Percent Voluntary Activation Research Articles

The Effects of Short-Term Resistance Training and Subsequent Detraining on Neuromuscular Function, Muscle Cross-Sectional Area, and Lean Mass

PurposeTo examine and compare the effects of three days of dynamic constant external resistance (DCER) and isokinetic (ISOK) training and subsequent detraining on thigh muscle cross-sectional area (TMCSA) and thigh lean mass (TLM), ISOK peak torque (PT), DCER strength, isometric force, muscle activation, and percent voluntary activation (%VA).MethodsThirty-one apparently-healthy untrained men (mean ± SD age = 22.2 ± 4.2 years; body mass = 77.9 ± 12.9 kg; height = 173.9 ± 5.4 cm) were randomly assigned to a DCER training group (n = 11), ISOK training group (n = 10) or control (CONT) group (n = 10). Subjects visited the laboratory eight times. The first visit was a familiarization session, the second visit was a pre-training assessment, the subsequent three visits were for unilateral training of the quadriceps (if assigned to a training group), and the last three visits were the post-training assessments conducted at three days, one week, and two weeks after training ended.ResultsDCER strength increased from pre- to post-training assessment 1 in both limbs for the DCER group only, and remained elevated during post-training assessments 2 and 3 (P < 0.05). In addition, surface EMG for the biceps femoris was higher at post-training assessment 3 than at the pre-training assessment, and post-training assessments 1 and 2 (P < 0.05). No other training-related changes were found.ConclusionThe primary finding of this study was that DCER strength of the trained and untrained limbs can be increased with three days of training. This has important implications for injury rehabilitation, where in the initial period post-injury strength gains on an injured limb can possibly be obtained with short-term contralateral resistance training.

Comparisons of muscle strength, size, and voluntary activation in pre- and post-pubescent males and females.

The purpose of this study was to compare measurements of muscle strength, size, and activation of the forearm flexors in pre- and post-pubescent males and females. Forty pre-pubescent (mean ± 95% confidence interval, age = 9.79 ± 0.35years, n = 10 males, n = 10 females) and post-pubescent (age = 17.23 ± 0.58years, n = 10 males, n = 10 females) youth participated. Subjects completed maximal voluntary isometric contractions (MVICs) of the forearm flexors, and submaximal isometric step muscle actions at 30, 50, and 70% of the peak MVIC. Percent voluntary activation (VA) was quantified during all isometric muscle actions. Forearm flexor (biceps brachii and brachialis) muscle cross-sectional area (CSA) was quantified from ultrasound images. MVIC strength was expressed in absolute terms and normalized to CSA. Post-pubertal males were 130% stronger, had 101% greater CSA, and 17% greater maximal VA than pre-pubertal males, while post-pubertal females were 72% stronger, had 54% greater CSA, and 23% greater maximal VA than pre-pubertal females. When MVIC strength was normalized to CSA, the post-pubertal males were still 15% stronger than the pre-pubertal males, while the post-pubertal females were only 12% stronger than the pre-pubertal females. The responses for VA across intensity reflected differences in muscle activation strategies between pre- and post-pubertal males and females. These results suggest that muscle size may account for a greater proportion of the growth and development-related differences in strength among males, while females may be more affected by changes in muscle activation. Regardless of sex, changes in muscle size and neuromuscular function influence strength increases during growth and development.

No acute effects of placebo or open-label placebo treatments on strength, voluntary activation, and neuromuscular fatigue.

Recent evidence suggests that deception may not be necessary for placebos to improve clinical outcomes. We tested the hypothesis that placebo and open-label placebo (OLP) treatments would acutely improve strength and voluntary activation, as well as minimize neuromuscular fatigue, in untrained participants. Twenty-one males (n = 11) and females (n = 10) visited the laboratory on three occasions (placebo, OLP, control) to receive each treatment in a randomized, counter-balanced manner. Trials involved a pretest, a 15-min intervention, and posttests. For the placebo trial, participants were informed that they would be ingesting a capsule that would improve their performance and make them feel more energetic. For the OLP intervention, participants were told that the capsules would have no effects. In "Experiment #1", knee extensor maximal voluntary contraction (MVC) peak torque and percent voluntary activation were evaluated. In "Experiment #2", participants performed 20 consecutive MVCs while surface electromyographic signals were detected from the vastus lateralis. Subjective assessments of energy and perceived exertion were examined. The interventions had no effect on strength or voluntary activation, but energy levels increased following treatments (p = 0.016, η2 = 0.257). Neither treatment influenced neuromuscular fatigue. Though some variables showed moderate-to-large effect sizes, these results were consistent for individuals with lower voluntary activation. Placebo and OLP treatments had minimal influence on strength, voluntary activation, and fatigue resistance. As these findings differ from recent reports, we speculate that placebos and OLPs are more likely to enhance muscle function in patient populations seeking medical care.

Greater Neural Adaptations following High- vs. Low-Load Resistance Training.

We examined the neuromuscular adaptations following 3 and 6 weeks of 80 vs. 30% one repetition maximum (1RM) resistance training to failure in the leg extensors. Twenty-six men (age = 23.1 ± 4.7 years) were randomly assigned to a high- (80% 1RM; n = 13) or low-load (30% 1RM; n = 13) resistance training group and completed leg extension resistance training to failure 3 times per week for 6 weeks. Testing was completed at baseline, 3, and 6 weeks of training. During each testing session, ultrasound muscle thickness and echo intensity, 1RM strength, maximal voluntary isometric contraction (MVIC) strength, and contractile properties of the quadriceps femoris were measured. Percent voluntary activation (VA) and electromyographic (EMG) amplitude were measured during MVIC, and during randomly ordered isometric step muscle actions at 10–100% of baseline MVIC. There were similar increases in muscle thickness from Baseline to Week 3 and 6 in the 80 and 30% 1RM groups. However, both 1RM and MVIC strength increased from Baseline to Week 3 and 6 to a greater degree in the 80% than 30% 1RM group. VA during MVIC was also greater in the 80 vs. 30% 1RM group at Week 6, and only training at 80% 1RM elicited a significant increase in EMG amplitude during MVIC. The peak twitch torque to MVIC ratio was also significantly reduced in the 80%, but not 30% 1RM group, at Week 3 and 6. Finally, VA and EMG amplitude were reduced during submaximal torque production as a result of training at 80% 1RM, but not 30% 1RM. Despite eliciting similar hypertrophy, 80% 1RM improved muscle strength more than 30% 1RM, and was accompanied by increases in VA and EMG amplitude during maximal force production. Furthermore, training at 80% 1RM resulted in a decreased neural cost to produce the same relative submaximal torques after training, whereas training at 30% 1RM did not. Therefore, our data suggest that high-load training results in greater neural adaptations that may explain the disparate increases in muscle strength despite similar hypertrophy following high- and low-load training programs.

Neuromuscular responses of the plantar flexors to whole-body vibration.

Enhanced physical performance following whole-body vibration (WBV) has been attributed to increased muscle activity; however, few studies have measured the mechanisms underlying these changes. The objective of this study was to measure the responsiveness of the Ia pathway as well as contractile properties in 16 young adults (24±2years, eight men, eight women) following repeated bouts of acute WBV (45Hz, 2mm). Hoffman reflexes (H-reflex), compound muscle action potentials (M-wave), and twitch contractile properties were measured prior to and immediately following five 1-minute WBV exposures, and at 3, 5, 10, and 20minute post-WBV. M-wave and H-reflex amplitudes decreased by 8% (P<.001) and by 46% (P<.05), respectively, whereas peak twitch torque decreased by 9% (P<.01) and rate of twitch torque development slowed 8% (P<.05). Percent voluntary activation and maximal plantar flexor torque were unchanged as a consequence of WBV (P>.05). In response to acute WBV, the root mean square of the soleus electromyography signal (EMGRMS ) increased by 8%, while the EMGRMS of the lateral gastrocnemius increased by 3% (P<.05). These data indicate that the responsiveness of the Ia pathway is diminished and contractile function is impaired immediately following WBV, and that the neural mechanisms underlying improved performance following WBV lie in alternative hypotheses possibly involving spindle disfacilitation or Golgi afferent modulation.

Neuromuscular Adaptations After 2 and 4 Weeks of 80% Versus 30% 1 Repetition Maximum Resistance Training to Failure.

Jenkins, NDM, Housh, TJ, Buckner, SL, Bergstrom, HC, Cochrane, KC, Hill, EC, Smith, CM, Schmidt, RJ, Johnson, GO, and Cramer, JT. Neuromuscular adaptations after 2 and 4 weeks of 80% versus 30% 1 repetition maximum resistance training to failure. J Strength Cond Res 30(8): 2174-2185, 2016-The purpose of this study was to investigate the hypertrophic, strength, and neuromuscular adaptations to 2 and 4 weeks of resistance training at 80 vs. 30% 1 repetition maximum (1RM) in untrained men. Fifteen untrained men (mean ± SD; age = 21.7 ± 2.4 years; weight = 84.7 ± 23.5 kg) were randomly assigned to either a high-load (n = 7) or low-load (n = 8) resistance training group and completed forearm flexion resistance training to failure 3 times per week for 4 weeks. Forearm flexor muscle thickness (MT) and echo intensity, maximal voluntary isometric (MVIC) and 1RM strength, and the electromyographic, mechanomyographic (MMG), and percent voluntary activation (%VA) responses at 10-100% of MVIC were determined at baseline, 2, and 4 weeks of training. The MT increased from baseline (2.9 ± 0.1 cm) to week 2 (3.0 ± 0.1 cm) and to week 4 (3.1 ± 0.1 cm) for the 80 and 30% 1RM groups. MVIC increased from week 2 (121.5 ± 19.1 Nm) to week 4 (138.6 ± 22.1 Nm) and 1RM increased from baseline (16.7 ± 1.6 kg) to weeks 2 and 4 (19.2 ± 1.9 and 20.5 ± 1.8 kg) in the 80% 1RM group only. The MMG amplitude at 80 and 90% MVIC decreased from baseline to week 4, and %VA increased at 20 and 30% MVIC for both groups. Resistance training to failure at 80 vs. 30% 1RM elicited similar muscle hypertrophy, but only 80% 1RM increased muscle strength. However, these disparate strength adaptations were difficult to explain with neuromuscular adaptations because they were subtle and similar for the 80 and 30% 1RM groups.

Anatomical and neuromuscular variables strongly predict maximum knee extension torque in healthy men.

This study examined the relative influence of anatomical and neuromuscular variables on maximal isometric and concentric knee extensor torque and provided a comparative dataset for healthy young males. Quadriceps cross-sectional area (CSA) and fascicle length (l f) and angle (θ f) from the four quadriceps components; agonist (EMG:M) and antagonist muscle activity, and percent voluntary activation (%VA); patellar tendon moment arm distance (MA) and maximal voluntary isometric and concentric (60°s(-1)) torques, were measured in 56men. Linear regression models predicting maximum torque were ranked using Akaike's Information Criterion (AICc), and Pearson's correlation coefficients assessed relationships between variables. The best-fit models explained up to 72% of the variance in maximal voluntary knee extension torque. The combination of 'CSA+θ f+EMG:M+%VA' best predicted maximum isometric torque (R (2)=72%, AICc weight=0.38) and 'CSA+θ f+MA' (R (2)=65%, AICc weight=0.21) best predicted maximum concentric torque. Proximal quadriceps CSA was included in all models rather than the traditionally used mid-muscle CSA. Fascicle angle appeared consistently in all models despite its weak correlation with maximum torque in isolation, emphasising the importance of examining interactions among variables. While muscle activity was important for torque prediction in both contraction modes, MA only strongly influenced maximal concentric torque. These models identify the main sources of inter-individual differences strongly influencing maximal knee extension torque production in healthy men. The comparative dataset allows the identification of potential variables to target (i.e. weaknesses) in individuals.

Contributions of voluntary activation deficits to hand weakness after stroke

Background: Hemiparetic stroke survivors often exhibit profound weakness in the digits of the paretic hand, but the relative contribution of potential biomechanical and neurological impairment mechanisms is not known. Establishing sources of impairment would help in guiding treatment.Objective: The present study sought to quantify the role of diminished capacity to voluntarily active finger flexor and extensor muscles as one possible neurological mechanism.Methods: Two groups of stroke survivors with “severe” (N = 9) or “moderate” (N = 9) hand impairment and one group of neurologically intact individuals (N = 9) participated. Subjects were asked to create isometric flexion force and extension force, respectively, with the tip of the middle finger. The maximum voluntary force (MVF) and the maximum stimulated force (MSF) produced by an applied train of electrical current pulses (MSF) were recorded for flexion and extension. Percent voluntary activation (PVA) was computed from MVF and MSF.Results: Significant deficits in both MVF and PVA were observed for stroke subjects compared to control subjects. For example, activation deficits were >80% for extensor digitorum communis (EDC) for the “severe” group. Maximum voluntary force and PVA deficits were greater for EDC than for flexor digitorum superficialis (FDS) for stroke subjects with severe impairment. Maximum voluntary force and PVA correlated significantly for stroke subjects but not for control subjects.Conclusions: Although extrinsic finger muscles could be successfully recruited electrically, voluntary excitation of these muscles was substantially limited in stroke survivors. Thus, finger weakness after stroke results predominantly from the inability to fully activate the muscle voluntarily.

Central Nervous System Fatigue During Upper-body Maximal Intensity, Intermittent Sprints.

PURPOSE: To investigate central nervous system fatigue by assessing 1) neuromuscular fatigue of the elbow flexors and 2) corticospinal excitability, during upper-body maximal intensity intermittent-sprints (IS). METHODS: 19 participants completed (Part A) 10 trials of 10s sprints on an arm cycle ergometer interspersed with 180s of recovery. Mean power was recorded for each IS. Elbow flexors maximum voluntary contraction (MVC) force and percent voluntary activation were recorded prior to the first IS and immediately after IS 5 and 10. 7 participants completed (Part B) the same sprint protocol, and we used transcranial magnetic stimulation, transmastoid electrical stimulation and brachial plexus electrical stimulation to produce motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs) and maximal M-waves (Mmax) in the biceps brachii during a 5% MVC elbow flexion contraction. All MEPs and CMEPs were normalized to Mmax. RESULTS: Part A: There was clear evidence of fatigue induced by the sprint protocol. Mean power decreased by 11.5% (p = 0.002), and 8.4% (p < 0.001) from IS 1 to 10 and IS 5 to 10, respectively. MVC force decreased by 7.2% (p = 0.003), 12.9% (p = 0.001), and 6.0% (p = 0.01) from pre-IS to post-IS 5, pre-IS to post-IS 10, and post-IS 5 to post-IS 10, respectively. Percent voluntary activation decreased by 1.85% (p = 0.027) and 3.92% (p = 0.042) at post-IS 5 and 10, respectively, compared to pre-IS. Part B: External manifestations of fatigue were matched by decreased supraspinal excitability compared to pre-IS. MEP amplitudes decreased by 28.7% (p = 0.002) and 42.2% (p = 0.002) at post-IS 1 and 5. Despite that, CMEP amplitudes did not differ (p > 0.142) at any time point from pre-IS. MEP/CMEP ratios decreased by 29.8% (p < 0.001) and 31.6% (p = 0.015) at post-IS 5 and 10, respectively compared to pre-IS. Mmax amplitudes did not differ (p > 0.264) at any time point. CONCLUSION: The IS protocol produced a decrease in power, force and percent voluntary activation, illustrating neuromuscular fatigue of the elbow flexors. The differential effect on MEPs and CMEPs suggests that the fatigue may largely be attributed to decreases in supraspinal drive that limits motor output during IS.

Acute effects of passive stretching of the plantarflexor muscles on neuromuscular function: the influence of age.

The acute effects of stretching on peak force (Fpeak), percent voluntary activation (%VA), electromyographic (EMG) amplitude, maximum range of motion (MROM), peak passive torque, the passive resistance to stretch, and the percentage of ROM at EMG onset (%EMGonset) were examined in 18 young and 19 old men. Participants performed a MROM assessment and a maximal voluntary contraction of the plantarflexors before and immediately after 20 min of passive stretching. Fpeak (-11 %), %VA (-6 %), and MG EMG amplitude (-9 %) decreased after stretching in the young, but not the old (P > 0.05). Changes in Fpeak were related to reductions in all muscle activation variables (r = 0.56-0.75), but unrelated to changes in the passive resistance to stretch (P ≥ 0.24). Both groups experienced increases in MROM and peak passive torque and decreases in the passive resistance to stretch. However, the old men experienced greater changes in MROM (P < 0.001) and passive resistance (P = 0.02-0.06). Changes in MROM were correlated to increases in peak passive torque (r = 0.717), and the old men also experienced a nonsignificant greater (P = 0.08) increase in peak passive torque. %EMGonset did not change from pre- to post-stretching for both groups (P = 0.213), but occurred earlier in the old (P = 0.06). The stretching-induced impairments in strength and activation in the young but not the old men may suggest that the neural impairments following stretching are gamma-loop-mediated. In addition, the augmented changes in MROM and passive torque and the lack of change in %EMGonset for the old men may be a result of age-related changes in muscle-tendon behavior.

The Effect of Creatine Loading on Neuromuscular Fatigue in Women

This study aimed to examine the effects of intermittent isometric fatigue on maximal voluntary contraction (MVC) strength, percent voluntary activation (%VA), peak twitch force (PTF), peak rate of force development (PRFD), half relaxation time (HRT), and maximal compound action potential (M-wave) amplitude of the soleus and medial gastrocnemius muscles before and after creatine (Cr) loading. Using a double-blinded, placebo-controlled, randomized design, 12 women were assigned to a Cr (n = 6; mean age ± SD = 23.3 ± 3.0 yr) or placebo (PL; n = 6; mean age ± SD = 21.3 ± 1.6 yr) group. Participants supplemented four times daily for 5 d with 5 g of Cr + 10 g of fructose or 10 g of fructose. At baseline and after testing, an isometric MVC and the twitch interpolation procedure were used before and after a 4-min isometric fatigue protocol of the plantarflexor muscles, which consisted of six intermittent duty cycles per minute (7-s contraction, 3-s relaxation) at 70% MVC. There were no interactions between the Cr and PL groups (P > 0.05) for any dependent variable. The fatigue protocol reduced voluntary strength (-17.8%, P < 0.001) and %VA (-3.7%, P = 0.005). Baseline PTF (P < 0.005) and PRFD (P < 0.001) values were less than those of all respective time points, but PTF value decreased from 3 min to 4 min and after testing (P < 0.005). HRT increased from baseline to minutes 1 and 2 and then returned to baseline at minutes 3 and 4 and after testing. The M-wave did not change (P > 0.05). Five days of Cr loading did not influence isometric force, %VA, evoked twitch properties, or the central and peripheral aspects of fatigue measured in this study.

The Reliability of the Interpolated Twitch Technique During Submaximal and Maximal Isometric Muscle Actions

The reliability of the interpolated twitch technique during submaximal and maximal isometric muscle actions. J Strength Cond Res 27(10): 2909-2913, 2013-The purpose of this study was to examine the test-retest reliability of the percent voluntary activation (%VA) vs. force relationships. Fourteen healthy men (mean ± SD age = 21 ± 2.6 years) and 8 women (age = 21 ± 1.8 years) completed 4 maximal voluntary contractions (MVCs) and 9 randomly ordered submaximal isometric plantar flexions from 10 to 100% of the MVC. Transcutaneous electrical stimuli were delivered to the tibial nerve using a high-voltage constant-current stimulator (DS7AH; Digitimer, Herthfordshire, United Kingdom). The %VA was calculated for each maximal and submaximal MVC. Paired-samples t-tests were used to quantify systematic variability, whereas the intraclass correlation coefficients (ICCs), standard error of the mean (%SEM), and minimum differences (%MD; expressed as a percentage of the means) were used for test-retest reliability. Systematic variability was not present at any of the contraction intensities (p > 0.05). The ICCs ranged from 0.52 to 0.84, whereas the %SEM ranged from 6.75 to 38.45%, and the %MD ranged from 18.71 to 106.58%. The ICCs were ≥0.74 at contraction intensities ranging from 40 to 100% MVC (6.75-16.78% SEM), whereas the ICCs were ≤0.65 (20.95-38.45% SEM) for the contraction intensities ≤30% MVC. Although not statistically tested, the ICCs tended to be higher, whereas the %SEMs lower for contractions ≥40% MVC. Future research using %VA during submaximal contraction intensities to predict a true maximal force may want to exclude contraction intensities <40% MVC. In addition, caution is warranted when interpreting the changes in the %VA during MVCs after an experimental intervention.

Contribution of central vs. peripheral factors to the force loss induced by passive stretch of the human plantar flexors

The purpose of the present research was to identify the contribution of central vs. peripheral factors to the force loss after passive muscle stretching. Thirteen men randomly performed both a 5-min constant-torque stretch of the plantar flexors on an isokinetic dynamometer and a resting condition on 2 separate days. The triceps surae electromyogram (EMG) was recorded simultaneously with plantar flexor isometric torque. Measures of central drive, including the EMG amplitude normalized to the muscle compound action potential amplitude (EMG/M), percent voluntary activation and first volitional wave amplitude, and measures of peripheral function, including the twitch peak torque, 20-to-80-Hz tetanic torque ratio and torque during 20-Hz stimulation preceded by a doublet, were taken before and immediately and 15 min after each condition. Peak torque (-15.7%), EMG/M (-8.2%), and both twitch (-9.4%) and 20-Hz peak torques (-11.5%) were reduced immediately after stretch but recovered by 15 min. There were strong correlations between the torque loss and the reductions in central drive parameters (r = 0.65-0.93). Torque recovery was also strongly correlated with the recovery in EMG/M and percent voluntary activation (r = 0.77-0.81). The moderate decreases in measures of peripheral function were not related to the torque loss or recovery. These results suggest that 1) central factors were strongly related to the torque reduction immediately after stretch and during torque recovery; and 2) the muscle's contractile capacity was moderately reduced, although these changes were not associated with the torque reduction, and changes in excitation-contraction coupling efficiency were not observed.

Electromyographic, but not mechanomyographic amplitude–force relationships, distinguished differences in voluntary activation capabilities between individuals

The purpose of the present study was to examine the influence of activation capabilities on the electromyography (EMGRMS) and mechanomyography amplitude (MMGRMS)–force relationships of the vastus lateralis (VL) and rectus femoris (RF). Thirteen men (mean±SD; age=22±3 year) performed nine submaximal contractions (10–90% maximal voluntary contraction [MVC]) with the interpolated twitch technique performed during a separate contraction at 90% MVC to calculate percent voluntary activation (%VA). Nine participants with >90% VA were categorized into the high-activated group with the remaining categorized into the moderate-activated group. Slopes (b terms) were calculated from the log-transformed EMGRMS and MMGRMS–force relationships. The b terms (collapsed across the VL and RF) for the EMGRMS–force relationships were greater for the high- (1.29±0.31) than the moderate-activated (1.10±0.20) group. In contrast, there were no differences in the b terms for the MMGRMS–force relationships between the high- and moderate-activated groups. For the EMGRMS and MMGRMS–force relationships, the b terms were greater for the RF (1.38±0.30, 0.81±0.20) than the VL (1.08±0.19, 0.60±0.13) collapsed across groups. The b terms from the EMGRMS–force relationships, but not the MMGRMS–force relationships, reflected differences in %VA.

Differences in the log-transformed electromyographic–force relationships of the plantar flexors between high- and moderate-activated subjects

The present study examined the log-transformed electromyographic amplitude (EMG) versus force relationships for the medial gastrocnemius (MG) and soleus (SOL) in high- and moderate-activated subjects. Twenty-five (age = 21 ± 2 year; mass = 62 ± 12 kg) participants performed six submaximal contractions (30–90% maximal voluntary contraction [MVC]) with the interpolated twitch technique (ITT) performed at 90% MVC to calculate percent voluntary activation (% VA). Sixteen participants with > 90% VA at 90% MVC were categorized high-activated group; the remaining nine were the moderate-activated group. Linear regression models were fit to the log-transformed EMG–force relationships. The slope ( b value) and the antilog of the Y-intercept ( a value) were calculated. The b values from the MG EMG–force relationships were higher ( P < 0.05) for the high-activated group (1.27 ± 0.13) than the moderate-activated group (0.88 ± 0.06). The a values and p–p M-wave amplitude values (collapsed across twitches [superimposed and potentiated]) were larger ( P < 0.05) for the MG (1.17 ± 0.40 and 8.98 ± 0.46 mV) than the SOL (0.24 ± 0.07 and 4.48 ± 0.20 mV) when collapsed across groups. The b value from the log-transformed EMG–force relationships is an attractive model to determine if a subject has the ability to achieve high activation of their MG without muscle or nerve stimulation.

The Effect of Creatine Loading on Neuromuscular Function

Although the existing body of creatine (Cr) literature is robust and supports its role as a metabolic ergogenic aid, a limited number of studies have examined the effects of Cr supplementation on neuromuscular function. PURPOSE: The purpose of the present study was to examine the effects of fatigue on voluntary peak force (PF), percent voluntary activation (%VA), peak twitch force (PTF), peak twitch rate of force development (RFD), and electromyographic (EMG) amplitude of the soleus (SOL) and medial gastrocneimus (MG) muscles prior to and following short term creatine loading. METHODS: Using a double-blind, placebo-controlled, randomized design, 29 moderately trained participants were assigned to a creatine (Cr; n = 16, age: 21.9 ± 2.6 yrs, stature: 174.6 ± 10.9 cm, body mass: 73.0 ± 10.9 kg) or placebo (Pl; n = 13, age: 22.2 ± 3.5 yrs, stature: 175.7 ± 9.5 cm; body mass: 71.9 ± 14.2 kg) group. Subjects supplemented their diet four times daily for five days with 5g of Cr + 15g dextrose (Cr) or 20g dextrose (Pl). At baseline (week 1) and post-testing (week 2) all subjects performed an isometric maximal voluntary contraction (MVC) and the twitch interpolation technique prior to and immediately following a four minute fatigue protocol of the plantar flexor muscles on a custom-built load cell apparatus attached to a calibrated isokinetic dynamometer. The fatiguing protocol included four sets of 10 intermittent isometric contractions (7-sec on / 3-sec off) at 70% of MVC. Additionally, a familiarization session was completed prior to baseline measurements. Separate three-way mixed factorial ANOVAs [acute (pre vs. post) x chronic (week 1 vs. week 2) x treatment (Cr vs. Pl)] were used to analyze the PF, %VA, PTF and RFD. A four-way mixed factorial ANOVA [muscle (SOL vs. MG) x acute (pre vs. post) x chronic (week 1 vs. week 2) x treatment (Cr vs. Pl)] was used to analyze EMG values. RESULTS: The acute bout of fatiguing contractions resulted in a significant main effect for acute resulting in a loss in voluntary strength (–15.7%, P < 0.001), %VA (−7.3%, P = 0.003) and an increase in RFD (4.7%, P = 0.037) for both groups. There was a significant chronic × treatment interaction for MVC. The chronic effects of Cr supplementation from week 1 to week 2 demonstrated a maintenance in voluntary strength (MVC) for the Cr group (−0.5%, P = 0.710) while the PL group showed a significant decline in strength (−5.8%, P = 0.035). There was also a significant chronic × treatment interaction for the evoked twitch properties, resulting in an increase in PTF (5.7%, P = 0.019) and a non-significant increase in RFD (4.6%, P = 0.071) for the Cr group only. In addition, there was a main effect for treatment for %VA where the Cr group was 7.3% more active than the PL group across all time points (P = 0.023). Furthermore, there was a main effect for chronic where EMG amplitude was higher (20.0%, P = 0.023) for both groups at week 2 compared to week 1. CONCLUSIONS: These findings indicated that chronic Cr supplementation preserved maximal voluntary strength and enhanced evoked twitch properties. However, there were no acute differences between groups before and after the fatiguing protocol. PRACTICAL APPLICATIONS: Cr supplementation enhanced maximal contractile properties, but had a limited influence during a fatiguing bout of exercise in the present study. The ability to maintain maximal force between training sessions may lead to practical increases in strength.

Acute effects of passive stretching vs vibration on the neuromuscular function of the plantar flexors

This study examined the acute effects of passive stretching (PS) vs prolonged vibration (VIB) on voluntary peak torque (PT), percent voluntary activation (%VA), peak twitch torque (PTT), passive range of motion (PROM), musculotendinous stiffness (MTS), and surface electromyographic (EMG) and mechanomyographic (MMG) amplitude of the medial gastrocnemius (MG) and soleus (SOL) muscles during isometric maximal voluntary contractions (MVCs) of the plantar flexors. Fifteen healthy men performed the isometric MVCs and PROM assessments before and after 20 min of PS, VIB, and a control (CON) conditions. There were 10% and 5% decreases in voluntary PT, non-significant 3% and 2% decreases in %VA, 9-23% decreases in EMG amplitude of the MG and SOL after the PS and VIB, respectively, with no changes after the CON. PROM increased by 19% and MTS decreased by 38% after the PS, but neither changed after the VIB or CON conditions. Both PS and VIB elicited similar neural deficits (i.e., gamma loop impairment) that may have been responsible for the strength losses. However, mechanical factors related to PROM and MTS cannot be ruled out as contributors to the stretching-induced force deficit.

RESPONSE

Dear Editor-in-Chief: First, we would like to thank you for allowing us to respond to Dr. Knudson's comments. Second, we wish to thank Dr. Knudson for his interest in our paper and acknowledge our respect for his contributions to the literature on this topic (6-8). Dr. Knudson's first concern was that we did not acknowledge his previous study (7), two studies that were published while ours was in press (5,10), an abstract (9), and a paper by Brandenburg (1). However, none of the studies cited by Dr. Knudson (1,5,7,9,10) involved the plantar flexors or the twitch interpolation technique; therefore, these studies were not directly related to our experimental approach. Dr. Knudson's second concern was that our study was underpowered because of a sample size of n = 13. Our observed statistical power values for the omnibus F statistics in the ANOVA models for each of our dependent variables (peak torque, range of motion, peak twitch torque, rate of torque development, EMG amplitude, and percent voluntary activation) were 0.75, 1.00, 0.88, 0.95, 0.76, and 0.65, respectively. We would consider these power values acceptable and high enough to detect a statistical difference if there was one. Furthermore, the goal of our study was to extend the findings of Fowles et al. (2) and other studies (4,13) on the plantar flexors, which each had sample sizes of n = 10, n = 15, and n = 15, respectively. We believe our sample size of n = 13 was consistent with these studies. Dr. Knudson's third concern was his recommendation to correct for Type I error inflation. Such statistical adjustments of the alpha level, such as the Bonferroni correction, are typically reserved for multiple ANOVA or t-tests on a single dependent variable (12). We used the highest-order ANOVA, which was also the most powerful univariate ANOVA model (12) that was possible for each of our six dependent variables. Therefore, we would argue that no such correction was necessary. In addition, when reducing the alpha level to widen the confidence interval as suggested by Dr. Knudson, there is an increased risk of a Type II error (3), which occurs when the study fails to detect differences that really do exist (12). Therefore, our 95% confidence interval (α = 0.05) combined with the observed statistical power values in the present study suggested that if there was a significant stretching-induced force deficit beyond the control condition, we would have detected it. In conclusion, we agree with Dr. Knudson's suggestion that not only our study (11) but also all original research studies should be evaluated with caution.

Effects of Vascular Restriction on Muscular Function during Intermittent Submaximal Isometric Exercise

It is generally accepted that a threshold intensity of 65% 1-RM is needed to induce muscle hypertrophy; however, recent studies using low-intensity resistance exercise (20% 1-RM) combined with vascular restriction (KAATSU) have also been able to demonstrate increases in muscle size and strength. PURPOSE: To investigate EMG and MMG responses of the vastus lateralis (VL) before, during, and after low intensity intermittent isometric exercise in combination with moderate vascular restriction and to examine the percent voluntary activation (PVA) of the VL. METHODS: Twelve healthy males volunteered to participate in this study (mean + SD age = 23.7±4.1 years). Each participant visited the laboratory 3 times: one familiarization trial and two experimental trials, separated by at least 48 h. The two experimental trials consisted of the same testing and isometric exercise protocol; however, the participants experienced either the KAATSU or no-KAATSU conditions in random order. For the experimental trials, the following procedures were performed in order: a) Resting blood pressure assessment after 5 min rest (for the KAATSU trial only), b) five-minute warm-up on a stationary cycle ergometer with a power output of 50 W and a pedaling cadence of 50–70 rpm, c) two pre-exercise 5-s isometric MVCs with 1 min rest between trials, d) five sets of 20 intermittent isometric contractions (2-s on and 1 -s off) at 20% of MVC with a 30-s interset rest period, and e) two post-exercise isometric 5-s MVCs with 1 min rest between trials. RESULTS: For MVC torque, PVA, EMG amplitude, EMG mean power frequency (MPF), MMG amplitude, and MMG MPF, there was no interaction and no main effects for time or session for the pre- and post-exercise isometric MVCs with and without KAATSU. The average normalized EMG amplitude (%MVC) increased from repetitions 1–4 to 5–8 to 9–12 and MMG amplitude increased significantly from set 1 to 2 for both the KAATSU and no-KAATSU sessions. CONCLUSIONS: The present study demonstrated no significant change in post-exercise torque values and % activation, suggesting that the exercise task was not intense enough to cause muscular fatigue in either treatment condition.

Time Course for the Acute Effects of Passive Stretching on Isometric Strength and Neuromuscular Function

PURPOSE: To examine the time course for the acute effects of eight min of passive stretching on isometric peak torque (PT), percent voluntary activation (%VA), electromyography (EMG), and mechanomygraphy (MMG) of the plantarflexor muscles. METHODS: Fourteen (mean age ± SD = 22 ± 3 yr) subjects performed sixteen consecutive 30-s passive stretches (8 min of time under stretch) on an active isokinetic dynamometer. PT, %VA, EMG, and MMG amplitude were assessed before, immediately after, and at 10, 20, and 30 min post-stretching. Torque (Nm) values were sampled from the dynamometer, while normalized EMG and MMG amplitude values were recorded from the soleus and medial gastrocnemius muscles. %VA was assessed by tibial nerve stimulation and the twitch interpolation technique. RESULTS: There was a 9% decrease (P<0.05) in PT immediately after the stretching. PT remained non-significantly (P>0.05) depressed at 10 (5%), 20 (3%), and 30 (6%) min post-stretching. Normalized MMG amplitude was unchanged immediately after the stretching, but progressively increased at 10 (20%, P>0.05), 20 (51%, P<0.05), and 30 min (40%, P<0.05) post-stretching. %VA and normalized EMG amplitude were unaltered (P>0.05). CONCLUSIONS: Eight min of passive stretching reduced plantarflexor strength immediately after the stretching, however, most of the force deficit had recovered after 10 min. Interestingly, the stretching-induced force deficit was not accompanied by decreases in muscle activation (%VA or EMG amplitude) or changes in MMG amplitude. However, the delayed increase in MMG amplitude may have reflected increases in active musculotendinous compliance, which may have been masked by inflammation or swelling shortly after the stretching protocol. Therefore, these findings suggested that decreases in plantarflexor strength after only 8 min of passive stretching may be attributed to increases in musculotendinous compliance, rather than decreases in muscle activation.