
the fatigue protocols (A, B, and C) and the diVerences
between pre- and post-tests measures. F ratios that
reached P · 0.05 were considered signiWcant. Student’s
paired t tests were performed, with a Bonferoni correction, where signiWcant main eVects were detected. One
set of values for the twitch contractile properties was
removed due to incomplete data.
123
No signiWcant interaction was detected for percent
diVerence muscle activation from pre- to post-values
between protocols (F = 0.67). Mean muscle activation
values, across all protocols, indicated that participants
were able to achieve full or near full motor unit recruit-
450
Maximal Voluntary Isometric Contraction (N)
Participants were instructed to remain relaxed and to
close their eyes to prevent anticipation of the stimulus to eliminate neural activation. All twitch contractile properties were measured from a single
stimulation (singlet): Peak twitch (PT) force was
recorded as the greatest force evoked by the singlet,
Time to peak twitch (TPT) was the time from the
onset of stimulation to the PT, and half relaxation
time (0.5RT) was the time from PT to decrease in half
the amplitude. The mean rate of force development
(PT/TPT) is deWned as the average rate for peak
force development evoked from the singlet in a
rested state and was calculated by dividing the PT by
TPT. Similarly the mean rate of twitch relaxation is
deWned as the average rate for the PT to decrease in
tension by half and was calculated by dividing
¡0.5PT by 0.5RT. All twitch characteristics were
included (Acknowledge 3.7. Biopac Systems Inc.) and
averaged (three trials).
pre
post
400
350
*a
*
*a
300
250
200
A
B
C
Fatigue Protocol
Fig. 2 Maximal voluntary isometric contraction measured preand 1 min post-completion of each fatiguing protocol. Vertical
lines represent standard error of the means. Asterisk (*) denotes
signiWcant diVerence from pre- to post-completion (P < 0.05).
Letter “a” denotes signiWcant percent diVerence from each other
(P < 0.05)
Eur J Appl Physiol (2006) 98:402–410
407
ment (96.5 § 0.6%). Initial muscle activation of protocols
A, B, and C (95.7 § 1.3, 96.5 § 0.73, and 97.3 § 0.85%,
respectively) were not signiWcantly diVerent from
post-protocol values (95.1 § 1.54, 97.3 § 0.70, and
96.2 § 1.1, respectively).
A signiWcant interaction was found for the twitch contractile properties (F = 5.01, P < 0.05). The high volume load and high TUT of protocol A resulted in
signiWcantly greater (P < 0.05) percent reduction in PT
(¡57.2 § 5.1%) than protocol B and C (¡11.8 § 6.5
and ¡30.3 § 8.6%, respectively). Protocol C, which
involved 2.5 times greater concentric TUT, resulted in
signiWcantly greater percent decreases in PT compared
to protocol B (P < 0.05) (Fig. 3).
No signiWcant interaction occurred between the
fatiguing protocols in TPT or 0.5RT but there was a
signiWcant main eVect from the pre- to post-protocol
values (F = 33.5, P < 0.05 and F = 3.6, P < 0.05, respectively). Therefore, all protocols resulted in similar
deceases from pre- to post-protocol values in TPT
(¡18.4 § 3.13, ¡16.6 § 2.2, and ¡20.1 § 3.35%,
respectively) and 0.5RT (¡37.5 § 8.8, ¡33.1 § 7.1, and
¡30.2 § 7.0%, respectively) (Figs. 4, 5).
A signiWcant interaction was detected for the mean
rate of force development (F = 4.51, P < 0.05). The
40
pre
post
100
Time to Peak Twitch (ms)
Peripheral measures
120
80
60
40
20
0
A
B
C
Fatigue Protocol
Fig. 4 Time to peak twitch measured, pre- and post-completion
of each fatiguing protocol. Vertical lines represent standard error
of the means. All protocols showed similar signiWcant decreases
between the pre- and post-conditions
100
pre
post
90
pre
post
35
Peak Twitch (N)
30
*a
25
*a
20
*a
15
Half Relaxation Time (ms)
80
70
60
50
40
30
10
20
5
10
0
A
B
Fatigue protocol
C
Fig. 3 Peak twitch forces measured pre- and post-completion of
each fatiguing protocol. Vertical lines represent standard error of
the means. Asterisk (*) denotes signiWcant diVerence from pre- to
post- (P < 0.05). Letter “a” denotes signiWcant percent diVerences
from each other (P < 0.05)
0
A
B
C
Fatigue Protocol
Fig. 5 Half relaxation time measured pre- and post-completion
of each fatiguing protocol. Vertical lines represent standard error
of the means. All protocols showed similar signiWcant decreases
between the pre- and post-conditions
123
408
Eur J Appl Physiol (2006) 98:402–410
-350
pre
post
-300
Mean Rate of Twitch Relaxation (N/s)
mean rate of force development for protocol A
decreased by 45.9 § 6.6%, which was signiWcantly
diVerent (P < 0.05) than protocol B (8.64 § 9.4%) and
C (¡12.8 § 10.5%). Protocol B and C were not signiWcantly diVerent. Protocol B was not signiWcantly diVerent from pre- to post-protocol values (Fig. 6).
A signiWcant interaction was detected for the mean
rate of twitch relaxation (F = 4.01, P < 0.05). The mean
rate of twitch relaxation of protocol A decreased by
20.3 § 8.3% and was signiWcantly diVerent (P < 0.05)
compared to the increased rates observed from B
(74.5 § 32.7%) and C (6.3 § 11.8%). Protocol B and C
were statistically diVerent (p < 0.05). Protocol C was
not signiWcantly diVerent from pre- to post-protocol
values (Fig. 7).
*a
-250
a
-200
*a
-150
-100
-50
0
Discussion
A
B
C
Fatigue Protocol
The major Wnding of the study was that manipulating
TUT or volume load inXuenced acute markers of
fatigue when equated for volume (either by the TUT
or volume load method). Greater concentric TUT,
when volume load was equated, resulted in signiWcantly greater neuromuscular fatigue (A vs. B). Substantially greater impairments to twitch force suggest
450
pre
post
400
Mean Rate of Force Development (N/s)
a
350
Fig. 7 Mean rate of twitch relaxation measured pre- and postcompletion of each fatiguing protocol. Vertical lines represent
standard error of the means. Asterisk (*) denotes signiWcant
diVerence from pre- to post- (P < 0.05). Letter “a” denotes signiWcant percent diVerence from each other (P < 0.05)
greater training stresses are placed on the peripheral
(muscle) rather than the central nervous component
with a longer TUT (A and C vs. B). Similarly, more
volume load resulted in signiWcantly greater peripheral
fatigue, as reXected by evoked muscle twitch measurements, when TUT was equated (A vs. C). Therefore,
resistance training programmes or protocols that fail to
control for either volume load and TUT cannot conWdently assume that volume is equated.
*b
300
Muscle contractile properties
250
Varying TUT
* ab
200
150
100
50
0
A
B
C
Fatigue Protocol
Fig. 6 Mean rate of force development measured pre- and postcompletion of each fatiguing protocol. Vertical lines represent
standard error of the means. Asterisk (*) denotes signiWcant
diVerence from pre- to post-completion (P < 0.05). Letters “a”
and “b” denote signiWcant percent diVerences from each other
(P < 0.05)
123
Twitch contractile properties exhibited signiWcant
interactions with diVerent TUT. The greater concentric
TUT from performing protocol A, which was 2.5 times
greater than protocol B, resulted in a signiWcantly
greater percent decrease in PT suggesting greater
impairment of muscle contractile properties. According to Ingalls et al. (1998) the majority of immediate
deWcits in twitch force production may be attributed to
impairments within the excitation–contraction-coupling processes. The data are consistent with Behm
et al. (2002) who observed greater PT deWcits following
a fatiguing protocol that involved four times the TUT
than another protocol.
Performing all protocols resulted in signiWcant
decreases in TPT and 0.5RT from initial values but no
Eur J Appl Physiol (2006) 98:402–410
interactions were detected between protocols. These
results were unexpected because temporal twitch characteristics are usually lengthened due in part to disrupted Ca++ kinetics as a consequence of fatigue
(Ingalls et al. 1998; Ørtenblad et al.2000). The lack of
signiWcant interaction of TPT and 0.5RT, despite diVerent fatigue responses, might be attributed to the signiWcantly diVerent post-PTs (P < 0.01) of protocol A and
B (Fig. 3). Behm et al. (2002) also detected signiWcant
reductions in TPT and 0.5RT following a bout of resistive exercises. However, the large magnitude of PT
deWcits (44.1–46.8%), similar to the present study, may
have contributed to the decreased TPT. Behm and StPierre (1997) found that increased PT resulted in
increased TPT and vice versa, thus, TPT and 0.5RT can
interact with and be inXuenced by twitch amplitude.
DiVerences may be present when TPT or 0.5RT are
normalized to their respective PT. SigniWcant interactions were detected between protocols A and B when
the mean rates of force development and twitch relaxation were calculated, which may be a superior measure
than TPT or 0.5RT because it addresses the potential
problem of varying PT.
The greater decreases in mean rates of force development and twitch relaxation after performing protocol A, compared to protocol B, suggest disturbances in
the rate of contractile property characteristics. The
non-signiWcant change in mean rate of force development from pre- to post-protocol in performing protocol
B suggest no temporal impairment, which is consistent
with the minimal fatigue that was found. However,
protocol B resulted in a signiWcant increase in the mean
rate of relaxation (74.5 § 32.74%) suggesting the contractile mechanisms may be more eYcient. The
increased rate of 0.5RT may be due to potentiation as a
result of the less fatiguing protocol. Behm and StPierre (1997) found that isometric fatigue of the plantar Xexors resulted in potentiated twitch contractile
properties. Garland et al. (2003) suggested that the
mechanisms of fatigue and potentiation can coexist,
and it is possible to have an increased rate of Ca++
reuptake at the sarcoplasmic reticulum (SR) despite an
overall decrease in force output.
Varying volume load
Twitch contractile properties were also inXuenced by
volume load. Performing protocol A, which involved
twice the volume load compared to performing protocol C, resulted in a greater reduction in PT. Consequently, performing protocol A may have produced
greater impairments in muscle contractile properties.
No signiWcant diVerences in TPT and 0.5RT were
409
detected between protocols A and C. However, performing protocol A resulted in signiWcantly greater
reduction in mean rates of force development and
twitch relaxation than C, which suggests that greater
decreases in PT may be a result of time-related contractile properties. Therefore, performing a greater
volume load, when TUT and load were equated,
resulted in greater deWcits in contractile properties.
Volume load versus TUT
Comparisons between performing protocols B and C
suggest that varying TUT was more inXuential on acute
peripheral fatigue than volume load when training load
was equated. Reduction in PT following protocol C
was signiWcantly greater than B, which suggests that
the observed trends of greater fatigue may be a result
of greater disruption in contractile properties.
The signiWcant changes in twitch contractile properties despite non-signiWcant changes in MVIC may be a
result of low frequency fatigue (LFF), which is more
consistent with human voluntary exercise (Jones 1996).
Low frequency fatigue results in preferential muscle
force impairment at low frequency stimulation whereas
maximal strength production remains intact or is minimally impaired. Increased motor unit discharge has
been proposed to partially explain maintenance of
force production following post-exercise development
of LFF (de Ruiter et al. 2005). This mechanism is consistent with the non-signiWcant changes in central
fatigue observed in the present study.
Extent of full activation
All participants were able to achieve full or near full
muscle activation (96.5 § 0.56%) of the elbow Xexors.
These results agree with other studies that have
reported high levels of muscle activation of the elbow
Xexors (Allen et al. 1998 [99.1%]; Gandevia et al. 1998
[98%]). All fatiguing protocols resulted in non-signiWcant changes in muscle activation and are consistent
with other studies (Gandevia et al. 1998; Plaskett and
Cafarelli 2001). The results suggest that within the context of the present study full or near full muscle activation can be maintained during the development of
fatigue during dynamic eVorts of the elbow Xexors.
Conclusion
Manipulating the TUT or the volume load during
dynamic resistance training inXuences acute fatigue
which may have an impact on chronic neuromuscular
123
410
adaptations. The majority of force deWcits appear to be
due to peripheral rather than central factors. The data
suggest that full muscle activation can be maintained
during the development of fatigue of the elbow Xexors.
Increased TUT or volume load resulted in greater
fatigue that would appear to be a result of impairments
in muscle contractile properties.
The results of this study suggest that, in order to
clarify the inXuence of resistance-training protocols,
the load and contraction velocities of the repetitions
for the exercise must be clearly deWned when describing training volume. The diVerences in the acute neuromuscular responses to the way training volume is
manipulated may lead to diVerences in chronic neuromuscular adaptations. If neuromuscular fatigue is an
important variable in the development of muscular
strength and hypertrophy, then greater TUT may yield
superior strength and hypertrophic gains as long as the
training load is not severely compromised.
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