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Muscle time under tension during resistance exercise stimulates differential muscle protein sub-fractional synthetic responses in men

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We aimed to determine if the time that muscle is under loaded tension during low intensity resistance exercise affects the synthesis of specific muscle protein fractions or phosphorylation of anabolic signalling proteins. Eight men (24 ± 1 years (sem), BMI = 26.5 ± 1.0 kg m(-2)) performed three sets of unilateral knee extension exercise at 30% of one-repetition maximum strength involving concentric and eccentric actions that were 6 s in duration to failure (SLOW) or a work-matched bout that consisted of concentric and eccentric actions that were 1 s in duration (CTL). Participants ingested 20 g of whey protein immediately after exercise and again at 24 h recovery. Needle biopsies (vastus lateralis) were obtained while fasted at rest and after 6, 24 and 30 h post-exercise in the fed-state following a primed, constant infusion of l-[ring-(13)C(6)]phenylalanine. Myofibrillar protein synthetic rate was higher in the SLOW condition versus CTL after 24-30 h recovery (P < 0.001) and correlated to p70S6K phosphorylation (r = 0.42, P = 0.02). Exercise-induced rates of mitochondrial and sarcoplasmic protein synthesis were elevated by 114% and 77%, respectively, above rest at 0-6 h post-exercise only in the SLOW condition (both P < 0.05). Mitochondrial protein synthesis rates were elevated above rest during 24-30 h recovery in the SLOW (175%) and CTL (126%) conditions (both P < 0.05). Lastly, muscle PGC-1α expression was increased at 6 h post-exercise compared to rest with no difference between conditions (main effect for time, P < 0.001). These data show that greater muscle time under tension increased the acute amplitude of mitochondrial and sarcoplasmic protein synthesis and also resulted in a robust, but delayed stimulation of myofibrillar protein synthesis 24-30 h after resistance exercise.
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J Physiol 590.2 (2012) pp 351–362 351
The Journal of Physiology
Muscle time under tension during resistance exercise
stimulates differential muscle protein sub-fractional
synthetic responses in men
Nicholas A. Burd1,RichardJ.Andrews
1, Daniel W.D. West1,JonathanP.Little
1, Andrew J.R. Cochran1,
Amy J. Hector1,JoshuaG.A.Cashaback
2, Martin J. Gibala1,JamesR.Potvin
2,StevenK.Baker
3
and Stuart M. Phillips1
1Exercise Metabolism Research Group, 2Occupational Biomechanics Laboratory, Department of Kinesiology, and 3Michael G. DeGroote School of
Medicine, Department of Neurology, McMaster University, Hamilton, Ontario, Canada
Non-technical summary A single bout of resistance exercise stimulates the synthesis of new
muscle proteins. Chronic performance of resistance exercise (i.e. weight training) is what makes
your muscles grow bigger; a process known as hypertrophy. However, it is unknown if increasing
the time that muscle is under tension will lead to greater increases in muscle protein synthesis. We
report that leg extension exercise at 30% of the best effort (which is a load that is comparatively
light), with a slow lifting movement (6 s up and 6 s down) performed to fatigue produces greater
increases in rates of muscle protein synthesis than the same movement performed rapidly (1 s up
and 1 s down). These results suggest that the time the muscle is under tension during exercise
may be important in optimizing muscle growth; this understanding enables us to better prescribe
exercise to those wishing to build bigger muscles and/or to prevent muscle loss that occurs with
ageing or disease.
Abstract We aimed to determine if the time that muscle is under loaded tension
during low intensity resistance exercise affects the synthesis of specific muscle protein
fractions or phosphorylation of anabolic signalling proteins. Eight men (24 ±1years (
SEM),
BMI =26.5 ±1.0 kg m2) performed three sets of unilateral knee extension exercise at 30% of
one-repetition maximum strength involving concentric and eccentric actions that were 6 s in
duration to failure (SLOW) or a work-matched bout that consisted of concentric and eccentric
actions that were 1 s in duration (CTL). Participants ingested 20 g of whey protein immediately
after exercise and again at 24 h recovery. Needle biopsies (vastus lateralis) were obtained while
fasted at rest and after 6, 24 and 30 h post-exercise in the fed-state following a primed, constant
infusion of L-[ring-13 C6]phenylalanine. Myofibrillar protein synthetic rate was higher in the
SLOW condition versus CTL after 24–30 h recovery (P<0.001) and correlated to p70S6K
phosphorylation (r=0.42, P=0.02). Exercise-induced rates of mitochondrial and sarcoplasmic
protein synthesis were elevated by 114% and 77%, respectively, above rest at 0–6 h post-exercise
only in the SLOW condition (both P<0.05). Mitochondrial protein synthesis rates were elevated
above rest during 24–30 h recovery in the SLOW (175%) and CTL (126%) conditions (both
P<0.05). Lastly, muscle PGC-1αexpression was increased at 6 h post-exercise compared to
rest with no difference between conditions (main effect for time, P<0.001). These data show
that greater muscle time under tension increased the acute amplitude of mitochondrial and
sarcoplasmic protein synthesis and also resulted in a robust, but delayed stimulation of myo-
fibrillar protein synthesis 24–30 h after resistance exercise.
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2012 The Authors. The Journal of Physiology C
2012 The Physiological Society DOI: 10.1113/jphysiol.2011.221200
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352 N. A. Burd and others J Physiol 590.2
(Resubmitted 25 September 2011; accepted after revision 15 November 2011; first published online 21 November 2011)
Corresponding author S. M. Phillips: McMaster University, 1280 Main Street West, Hamilton, ON, Canada, L8S 4K1.
Email: phillis@mcmaster.ca
Abbrevations Akt, protein kinase B; 4E-BP1, eukaryotic initiation factor 4E binding protein 1; eIF2Bε,eukaryotic
translation initiation factor 2B epsilon; FSR, fractional synthetic rate; MAPK, mitogen-activated protein kinase; mTOR,
mammalian target of rapamycin; p38 MAPK, p38 mitogen-activated protein kinase; p70S6K, 70 kDa S6 protein kinase;
p90RSK, 90 kDa ribosomal S6 protein kinase; PGC-1α, peroxisome proliferator-activated receptor-γcoactivator-1α;
rpS6, ribosomal protein S6.
Introduction
High intensity resistance exercise is an effective stimulus
to increase muscle protein synthesis rates (Chesley et al.
1992; Phillips et al. 1997; Kumar et al. 2009). We recently
demonstrated that lifting relatively low loads (30%
of maximum strength) to the point of fatigue was
equally effective as high intensity resistance exercise for
stimulating rates of myofibrillar protein synthesis (Burd
et al. 2010b). In fact, performance of lower intensity
contractions allows for higher total work by the time
fatigue is achieved because the rate of fatigue increases
exponentially with intensity (Fuglevand et al. 1993).
Hence, the stimulus for myofibrillar protein synthesis,
in its basic physiological nature, appears to be a matter
of muscle fibre recruitment, with exercise performed to
fatigue resulting in eventual maximal fibre recruitment
(Burd et al. 2010b). This thesis is consistent with the work
of Henneman et al. (1965) which demonstrated that motor
units, and thus muscle fibres, are recruited in accordance
with the size principle during voluntary contractions. Also,
ourrecentworksuggeststhattheeventualelicitationoffull
muscle fibre recruitment, during fatiguing acute resistance
exercise, is integral to inducing an enhanced sensitivity of
myofibrillar protein synthesis to protein feeding during
longer-term (i.e. 24 h later) exercise recovery (Burd et al.
2011a).
Based on our previous findings (Burd et al. 2010b,
2011a) and as a further test of the thesis that complete
muscle fibre recruitment is an important driver of myo-
fibrillar protein synthesis rates, we manipulated the
time that loaded muscle was under tension during low
intensity resistance exercise using a slow cadence to achieve
fatigue (i.e. maximal fibre activation) and compared
post-exercise muscle protein synthetic responses, intra-
muscular signalling, and PGC1αmRNA responses with
a work-matched condition that was performed using a
faster lifting cadence that did not result in fatigue. We did
not confine our conclusions to the mixed muscle protein
synthetic response. Instead, we studied the responses
in the myofibrillar, sarcoplasmic and mitochondrial
enriched protein fractions. This approach allowed for the
characterization of the muscle protein synthetic responses
at the fraction-specific level to underpin the true acute
phenotypic response during exercise recovery (Wilkinson
et al. 2008; Burd et al. 2010b). We hypothesized that
a longer time under muscle tension leading to fatigue,
and thus ‘full’ muscle fibre recruitment, will result in
greater increases in rates of muscle protein synthesis
(i.e. myofibrillar, mitochondrial and sarcoplasmic),
intramuscular signalling protein phosphorylation, and
PGC-1αmRNA responses compared to a low-intensity
external work-matched control condition. Certainly, to
study the effects of contractile intensity and muscle fibre
recruitment on muscle protein synthesis rates in vivo in
humans has its inherent limitations. An assumption made
with the methodology employed is that a small population
of sampled muscle fibres reflects that of the entire vastus
lateralis.
Methods
Participants
Eight recreationally resistance-trained men (23.5 ±
1years(
SEM); 88.3 ±5 kg; BMI =26.5 ±1.0 kg m2)were
recruited for the study. Participants were habitually active
and engaged in lower body resistance exercise at least 2
times per week for 2 years at the time of the study.
All participants were deemed healthy based on their
response to a routine medical screening questionnaire.
We chose to recruit resistance-trained subjects to increase
the reliability of our strength measurements and to
eliminate the potential for non-specific muscle protein
synthetic responses due to the novelty of a resistance
exercise stimulus (Wilkinson et al. 2008). Participants
were informed of the purpose of the study, experimental
procedures, and all its potential risks prior to providing
written consent to participate. The study was approved by
the local Research Ethics Board of McMaster University
and Hamilton Health Sciences and conformed to
standards for the use of human subjects in research
as outlined in the fifth Declaration of Helsinki and
with current Tri-Council Canadian Government funding
agency guidelines for use of human subjects in research.
Experimental protocol
Two weeks prior to the infusion trials, all participants
reported to the laboratory for familiarization with the
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J Physiol 590.2 Time under muscle tension and muscle protein synthesis 353
exercise protocol and to establish their unilateral one
repetition maximum (1RM) on each leg for knee extension
exercise. Participants’ unilateral 1RM for the right and
left legs were 105 ±6 kg and 101 ±6kg, respectively
(P=0.7). Each participant recorded his dietary intake for
3 days prior to the resting and exercise infusion trial (trial
1). A unilateral model was used, whereby each individual
served as his own comparison with the opposite condition,
and including a rested fasted control measurement. This
ensured that acute changes in muscle protein synthesis
and protein phosphorylation after exercise and feeding
were due to the imposed stimuli. On the morning of
trial 1 (Fig. 1), participants reported to the laboratory
at 07.00 h after an overnight fast and after refraining
from any physical activity for the preceding 3 days. An
18-gauge catheter was inserted in an antecubital vein
of one arm for blood sampling and kept patent with
0.9% saline drip for repeated blood sampling. After a
baseline blood sample was drawn, a second catheter was
inserted in the contralateral arm for the primed constant
infusion (PHD 2000; Harvard Apparatus, Natick, MA,
USA) of L-[ring-13 C6]phenylalanine (prime: 2 μmol kg1;
0.05 μmol kg1min1; Fig. 1), which was passed through
a0.2μm filter. Except for during the exercise bout, the
participants rested comfortably on a bed throughout the
infusions. At 3.5 ±0.1 h after the start of the infusion, a
single muscle biopsy was obtained from the dominant leg
to measure fasting rates of muscle protein synthesis (Fast).
After the biopsy, the participants’ legs were shaved with
a hand razor and cleaned with isopropyl alcohol prior
to electrode placement. Bipolar self-adhesive Ag/AgCl
monitoring electrodes (Kendall Meditrace 133, Chicopee,
MA, USA) were placed on the medial portion of the
muscle bellies of the vastus lateralis, vastus medialis, and
rectus femoris in line with the direction of muscle fibre
orientation. The reference electrode was placed on the
head of the fibula and electromyography (EMG) was
measured during exercise.
Participants subsequently performed bouts of unilateral
leg extension exercise at 30% of their previously
established concentric 1RM. Legs were randomized and
balanced for dominance based on maximal strength
to perform exercise at a slow lifting (SLOW) or an
external work-matched control (CTL) conditions. The
leg assigned to the SLOW condition performed exercise
with a lifting/lowering cadence of 6 s concentric phase
and a 6 s eccentric phase with no pauses until volitional
fatigue (i.e. failure). Failure was defined as the point
at which the participant could not lift through the full
range, or his technique to lift the load included motions
at joints other than the knee. The CTL condition was
completed with the contralateral leg and was matched to
the experimental condition for contraction volume such
that the leg performed an identical number of repetitions
at an equivalent load, but not to failure, and was performed
with a lifting cadence of 1 s concentric phase and a 1 s
eccentric phase. Participants performed three sets with
2 min rest between each set for each condition and were
instructed on proper lifting cadence using verbal cues and
a metronome. A goniometer was positioned on the leg
Figure 1. Schematic diagram of the experimental infusion protocols
Double arrows indicate bilateral biopsies were obtained at corresponding time points. Subjects consumed 20 g of
whey protein isolate at the feeding time points.
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354 N. A. Burd and others J Physiol 590.2
extension machine to record knee joint angle. The time
derivative of the flexion angle (angular velocity) was used
to identify the concentric, isometric and eccentric phases
of each repetition.
After completion of the exercise, participants returned
to the resting position and a blood sample was collected
and placed on ice. Subsequently, participants consumed a
drink containing 20 g of whey protein isolate (Fonterra
Alacen-895-I, Auckland, New Zealand). This amount
of protein has been established to maximally stimulate
rates of muscle protein synthesis after resistance exercise
in young men (Moore et al. 2009a). To minimize
disturbances in isotopic equilibrium, the drinks were
enriched to 4% with tracer according to a measured
phenylalanine content of 3.5% in the whey protein;
this approach is explained in detail elsewhere (Burd
et al. 2011b). Bilateral biopsies were taken at 6 h
after completion of unilateral resistance exercise. After
completion of trial 1, participants were fed a meal from the
cafeteria that represented 2500–3000 kJ and instructed
to eat a subsequent meal that was representative of the
meals they previously recorded on the 3 day dietary log.
This meal was to be consumed no later than 22.00 h
to ensure a 10 h fast prior to the beginning of the
24 h post-exercise protein synthesis measurement (trial
2). Participants were instructed to refrain from physical
activity for the evening.
In the morning participants returned to the laboratory
for trial 2 and underwent the previously described infusion
trial procedures. Bilateral biopsies were obtained at 1.5 h
after the start of the infusion, followed by the consumption
of a tracer-enriched protein drink containing 20 g of
whey isolate. Infusion trial 2 was concluded by bilateral
biopsies at 6.5 h. Muscle biopsies, all via separate incisions
separated by 4 cm, were performed with a Bergstr¨
om
needle that was custom-modified for manual suction
under local anaesthesia (2% xylocaine). All biopsies were
obtained from the vastus lateralis. Biopsy samples were
blotted and freed of any visible fat and connective tissue,
immediately frozen in liquid nitrogen and stored at 80C
until further analysis. During trials 1 and 2, blood samples
drawn every 0.5 or 1 h and were processed as previously
described (Moore et al. 2009a)(Fig.1).
Blood analyses
Plasma [ring-13 C6] phenylalanine enrichments were
determined as previously described (Glover et al. 2008).
Blood amino acid concentrations were analysed by HPLC
as previously described (Moore et al. 2005). Blood glucose
concentrations were analysed using a blood glucose meter
(OneTouch Ultra 2, Lifescan Inc., Milpitas, CA, USA)
within 2 min of blood collection. Plasma insulin was
measured using a commercially available immunoassay
kit (ALPCO Diagnostics, Salem, NH, USA).
Electromyography analyses
The raw electromyographic (EMG) signals were sampled
at 1024 Hz, using a custom-made bioamplifier, and were
collected with acquisition software (LabVIEW v 8.2;
National Instruments, Austin, TX, USA). All raw EMG
signals were digitized and stored on an external hard drive
and analysed as previously described (Burd et al. 2010a).
Muscle protein synthesis
A piece of wet muscle (100 mg) was homogenized with
a Dounce glass homogenizer on ice in an ice-cold homo-
genizing buffer (10 μlmg
1;1Msucrose, 1 MTr i s/ HCl ,
1MKCl , 1 MEDTA) supplemented with a Complete
Mini, protease inhibitor and phosphatase cocktail tablets
(PhosSTOP, Roche Applied Science, Mannhein, Germany)
per 10 ml of buffer. The homogenate was transferred
to an Eppendorf tube and spun at 700 gfor 15 min at
4C to pellet a fraction enriched with myofibrillar and
cytoskeleton proteins. The supernatant was transferred to
another Eppendorf tube and spun at 12,000 gfor 20 min at
4C to pellet the mitochondrial enriched protein fraction.
The supernatant was placed in two separate aliquots
for intramuscular signalling and sarcoplasmic protein
fraction. The sarcoplasmic proteins were precipated by
the addition of 1 ml of 1 Mperchloric acid (PCA). The
mitochondrial enriched pellet was washed twice with
500 μl of an ice-cold homogenizing buffer (1 Msucrose,
1MTri s /HC l , 1 MKCl, 1 MEGTA/Tris) and spun at 12,000 g
for 5 min at 4C. The supernatant was discarded and the
pellet washed with 95% ethanol and spun at 12,000 gfor
5 min. The supernatant was discarded and the pellet was
lyophilized. Amino acids were liberated by adding 1.5 ml
of 6 MHClandheatingto110
C for 24 h. The myofibrillar
and collagen pellets that remained from the initial 700 g
spin were washed twice with the homogenization buffer
and spun at 700 gfor 5 min at 4C. The supernatant
was discarded. The myofibrillar enriched proteins were
solubilised by adding 1.5 ml of 0.3 MNaOH and heating to
37C for 30 min with vortex mixing every 10 min. Samples
were centrifuged at 700 gfor 5 min at 4C and the super-
natant containing the myofibrillar-enriched fractions was
collected and the collagen pellets were discarded. Myo-
fibrillar proteins were precipitated by the addition of
1mlof1
MPCA and spinning at 700 gfor 10 min at
4C. The myofibrillar and sarcoplasmic enriched fractions
were washed twice with 70% ethanol and the latter was
lyophilized. The amino acids were liberated from the
myofibrillar, sarcoplasmic, and mitochondrial enriched
fractions by adding 1.5 ml of 6 MHClandheatingto110
C
for 24 h.
Free amino acids from myofibrillar, mitochondrial
and sarcoplasmic enriched fractions were purified using
cation-exchange chromatography (Dowex 50WX8-200
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J Physiol 590.2 Time under muscle tension and muscle protein synthesis 355
resin; Sigma-Aldrich Ltd) and converted to their
N-acetyl-n-propyl ester derivatives for analysis by
gas chromatography combustion-isotope ratio mass
spectrometry (GC-C-IRMS: Hewlett Packard 6890; IRMS
model Delta Plus XP, Thermo Finnagan, Waltham, MA,
USA). Muscle intracellular amino acids were extracted
from a separate piece of wet muscle (20 mg) with
ice-cold 0.6 MPCA. Muscle was homogenized on ice
with a Teflon-coated pestle and then centrifuged at
12,000 gfor 10 min at 4C. The supernatant was
then collected and this process was repeated two
more times. All three supernatants were combined and
taken as the intracellular amino acids and purified by
cation-exchange chromatography and converted to their
heptafluorobutyrate (HFB) derivatives before analysis by
GC-MS (models 6890 GC and 5973 MS; Hewlett-Packard,
Palo Alto, CA, USA) as previously described (Moore et al.
2009b).
Intramuscular signalling
The methods for determination of the extent of
phosphorylation of Akt on Ser473, mTOR on Ser2448,
p70S6K on Thr389, rps6 on Ser240/244, 4EBP1
on Thr37/46, eIF2Bεon Ser539, p38 MAPK on
Thr180/Try182, Erk1/2 on Thr202/Tyr204, p90RSK on
Thr573, rps6 on Ser235/236 and total protein were
performed as described in our previous work (Burd
et al. 2010a). All data are expressed as the ratio between
the phosphorylated to the total protein and analysed
accordingly.
Real-time quantitative polymerase chain-reaction
Total RNA was isolated from wet muscle samples
(20 mg) as described in previous work (Cochran
et al. 2010). RNA was transcribed and quantitative
RT-PCR reactions were conducted as described pre-
viously (Cochran et al. 2010). Fold change in
PGC-1αexpression were calculated using the Ct
method (Livak & Schmittgen, 2001), normalized to
the housekeeping gene glyceraldehyde 3-phosphate
dehydrogenase (GAPDH), which did not change
across time (P=0.75). GAPDH primers were as
follows: forward 5-CCTCCTGCACCACCAACTGCTT-3
and reverse 5-GAGGGGCCATTCACAGTCTTCT-3.
Calculations
The fractional synthetic rates (FSR) of muscle proteins
were calculated using the standard precursor–product
equation as described (Moore et al. 2009b;Westet al. 2009;
Burd et al. 2010a,b). The recruitment of ‘tracer-naive’
participants allowed us to use the pre-infusion blood
sample, which we have measured as being equivalent in
enrichment to a pre-infusion biopsy (unpublished), as the
pre-infusion baseline enrichment (Ep1) for the calculation
of resting muscle protein synthesis (Fast). This single
biopsy approach to determine basal muscle protein FSR
has been validated within our laboratory (Burd et al.
2011b). Moreover, this approach has been shown to be
valid by others, but instead the workers used a baseline
(i.e. pre-infusion and thus non-enriched) muscle biopsy
(Smith et al. 2010).
Statistics
A within-subject repeated measures design was used for
the current study. Differences in muscle protein synthesis,
mRNA responses, electromyography and intramuscular
signalling were tested using a two-factor (condition ×
time) analysis of variance (ANOVA) with repeated
measures on time. Muscle time under tension was analysed
using a one-way ANOVA. Blood glucose, plasma insulin,
and blood amino acid concentrations were analysed
using one-factor (time) repeated measures ANOVA.
Where significant interactions were identified in the
ANOVA, Tukey’s post hoc test was performed to determine
differences between means for all significant main effects
and interactions. Linear regression lines were fitted to
plasma enrichments to assess the existence of any deviation
in enrichment indicated by lines with a significant
positive or negative slope. Pearson’s rproduct–moment
correlation was used to examine associations between
different variables. For all analyses, differences were
considered significant at P<0.05. All results are presented
as means ±standard error of the mean (SEM).
Results
Resistance exercise
There was no difference in the load lifted for SLOW
(31 ±2 kg) or CTL (30 ±2 kg) conditions (P=0.7). The
repetitions performed during SLOW and CTL conditions
were 12 ±1, 7 ±1and6±1forset1,2and3,respectively.
Muscle time under muscle tension was greater for each
exercise set (all, P<0.05) in the SLOW condition (set 1,
198 ±10 s; set 2, 119 ±9 s; set 3, 90 ±7s) compared to
the CTL condition (set 1, 25 ±2 s; set 2, 14 ±1s; set 3,
11 ±1 s) with a similar 8:1 ratio between contraction
times for each set in the SLOW condition as compared
to CTL. The total time the muscle was under tension was
greater (P<0.001) in the SLOW (407 ±23 s) as compared
to the CTL (50 ±3 s) condition.
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356 N. A. Burd and others J Physiol 590.2
Table 1. Blood amino acid concentrations, blood glucose, and plasma insulin concentrations in the fasted-state and after ingestion
of 20 g of whey protein isolate during trial 1 and trial 2
After drink
Fasted 0 h 0.5 h 1.0 h 1.5 h 2.0 h 3.0 h
Tri a l 1:
EAA (μmol l1) 554 ±34 494 ±44 1063 ±661071 ±95689 ±59 611 ±42 503 ±45
Leucine (μmol l1)89±579±7 233 ±15249 ±23145 ±13119 ±8 103 ±10
Insulin (μUml
1)4.4±0.94.3±0.718.9±0.920.9±3.66.6±1.34.4±1.34.03 ±0.7
Glucose (mm) 5.0±0.25.4±0.35.1±0.15.3±0.15.2±0.35.4±0.14.9±0.2
Tri a l 2:
EAA (μmol l1) 608 ±26 562 ±45 870 ±49 1197 ±159970 ±110821 ±128 726 ±199
Leucine (μmol l1)99±490±7 190 ±11221 ±24163 ±16125 ±19 102 ±17
Insulin (μUml
1)4.1±0.64.2±1.012.2±2.219.2±1.36.1±0.63.4±0.23.7±0.5
Glucose (mm) 5.3±0.1— 5.3±0.25.1±0.24.9±0.24.9±0.25.0±0.2
Values are means ±S.E.M. (n=8). Drink composed of 20 g of whey protein isolate. EAA are sum of His, Ile, Leu, Lys, Met, Phe, Thr,
Val (note: neither Trp nor Cys was measured). Significantly different from Fast, P<0.05.
Blood glucose, plasma insulin and amino acid
concentrations
Blood glucose remained stable during infusion trial 1 and 2
(Table 1). Plasma insulin concentration peaked (P<0.05)
at 1 h post-drink ingestion in trial 1 (4.7-fold increase
above baseline) and in trial 2 (4.6-fold increase), but
returnedtobasalconcentrationsby2hinbothtrials.
Blood essential amino acid (EAA) concentrations peaked
(P<0.001) at 1 h post-drink ingestion and returned to
basal by 2 h in both trials (Table 1). Similarly, blood leucine
concentration peaked (P<0.001) at 1 h for trial 1 and trial
2andreturnedtobasallevelsby2hinbothtrials(Table1).
Electromyography
All muscles of the quadriceps that were measured (i.e.
vastus lateralis, vastus medialis and rectus femoris) showed
similar EMG results and therefore only the results for
the vastus lateralis are reported. EMG amplitude for the
concentric phase of exercise significantly increased from
the start of the set (0% set completion) to 60–100% set
completion for set 1 (all, P<0.001), whereas set 2 and
set 3 significantly increased at 50–100% set completion
(all P<0.001) in the SLOW condition. EMG amplitude
in the CTL condition significantly increased from 0% set
completion at 50–100% set completion for set 1 and 2 (all
P<0.001), whereas set 3 significantly increased from the
start of the set to 60–100% set completion (all P<0.05).
EMG amplitude (Fig. 2A) for the SLOW condition was
greater than the CTL condition at 90–100% set completion
(both P<0.05) for set 1 and at 80–100% for set 2 (all
P<0.05). EMG amplitude for the third set of the SLOW
condition was greater than the CTL condition at 0–100%
set completion (all P<0.001). There was a significant
decrease in isometric mean power frequency (MPF) from
the first repetition to the last repetition of the last set
completion only after performing the SLOW condition
(P<0.001), whereas the CTL condition did not show a
significant reduction in MPF from beginning to the end
of the contractile protocol (P=0.09).
Plasma and intracellular precursor enrichments
Intracellular precursor enrichments were similar across
time during trial 1 in SLOW (0.051 ±0.003 tracer/
tracee and 0.052 ±0.002 tracer/tracee) and CTL
conditions (0.051 ±0.003 tracer/tracee and 0.050 ±
0.002 tracer/tracee). Intracellular enrichments were
also similar during trial 2 for SLOW (0.048 ±0.002
and 0.049 ±0.003 tracer/tracee at 1.5 and 6.5 h,
respectively; P=0.9) and CTL conditions (0.054 ±0.3
and 0.053 ±0.2 tracer/tracee at 1.5 and 6.5 h, respectively;
P=0.7). Furthermore, linear regression analysis
indicated that the slopes of the plasma enrichments were
not significantly different from zero during trial 1 or trial
2(P=0.7), indicating that isotopic plateau was achieved
and that the use of the steady-state precursor product
equation was appropriate.
Muscle protein synthesis
There was no detectable increase in rates of myofibrillar
protein synthesis during 6 h of recovery in the SLOW or
CTL conditions (both P>0.05). The SLOW condition
resulted in a stimulation of myofibrillar protein synthesis
during 24–30 h recovery as indicated by a 2.3-fold increase
above fasted rates (P<0.001) and was greater than the
0–6 h response and the CTL condition at that time
point (Fig. 3A). Mitochondrial protein synthesis rates
were stimulated 2.1-fold above fast (P=0.018) during
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J Physiol 590.2 Time under muscle tension and muscle protein synthesis 357
0–6 h recovery only in the SLOW condition; however,
at 24–30 h post-exercise both SLOW (P<0.001) and
CTL (P=0.002) were stimulated above fast by 2.8- and
2.3-fold, respectively (Fig. 3B). The mitochondrial protein
synthetic responses at 24–30 h recovery were maintained
in the SLOW condition and increased from 0–6 h in
the CTL condition (P=0.002). Sarcoplasmic protein
synthesis rates were stimulated 1.8-fold during 0–6 h
exercise recovery only in the SLOW condition (P<0.001)
and were greater than the CTL condition at this time point
(P=0.001).
Intramuscular signalling
Phosphorylation of p70S6K (Fig. 4A) was increased above
fast by 3.4-fold at 24 h post-exercise only in the SLOW
condition (P=0.002) and was phosphorylated more than
Figure 2. Percentage increase in vastus lateralis activation
during the concentric phase of resistance exercise (A)andthe
change in mean power frequency (MPF) during the isometric
phase of resistance exercise from the first repetition to the
last repetition (B)
Numbers in parentheses following SLOW indicate set number. Lower
case letter indicates significantly different from CTL for sets 1–3: a,
SLOW(1); b, SLOW(2); c, SLOW(3); P<0.05. Significantly different
from 0% set completion, P<0.05. Significantly different from CTL
at that time point, P<0.05.
CTL at this same time point (P=0.004). There was
a significant association between the extent of p70S6K
phosphorylation at 24 h post-exercise and rates of myo-
fibrillar protein synthesis during 24–30 h of recovery
(r=0.42, P=0.02).
Phosphorylation of 4EBP1 was increased by 1.6-
and 1.5-fold above fast at 6 h and 24 h post-exercise,
respectively, only in the CTL condition (both P<0.05);
Figure 3. Myofibrillar (A), mitochondrial (B), and sarcoplasmic
(C) protein fractional synthetic rates (FSR) during protocols
Note different scales on y-axes between graphs. Rates are from
rested fasted and after resistance exercise with slow (SLOW) or
external work match control (CTL) muscle time under tension. Values
are means ±S.E.M. Significantly different from fasting, P<0.05.
Significantly different from CTL at that same time point, P<0.05.
Significantly different from the 0–6 h response in the same
condition, P<0.05.
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358 N. A. Burd and others J Physiol 590.2
however, at 30 h post-exercise 4EBP1 was phosphorylated
above fast (1.6-fold) only in the SLOW condition
(P=0.02). The phosphorylated-state of 4EBP1 in CTL
was greater than SLOW condition at 6 h post-exercise
(P=0.004). Phosphorylation of p90RSK was significantly
increased above fast by 2.5-fold only in the SLOW
condition (P=0.001). There was no change (P>0.05)
in phosphorylation of Erk1/2, p38 MAPK, Akt, mTOR,
Figure 4. Ratio of phosphorylated to total of p70S6KThr389 (A),
4E-BP1Thr37/46 (B) and p90RSKThr573 (C) during the protocols
Ratios are from rested fasted and after resistance exercise with slow
(SLOW) or external work match control (CTL) muscle time under
tension. Values are means ±S.E.M. Data are expressed in arbitrary
units (AU). Significantly different from fast, P<0.05. Significantly
different from CTL within that time point, P<0.05. Significantly
different from SLOW within that time point, P<0.05.
rps6 on Ser240/244 or 235/236, or eIF2Bε(supplemental
figure).
PGC-1αmRNA
PGC-1αmRNA was increased by 3-fold above rest at
6 h post-exercise, with no difference between conditions
(main effect for time, P=0.001) and returned to baseline
by 24 h (Fig. 5).
Discussion
Our study is the first to demonstrate that a prolonged
time under muscle tensi during resistance exercise did
not stimulate an immediate rise in myofibrillar protein
synthesis rates, but did result in a delayed stimulation
that was significant at 24–30 h recovery with a sub-
sequent feeding-induced increase of myofibrillar protein
synthesis. We also report evidence that resistance exercise
has a potent stimulatory effect on mitochondrial protein
synthesis rates that were only partially dependent on
muscle time under tension with a greater stimulation
of the response early after a longer time under tension.
However, there was a robust stimulation of mitochondrial
protein synthesis rates in the SLOW and CTL conditions
during 24–30 h recovery. Further, a longer time under
muscle tension increased the acute (0–6 h) amplitude of
sarcoplasmic protein synthesis rates; however, it had no
influence on extending the duration of this response. Inter-
estingly, acute low intensity resistance exercise increased
muscle PGC-1 mRNA expression at 6 h post-exercise and
this elevation was independent of muscle time under
tension during the bout.
It is generally accepted that exercise-induced rates of
muscle protein synthesis, in the fasted-state, are greatest
immediately after an acute bout of resistance exercise
and gradually decline in the hours and days that follow
(Phillips et al. 1997). Thus, the effect of exercise per se,
Figure 5. Peroxisome proliferator-activated receptor γ
coactivator 1-α(PGC-1α) mRNA content at fast, 6 h and 24 h
post-exercise
Significantly different from fasting, P<0.05.
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J Physiol 590.2 Time under muscle tension and muscle protein synthesis 359
in the absence of feeding, is to stimulate a prolonged
elevation in muscle protein synthesis rates. It is becoming
clear that prior exercise affects nutrition-mediated myo-
fibrillar protein synthesis rates at time points later (i.e.
24 h) in exercise recovery. What we observed here was
a potentiated effect, from that seen in the fasted-state,
of prior exercise in enhancing the feeding-induced myo-
fibrillar protein synthetic rates. This effect appears to be
dependent on maximal fibre activation during exercise,
which is consistent with our previous observation (Burd
et al. 2011a). The current study is noteworthy in that an
enhanced effect of protein feeding during late exercise
recovery was induced by a longer time under muscle
tension rather than intensity-independent contraction
volume, which we have previously examined (Burd
et al. 2010a). These data, and our other observations
(Burd et al. 2010b, 2011a), clearly show that contractile
variables can be manipulated to affect responses of muscle
protein synthesis. Thus, we speculate that maximal fibre
activation, and not percentage of maximal strength, is
fundamental to induce maximal rates of muscle protein
synthesis and we would hypothesize other purportedly
important variables that are thought to dictate hyper-
trophy (Ratamess et al. 2009) are largely redundant in their
ability to elicit an anabolic response to exercise so long as
high levels of muscle fibre recruitment are attained.
An interesting question that arises from our current
work is why did we not observe an increase in
exercise-mediated rates of myofibrillar protein synthesis
during 0–6 h recovery? This outcome was unexpected
asourpreviousworkdemonstratedthatlowintensity
exercise performed to failure, using a faster lifting
cadence, stimulating robust increases in myofibrillar
protein synthesis rates (Burd et al. 2010b). Indeed, this
finding provided the basis for a thesis that achieving
maximal muscle fibre activation during resistance exercise
is fundamental to maximally stimulate rates of myofibrillar
protein synthesis during acute exercise recovery. Certainly,
our current protocol was successfulin eliciting full muscle
fibre recruitment using a prolonged time under tension
(Fig. 2A); however, we did not find an immediate acute
stimulation in myofibrillar protein synthesis rates. The
explanation for this result is likely to relate to the timing
of the muscle biopsies, training status of subjects, and
the resistance exercise protocol. First of all, we specifically
chose to study the muscle protein synthetic responses over
6 h to minimize an overriding feeding effect, which would
peak at 3 h (Moore et al. 2009b), and thus capture a ‘true’
exercise effect, which normally is sustained for at least
5 h after high intensity resistance exercise (Moore et al.
2009b;Westet al. 2011). Secondly, resistance training
shortens the duration of the muscle protein synthetic
response (i.e. we used trained subjects) (Tang et al. 2008)
and may have further precluded our ability to detect
a response. Finally, the resistance exercise protocol we
employed is far from resembling any other resistance
protocol used in other studies studying muscle protein
metabolism in vivo in humans. Specifically, to minimize
the repetitions performed during each exercise set and
elicit fatigue with the low load, the time that the muscle
was under tension in the SLOW condition was 2min16s
for each set, a duration that far exceeded the low intensity
condition in our previous investigations (40 s each set)
(Burd et al. 2010a,b). Thus, it seems that the hallmark
response of loaded resistance exercise, as a stimulus for
myofibrillar protein synthesis, was shifted instead toward
increased synthesis of proteins in the mitochondrial and
sarcoplasmic pools (Fig. 3). What facilitates the differences
in synthesis of specific proteins within the muscle protein
pools is still, at least at the muscle protein synthetic level,
very much unclear. Importantly, such a finding would
likely have been missed had we measured mixed muscle
protein synthesis.
Despite the lack of an immediate stimulation of myo-
fibrillar protein synthesis in the current study, our data
do provide support that acute exercise until failure,
likely through maximal fibre activation, results in a
delayed sensitizing effect on myofibrillar protein synthesis
with nutrition during late exercise recovery and provide
further insight in the regulation of myofibrillar protein
synthesis during 24 h of exercise recovery (Burd et al.
2011a). The increased sensitivity to protein feeding at 24 h
post-exercise, reported previously (Burd et al. 2011a)and
in the current study, are perhaps not overly surprising.
However, since if basal fasting rates of muscle protein
synthesis can be elevated for up to 48 h (Phillips et al.
1997) then the feeding-induced potentiation of myo-
fibrillar protein synthesis over and above the fed-state
response itself (Moore et al. 2009b) should be evident
at 24 h and likely even at 48 h. Similar results have been
seen in aged men who, while unable to mount a significant
fed-state increase in mixed muscle protein synthesis in the
absence of exercise, showed a significant stimulation at
18 h after 40 min of walking (Fujita et al. 2007b).
Our general understanding of the influence of resistance
exercise on rates of mitochondrial protein synthesis in
humans is very limited. Our laboratory has reported that
an acute bout of resistance exercise has the capacity to
increase rates of mitochondrial protein synthesis in the
untrained state (Wilkinson et al. 2008), data which are
consistent with the notion that resistance exercise can
improve muscle oxidative potential (Tang et al. 2006). It
appears the responsiveness of the mitochondrial protein
pool, at least during acute recovery, to ‘conventional’ high
intensity resistance exercise is attenuated after a training
period when stimulated by the same absolute load as
used prior to training (Wilkinson et al. 2008). However,
our current data show that low intensity resistance
exercise can stimulate mitochondrial protein synthesis
rates during 0–6 h recovery in trained participants when
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360 N. A. Burd and others J Physiol 590.2
muscle time under tension is increased during the exercise
session. Indeed, it was unexpected that exercise induced
mitochondrial protein synthesis rates were elevated to a
similar extent at 24–30 h recovery between the SLOW
and CTL conditions. Due to the exhaustive nature of
the SLOW condition, we anticipated the mitochondrial
synthetic response to be more robust. However, this
finding may highlight just how sensitive the mitochondrial
proteinpoolistocontraction,regardlessoftheexercise
stimulus, during longer term recovery. In support, many
genes involved in mitochondrial function are up-regulated
at 48 h of recovery after endurance exercise, although
we admit it is not fair to compare resistance exercise
versus an endurance exercise stimulus (Rowlands et al.
2011). It is clear, however, that future investigations
are needed, which would include a time course of the
response, to underpin the physiological mechanism for
up-regulation of mitochondrial protein synthesis rates
during longer-term recovery. In a similar manner, we
had hypothesized that muscle PGC-1αexpression would
be more robust after the SLOW condition (Egan et al.
2010). We found that muscle time under tension, and
associated greater increase in metabolic work, had no
influence on the PGC-1αmRNA response. Notable, the
3-fold increase in PGC-1αmRNA expression observed
in the SLOW and CTL conditions are similar in amplitude
(Fig. 5) to that observed after four 30 s ‘all out’ cycling
sprints (Gibala et al. 2009). Also, it is worth highlighting
that that our previous investigations which examined rates
of sarcoplasmic protein synthesis (i.e. non-myofibrillar
proteins) would also have contained the mitochondria
protein pool (Moore et al. 2009b;Burdet al. 2010b). Here,
we present rates of sarcoplasmic protein synthesis that are
largely devoid of mitochondrial proteins and show that
the increased time the muscle was under tension affected
the acute amplitude of sarcoplasmic protein synthesis
rates with no effect extending to the 24 h post-exercise
period.
We studied candidate proteins within the Akt-mTOR
and MAPK pathways to ascertain if the phosphorylation
of intramuscular proteins involved in regulating mRNA
translation and elongation were influenced by muscle
time under tension. Indeed, it is difficult to fully under-
stand which of these signalling pathways are involved in
regulating the synthesis of specific muscle proteins (e.g.
is p70S6K activation specific toward the stimulation of
myofibrillar protein synthesis?), although there is likely
to be interplay between protein kinases such that each
coordinates the synthesis of more than one specific
muscle protein pool. However, more work is necessary
to address this question. Here, we found that 4E-BP1
phosphorylation was enhanced at 6 h post-exercise only
in the CTL condition, a finding that is consistent with
the notion that this signalling protein may be more
responsive to feeding (Fujita et al. 2007a;Athertonet al.
2010; Moore et al. 2011) rather than contraction (Dreyer
et al. 2006). We did not obtain muscle biopsies at
0.5–1.5 h after exercise, a time point when intramuscular
signalling protein activation is typically higher (Camera
et al. 2010), which may have precluded our ability to
distinguish whether muscle time under tension affected
the phosphorylation status of signalling proteins at a
potentially more relevant time than 6 h post-exercise.
There appears to be a substantial redundancy in the
intramuscular signalling protein activation that may, in
part, be mediating the delayed effect of resistance exercise
on muscle protein synthesis rates at 24 h. Specifically,
signalling proteins (i.e. p70S6K, 4EBP-1, and p90RSK) well
known to be phosphorylated immediately after resistance
exercise (Kumar et al. 2009; Camera et al. 2010; Terzis
et al. 2010; Moore et al. 2011) were also phosphorylated
at 24 and 30 h post-exercise. It remains to be clearly
established, but the current data suggest that certain
intramuscular signalling proteins may undergo relatively
prolonged changes in their phosphorylated states during
exercise recovery and may mediate rates of muscle protein
synthesis during late exercise recovery. The present results
also continue to add to the growing body of literature
supporting the phosphorylated state of p70S6K as a proxy
marker of myofibrillar protein synthesis rates after acute
resistance exercise in humans (reviewed in West et al.
2010).
Admittedly, the methods to study muscle protein
synthesis in vivo in humans only requires that a
small population of muscle fibres are sampled. Thus,
it is assumed that this small population of fibres is
representative of the entire thigh muscle. Indeed, all the
motor units, and the associated type I or II fibres, in a
muscle do not fire at the same time (Sale, 1987). Thus, there
is selective recruitment of the fast-twitch and slow-twitch
motor units to produce enough force to overcome the
load. In the current study, we employed a model that
allowed us to test how various levels of recruitment affect
specific protein pools within muscle. We are assuming that
type II muscle fibres were eventually activated, which is
supported by the EMG results, in the SLOW condition that
led to some of the superior responses (Figs 2–4). Certainly,
studying the response at the single fibre level would yield
valuable insight into how specific fibre types are affected
during low intensity resistance exercise. However, this
approach also takes into account a small population of
fibres and the feasibility of this methodology is difficult
to employ on a large-scale basis (examining multiple time
points post-exercise).
In summary, a prolonged muscle time under tension,
only when fatigue leads to full motor unit recruitment
(Fig. 2A), affects the acute amplitude of muscle
protein sub-fractional synthesis (i.e. mitochondria and
sarcoplasmic protein pools) and mediates a delayed effect
on rates of myofibrillar synthesis during 24–30 h recovery.
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J Physiol 590.2 Time under muscle tension and muscle protein synthesis 361
This delayed effect on myofibrillar protein synthesis
rates during longer-term recovery, when accompanied by
protein feeding, after fatiguing exercise highlights that
separate, yet undefined, mechanisms are facilitating a
nutrient enhancing effect on longer-term myofibrillar
protein synthetic responses as compared to immediately
after resistance exercise. Notable is that our current
data highlight, and substantiate our previous findings
(Burd et al. 2010b), that maximal fibre activation
cannot be viewed as the exclusive driver of myofibrillar
protein synthesis rates. It appears exercise volume is
yet another fundamental variable that promotes p70S6K
phosphorylation (Terzis et al. 2010) and a prolonged
elevation of myofibrillar protein synthesis rates (Burd
et al. 2010a,b). We are the first to provide a further time
course (i.e. 24 h later) of mitochondrial protein synthesis
rates after acute resistance exercise and report that low
intensity resistance exercise has a potent stimulatory effect
on the response at 24–30 h recovery. Additionally, low
intensity resistance exercise has the capacity to increase
muscle PGC-1αmRNA responses at 6 h post-exercise
recovery. Our data provide further evidence of the value
of studying muscle protein synthetic responses at the
muscle fraction specific level in order to gain a clear
understanding of the phenotypic response to an exercise
stimulus.
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Author contributions
N.A.B. and S.M.P. contributed to the conception and the design
of the experiment. All authors contributed to collection, analysis,
and interpretation of data. All authors contributed to drafting
or revising intellectual content of the manuscript. All authors
approved the final version of this manuscript.
Acknowledgements
The authors wish to thank Todd Prior, Tracy Rerecich, and
Mike Percival for analytical assistance. We also thank Andrew
Holwerda, Nathan Cain and Keegan Selby for their help in data
collection and the participants for their time and effort. We wish
to thank Colin De France of Inbalance Nutrition (Burlington,
ON) for his generous gift of whey protein isolate used in the
study. Lastly, we thank Justin Crane and Mark Tarnopolsky for
confirming the purity of our enriched protein fractions. This
research was supported by a researchg rant from Natural Sciences
and Engineering Research Council of Canada to S.M.P.
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... Metabolic stress within a muscle through a resistance exercise stimulus of sufficient magnitude results in the upregulation of muscle protein synthesis and, subsequently, protein accretion and changes in muscle size (Schoenfeld et al., 2021). Increases in muscle protein synthesis rates within the day following lower-body resistance exercise involving submaximal intensities performed with a 6-sec lowering phase until volitional fatigue have been reported (Burd et al., 2012). This suggests that maximal fibre activation may not exclusively drive muscle protein synthesis rates (Burd et al., 2012). ...
... Increases in muscle protein synthesis rates within the day following lower-body resistance exercise involving submaximal intensities performed with a 6-sec lowering phase until volitional fatigue have been reported (Burd et al., 2012). This suggests that maximal fibre activation may not exclusively drive muscle protein synthesis rates (Burd et al., 2012). Increasing exercise volume, through increasing time under tension during the eccentric phase of movement, may also stimulate protein accretion and lead to muscle growth despite lower resistance exercise loads (Burd et al., 2010;Schoenfeld, 2010;Terzis et al., 2010). ...
... Evidence suggests that sarcoplasmic hypertrophy (i.e., an increase in sarcoplasmic volume and its constituents [e.g., fluid, enzymes, organelles] [Roberts, Haun, Vann, Osburn, & Young, 2020]) contributes to increases in muscle size following higher volume resistance training (Haun et al., 2019a) and that these changes support subsequent myofibril hypertrophy (Roberts et al., 2020). Data show that increasing the time under tension during the eccentric phase of movement (~6 sec) increases the amplitude of sarcoplasmic protein synthesis within 6 h of the exercise bout and results in a delayed simulation of myofibril accretion 24-30 h post-training (Burd et al., 2012). Myofibrillar protein synthetic rate was also shown to be associated with p70S6K phosphorylation within this time frame following the training session (Burd et al., 2012). ...
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Successful performances in rugby league require the ability to engage in repeated contact efforts with minimal recovery while maintaining a high running intensity. The capacity to express high levels of time-limited force appears to underlie many important physical attributes required to meet the repeated-effort demands of rugby league play. If appropriately periodised and integrated into the training plan, resistance exercise that sufficiently loads the eccentric phase of movement may provide a beneficial stimulus to improve players' force-generating capacity. Comprehensive reviews relating to the adaptive effects of eccentric training and the methods most commonly prescribed in practical environments are available and may provide context for applying these strategies. However, no literature to date has specifically discussed the planning and programming of eccentric resistance exercise to enhance force production characteristics in elite athletes. Therefore, this narrative review focuses on the periodisation of eccentrically-integrated resistance training during a 17-week National Rugby League pre-season phase. To help guide programming during the pre-season period, the 17-week timeline is divided into several phases (i.e., general preparation, special preparation, active rest, and pre-competition). Within the periodised model, eccentric exercise parameters (i.e., volume, load [% 1RM]) are manipulated to progressively increase the rate of muscle lengthening velocity over the pre-season phase and sequentially elicit changes in muscle-tendon properties and neural function that culminate in improving muscular strength expression.
... [2][3][4] In consequence, this reduction in the repetitions promotes lower total volume (∑ repetitions x load) and time under tension (duration of sets). Given that total volume and time under tension are important variables to provide an increase in myofibrillar protein synthesis and long-term muscular adaptations, [5][6][7] it is additional strategy is needed when RM method is used. ...
... It has been shown that greater time under muscle tension during resistance exercise stimulates an increase in myofibrillar protein synthesis, 5 which may be important in optimizing muscle growth. In the present study, the session performed at 90% of 10-12 RM also produced greater time under tension (9.5%), compared to 100% of 10-12 RM. ...
... From a practical standpoint, our findings indicate that the use of lower loads may be an alternative to maximize the total volume and time under tension, which may cause greater long-term adaptations in muscle strength and hypertrophy. [5][6][7] The lower RPE observed in the first and second sets in the session performed with less load may contribute, in a certain way, to pleasure and adherence to the training program. On the other hand, higher loads may be adopted to promote total training stimulus, which could also be a strategy to promote long-term adaptations. ...
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Introduction Load reduction using the repetition maximum (RM) method may be necessary to promote higher numbers of repetitions, and consequently, higher total volume, time under tension, and perceived exertion ratings. Objective To compare the effects of different leg press exercise loads on number of repetitions, total volume, time under tension, and perceived exertion. Methods Eighteen women university students (23.9 ± 3.8 years) performed two experimental sessions with 90% and 100% of 10-12 RM in a balanced crossover design. Results The number of repetitions of the second and third sets, the total volume, and time under tension at 90% of 10-12 RM was statistically higher than at 100% of 10-12 RM ( p < 0.05). The perceived exertion of the first and second sets and the training load (perceived exertion x duration of sessions) were higher at 100% of the 10-12 RM session ( p < 0.05). Conclusion A small reduction in load results in a greater number of repetitions, total volume, and time under tension. The session with the higher load appeared to induce higher perceived exertion and training load. Thus, scientists and coaches might consider lower loads to maximize the number of repetitions, total volume, and time under tension, which may cause greater long-term muscular adaptations. Level of evidence II; Comparative prospective study. Keywords: Lower limb; Muscle fatigue; Muscle strength; Resistance training
... As a result, tension time and training volume (repetitions x load) increase, presumably stimulating protein synthesis and muscle hypertrophy (10,11). ...
... While low-volume RT results in modest increases in MM, it may increase engagement of older adult in RT (4). Lower intensity in low-volume RT may be an effective strategy for promoting muscle mass gains in the lower limbs, because women tend to choose training loads lower than those recommended by the guidelines (11). Higher intensity, on the other hand, should be considered for promoting gains in upper limb MS and MM. ...
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This study investigated the impact of intensity in a low-volume RT on sarcopenia indicators in postmenopausal women (PTW). Thirty-two participants were randomly assigned to either a control group (CT, n = 10), a LL-RT group (n = 10) that performed one set of 25–30 repetition maximum per exercise or a high-load RT group (HI-RT, n = 12) that performed one set of 8–12 repetition maximum per exercise. The RT groups performed 8 exercises, with 90 seconds of rest between exercises, 2 times a week for 24-weeks. Muscle mass (MM) of limbs (upper and lower) was assessed by DEXA, muscle strength (MS) was measured by the 1-RM leg press test, and physical performance by the TUG test and the 30-second sit to stand test. The ANCOVA (covariates: age, antihypertensive drugs, hormone replacement therapy and pre-time values) was used to analyze the gains (Δ) between groups, with a significance level of 5%. After 24-weeks of RT, lower and upper limb MM (together/summed) increased in both HI-RT (Δ = 0.60 kg; 95% CI: 0.23–1.0 kg) and LI-RT (Δ = 0.48 kg; 95% CI: 0.06–0.91 kg) in relation to CT (Δ=-0.03 kg; 95% CI: -0.43–0.37 kg) with no difference between them (p = 0.016; ƞ²=0.27 (large); observed power = 0.83). However, upper limb MM increased only in the HI-RT. For MS, the HI-RT group (Δ = 40 kg; 95% CI: 21–58 kg) showed greater gains compared to the CT (Δ = -5 kg; CI 95%: -24–14 kg) and LL (Δ = 12 kg; 95% CI: -8–33 kg) (p = 0.001 η ² = 0.35, Power = 0.98). Even though LI-RT promotes MM gains in lower limbs, HI-RT should be considered in low-volume training to promote gains in MS and also in MM in upper limbs in the PTW.
... The duration for which a muscle is under tension during a contraction as well as an external load is important for optimal adaption to training (Burd et al., 2012). Therefore, maximizing the duration and force produced in this phase during a training set could be essential for optimal physiological response. ...
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Aquatic resistance training has been proven to be beneficial to many people, in particular those struggling with degenerative joint diseases or recovering from other musculoskeletal issues as the reaction forces acting on the joints become lower, but without compromising the cardiovascular and neuromuscular benefit of the movement. Little has been written on the load produced by or measurements of the devices used in aquatic resistance training. Therefore, uncertainties exist regarding details of how much load can be applied onto the foot when performing the movements and how to quantify progression. In this study, an instrumented robotic arm was designed, built, and used to measure the load acting on the three different types of fins during a simulated flexion/extension movement of a knee. The angular velocities of the knee ranged from 25°/s to 150°/s, which represent the physiological range of in vivo movements. The results demonstrated that the load followed a second-order polynomial with the angular velocities. The load is therefore a function of the angular velocity, the surface area of the fins, and the location of the fins away from the joint center rotation. We modeled the progression of speeds at maximal voluntary movements based on previous studies. The maximum loads measured between 11 kg and 13 kg in extension and 6 kg and 9 kg in flexion at 150°/s rotational velocity.
... Furthermore, one can also speculate that the decrease in the number of repetitions per set will also contribute to a decrease in the repetition duration or the time the muscle is under tension (TUT) during exercise. According to several authors, the TUT is considered a determinant variable in optimizing muscle growth [17,18]. However, to our knowledge, no study that monitored repetition velocity has analyzed the effects of a 3 × 8RM protocol on TUT, meaning that further research is needed. ...
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This study analyzed the acute effects of heavy strength training on mechanical, hemodynamic, metabolic, and psychophysiological responses in adult males. Thirteen recreational level males (23.3 ± 1.5 years) randomly performed two heavy strength training sessions (3 sets of 8 repetitions at 80% of one repetition maximum [1RM]) using the bench press (HST-BP) or full squat (HST-FS)). The repetition velocity was recorded in both sessions. Moreover, before and after the sessions, the velocity attained against the ~1.00 m·s−1 load (V1Load) in the HST-BP, countermovement jump (CMJ) height in the HST-FS, blood pressure, heart rate, blood lactate, and psychophysiological responses (OMNI Perceived Exertion Scale for Resistance Exercise) were measured. There were differences between exercises in the number of repetitions performed in the first and third sets (both <8 repetitions). The velocity loss was higher in the HST-BP than in the HST-FS (50.8 ± 10.0% vs. 30.7 ± 9.5%; p < 0.001). However, the mechanical fatigue (V1Load vs. CMJ height) and the psychophysiological response did not differ between sessions (p > 0.05). The HST-FS caused higher blood pressure and heart rate responses than the HST-BP (p < 0.001 and p = 0.02, respectively) and greater blood lactate changes from pre-training to post-set 1 (p < 0.05). These results showed that the number of maximal repetitions performed in both sessions was lower than the target number and decreased across sets. Moreover, the HST-BP caused a higher velocity loss than the HST-FS. Finally, the HST-FS elicited higher hemodynamic and metabolic demand than the HST-BP.
... Hence, stretching immediately after eccentric loading may result in greater passive tension on the muscle following cross-bridge deactivation than what would be experienced in traditional static stretching, conceivably mediated via heightened titin stiffness (55). In addition to the heightened tension in the stretch, the strategy allows for a greater time-under-tension during a given session, which has been proposed as a driving factor in hypertrophy (56). ...
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Time is considered a primary barrier to exercise adherence. Therefore, developing time-efficient resistance training (RT) strategies that optimize muscular adaptations is of primary interest to practitioners. A novel approach to the problem involves combining intensive stretch protocols with RT. Conceivably, integrating stretch into the inter-set period may provide an added stimulus for muscle growth without increasing session duration. Mechanistically, stretch can regulate anabolic signaling via both active and passive force sensors. Emerging evidence indicates that both lengthening contractions against a high load as well as passive stretch can acutely activate anabolic intracellular signaling pathways involved in muscle hypertrophy. Although longitudinal research investigating the effects of stretching between RT sets is limited, some evidence suggests it may in fact enhance hypertrophic adaptations. Accordingly, the purpose of this paper is threefold: (1) to review how the active force of a muscle contraction and the force of a passive stretched are sensed; (2) to present evidence for the effectiveness of RT with inter-set stretch for muscle hypertrophy (3) to provide practical recommendations for application of inter-set stretch in program design as well as directions for future research.
Article
Surface electromyography (EMG) and mean force can be used to identify motor unit excitation and fatigue. Low-load resistance training with blood flow restriction (LL+BFR) may result in earlier fatigue and maximal muscle fibre recruitment compared to low-load resistance training (LL). The purpose of this investigation was to examine EMG and force responses during LL versus LL+BFR. Thirteen males (mean ± standard deviation = 24±4 years) completed a bout (1×30) of leg extension muscle actions at 30% of their 1 repetition maximum LL and LL+BFR while force, EMG amplitude, and EMG mean power frequency (EMG MPF) were recorded. EMG amplitude increased (74.2%) and EMG MPF decreased (22.6%) similarly during both conditions. There was no significant difference in mean force during the first 3 repetitions between LL+BFR (477.3±132.3 N) and LL (524.3±235.1 N) conditions, but mean force was lower during the last 3 repetitions for LL+BFR (459.7±179.3 N) compared to LL (605.4±276.4 N). The results of the present study indicated that a fatiguing bout of leg extension muscle actions performed LL and LL+BFR elicited similar neuromuscular responses. There was a significant difference in mean force during the last 3 repetitions (LL>LL+BFR) that may have been due to differences in the time spent near peak force.
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Klotho is an anti-aging protein with several therapeutic roles in the pathophysiology of different organs, such as the skeletal muscle and kidneys. Available evidence suggests that exercise increases Klotho levels, regardless of the condition or intervention, shedding some light on this anti-aging protein as an emergent and promising exerkine. Development of a systematic review and meta-analysis in order to verify the role of different exercise training protocols on the levels of circulating soluble Klotho (S-Klotho) protein. A systematic search of the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE through PubMed, EMBASE, CINAHL, CT.gov, and PEDro. Randomized and quasi-randomized controlled trials that investigated effects of exercise training on S-Klotho levels. We included 12 reports in the analysis, comprising 621 participants with age ranging from 30 to 65 years old. Klotho concentration increased significantly after chronic exercise training (minimum of 12 weeks) (Hedge’ g [95%CI] 1.3 [0.69–1.90]; P < 0.0001). Moreover, exercise training increases S-Klotho values regardless of the health condition of the individual or the exercise intervention, with the exception of combined aerobic + resistance training. Furthermore, protocol duration and volume seem to influence S-Klotho concentration, since the effect of the meta-analysis changes when subgrouping these variables. Altogether, circulating S-Klotho protein is altered after chronic exercise training and it might be considered an exerkine. However, this effect may be influenced by different training configurations, including protocol duration, volume, and intensity.
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Background Resistance exercise can be defined as the percentage of maximal strength (%1 repetition maximum) used for a particular exercise. Shear wave elastography (SWE) is a robust and novelty imaging technique that provides information regarding tissue stiffness. Superb microvascular imaging (SMI) is a non-irradiating technique that can provide quantitative measurement of muscle blood flow non-invasively. Purpose To compare the acute effects of low- and high-velocity resistance exercise on stiffness and blood flow in the biceps brachii muscle (BBM) using SWE and SMI. Material and Methods This prospective study included 60 healthy men (mean age=28.9 years; age range=26–34 years). BBM stiffness was measured by using SWE at rest, after low- and high-velocity resistance exercise, and muscle blood flow was also evaluated by SMI. Resistance exercise was performed using a dumbbell with a mass adjusted to 70%–80% of one-repetition maximum. Results The stiffness values increased significantly from resting to high- and low-velocity resistance exercises. There was no significant difference between the elastography values of the BBM after the high- and low-velocity resistance exercise. The blood flow increased significantly from resting to high- and low-velocity resistance exercises. Blood flow increase after low-velocity exercise was significantly higher compared to high-velocity exercise. Conclusion While muscle stiffness parameters and blood flow significantly increased from resting after both high- and low-velocity resistance exercises, blood flow significantly increased after low-velocity exercise compared to high-velocity exercise. This can mean that metabolic stress, an important trigger for muscle development, is more likely to occur in low-velocity exercise.
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Resistance training has been known to have a positive effect on muscle performance in exercisers. Whole-body electromyostimulation (WB-EMS) is advertised as a smooth, time-efficient, and highly individualized resistance training technology. The purpose of this study is to evaluate the effects of WB-EMS training on maximum isometric elbow muscle strength and body composition in moderately trained males in comparison to traditional resistance training. The study was a randomized controlled single-blind trial. Twenty, moderately trained, male participants (25.15 ± 3.84, years) were randomly assigned to the following groups: a WB-EMS training group ( n = 11) and a traditional resistance training group (the control group [CG]: n = 9). Both training intervention programs consisted of 18 training sessions for six consecutive weeks. All subjects performed dynamic movements with the WB-EMS or external weights (CG). The primary outcome variables included maximum isometric elbow flexor strength (MIEFS), maximum isometric elbow extensor strength (MIEES) and surface electromyography amplitude (sEMG RMS ). Secondary outcomes involved lean body mass, body fat content, arm fat mass, and arm lean mass. ANOVAs, Friedman test and post hoc t -tests were used ( P = 0.05) to analyze the variables development after the 6-week intervention between the groups. Significant time × group interactions for MIEFS (η ² = 0.296, P Bonferroni = 0.013) were observed, the increase in the WB-EMS group were significantly superior to the CG [23.49 ± 6.48% vs. 17.01 ± 4.36%; MD (95% CI) = 6.48 (1.16, 11.80); d = 1.173, P = 0.020]. There were no significant differences were observed between interventions regarding MIEES, sEMG RMS and body composition. These findings indicate that in moderately trained males the effects of WB-EMS were similar to a traditional resistance training, with the only exception of a significantly greater increase in elbow flexor strength. WB-EMS can be considered as an effective exercise addition for moderately trained males.
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Postexercise protein feeding regulates the skeletal muscle adaptive response to endurance exercise, but the transcriptome guiding these adaptations in well-trained human skeletal muscle is uncharacterized. In a crossover design, eight cyclists ingested beverages containing protein, carbohydrate and fat (PTN: 0.4, 1.2, 0.2 g/kg, respectively) or isocaloric carbohydrate and fat (CON: 1.6, 0.2 g/kg) at 0 and 1 h following 100 min of cycling. Biopsies of the vastus lateralis were collected at 3 and 48 h following to determine the early and late transcriptome and regulatory signaling responses via microarray and immunoblot. The top gene ontology enriched by PTN were: muscle contraction, extracellular matrix--signaling and structure, and nucleoside, nucleotide, and nucleic acid metabolism (3 and 48 h); developmental processes, immunity, and defense (3 h); glycolysis, lipid and fatty acid metabolism (48 h). The transcriptome was also enriched within axonal guidance, actin cytoskeletal, Ca2+, cAMP, MAPK, and PPAR canonical pathways linking protein nutrition to exercise-stimulated signaling regulating extracellular matrix, slow-myofibril, and metabolic gene expression. At 3 h, PTN attenuated AMPKα1Thr172 phosphorylation but increased mTORC1Ser2448, rps6Ser240/244, and 4E-BP1-γ phosphorylation, suggesting increased translation initiation, while at 48 h AMPKα1Thr172 phosphorylation and PPARG and PPARGC1A expression increased, supporting the late metabolic transcriptome, relative to CON. To conclude, protein feeding following endurance exercise affects signaling associated with cell energy status and translation initiation and the transcriptome involved in skeletal muscle development, slow-myofibril remodeling, immunity and defense, and energy metabolism. Further research should determine the time course and posttranscriptional regulation of this transcriptome and the phenotype responding to chronic postexercise protein feeding.
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Minimizing the number of muscle biopsies has important methodological implications and minimizes subject discomfort during a stable isotope amino acid infusion. We aimed to determine the reliability of obtaining a single muscle biopsy for the calculation of muscle protein fractional synthetic rate (FSR) as well as the amount of incorporation time necessary to obtain that biopsy after initiating a stable isotope infusion (Study 1). The calculation of muscle protein FSR requires tracer steady-state during the stable isotope infusion. Therefore, a second aim was to examine if steady-state conditions are compromised in the precursor pools (plasma free or muscle intracellular [IC]) after ingestion of a tracer enriched protein drink and after resistance exercise (Study 2). Sixteen men (23 ± 3 years; BMI = 23.8 ± 2.2 kg/m2, means ± SD) were randomized to perform Study 1 or Study 2 (n = 8, per study). Subjects received a primed, constant infusion of L-[ring-13C6]phenylalanine coupled with muscle biopsies of the vastus lateralis to measure rates of myofibrillar protein synthesis (MPS). Subjects in Study 2 were fed 25 g of whey protein immediately after an acute bout of unilateral resistance exercise. There was no difference (P = 0.3) in rates of MPS determined using the steady-state precursor-product equation and determination of tracer incorporation between sequential biopsies 150 min apart or using plasma protein as the baseline enrichment, provided the infusion length was sufficient (230 ± 0.3 min). We also found that adding a modest amount of tracer (4% enriched), calculated based on the measured phenylalanine content of the protein (3.5%) in the drink, did not compromise steady-state conditions (slope of the enrichment curve not different from zero) in the plasma free or, more importantly, the IC pool (both P > 0.05). These data demonstrate that the single biopsy approach yields comparable rates of muscle protein synthesis, provided a longer incorporation time is utilized, to that seen with a traditional two biopsy approach. In addition, we demonstrate that enriching protein-containing drinks with tracer does not disturb isotopic steady-state and thus both are reliable techniques to determine rates of MPS in humans.
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The two most commonly used methods to analyze data from real-time, quantitative PCR experiments are absolute quantification and relative quantification. Absolute quantification determines the input copy number, usually by relating the PCR signal to a standard curve. Relative quantification relates the PCR signal of the target transcript in a treatment group to that of another sample such as an untreated control. The 2(-DeltaDeltaCr) method is a convenient way to analyze the relative changes in gene expression from real-time quantitative PCR experiments. The purpose of this report is to present the derivation, assumptions, and applications of the 2(-DeltaDeltaCr) method. In addition, we present the derivation and applications of two variations of the 2(-DeltaDeltaCr) method that may be useful in the analysis of real-time, quantitative PCR data. (C) 2001 Elsevier science.
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SUMMARY In order to stimulate further adaptation toward specific training goals, progressive resistance training (RT) protocols are necessary. The optimal characteristics of strength-specific programs include the use of concentric (CON), eccentric (ECC), and isometric muscle actions and the performance of bilateral and unilateral single- and multiple-joint exercises. In addition, it is recommended that strength programs sequence exercises to optimize the preservation of exercise intensity (large before small muscle group exercises, multiple-joint exercises before single-joint exercises, and higher-intensity before lower-intensity exercises). For novice (untrained individuals with no RT experience or who have not trained for several years) training, it is recommended that loads correspond to a repetition range of an 8-12 repetition maximum (RM). For intermediate (individuals with approximately 6 months of consistent RT experience) to advanced (individuals with years of RT experience) training, it is recommended that individuals use a wider loading range from 1 to 12 RM in a periodized fashion with eventual emphasis on heavy loading (1-6 RM) using 3- to 5-min rest periods between sets performed at a moderate contraction velocity (1-2 s CON; 1-2 s ECC). When training at a specific RM load, it is recommended that 2-10% increase in load be applied when the individual can perform the current workload for one to two repetitions over the desired number. The recommendation for training frequency is 2-3 dIwkj1 for novice training, 3-4 dIwkj1 for intermediate training, and 4-5 dIwkj1 for advanced training. Similar program designs are recom- mended for hypertrophy training with respect to exercise selection and frequency. For loading, it is recommended that loads corresponding to 1-12 RM be used in periodized fashion with emphasis on the 6-12 RM zone using 1- to 2-min rest periods between sets at a moderate velocity. Higher volume, multiple-set programs are recommended for maximizing hypertrophy. Progression in power training entails two general loading strategies: 1) strength training and 2) use of light loads (0-60% of 1 RM for lower body exercises; 30-60% of 1 RM for upper body exercises) performed at a fast contraction velocity with 3-5 min of rest between sets for multiple sets per exercise (three to five sets). It is also recommended that emphasis be placed on multiple-joint exercises especially those involving the total body. For local muscular endurance training, it is recommended that light to moderate loads (40-60% of 1 RM) be performed for high repetitions (915) using short rest periods (G90 s). In the interpretation of this position stand as with prior ones, recommendations should be applied in context and should be contingent upon an individual's target goals, physical capacity, and training