Effects of endurance training on the maximal voluntary activation level of the knee extensor muscles

Purpose

The aim of this study was to investigate the neural adaptations to endurance training, and more specifically the adaptation of the cortical voluntary activation of the knee extensor (KE) muscles.

Methods

Sixteen sedentary men were randomly allocated into an endurance training (n = 8) or a control group (n = 8). All subjects performed a maximal aerobic speed test (MAS) before and immediately after the training period. Training lasted 8 weeks and was based on endurance running. During Pre- and Post-training testing sessions, maximal voluntary contraction (MVC) was measured and voluntary activation (VA) was calculated via peripheral nerve (PNS) and transcranial magnetic stimulations (TMS) superimposed to MVC. Electromyographic activity (EMG) of the KE muscles was also measured during MVC, PNS (M-wave) and TMS (motor evoked potentials—MEP). The cortical silent period following TMS was also assessed.

Results

Despite a significant improvement in endurance running performance, as suggested by the increase of MAS in the training group (Pre 15.4 ± 1.6 vs. Post 16.4 ± 1.6 km·h^?1), endurance training did not affect MVC or VA as measured with PNS and TMS. Similarly, the EMG of KE muscles during MVC did not show any significant changes. Furthermore, the MEP amplitude and the duration of the silent period also remained unchanged after endurance training.

Conclusions

The present study suggests an 8-week endurance-training program does not generate adaptations of neural factors in sedentary subjects.     

Neuromuscular adaptations following 12-week maximal voluntary co-contraction training

Purpose

The present study examined neuromuscular adaptations following 12-week maximal voluntary co-contraction training.

Methods

Sixteen young men were allocated to training (TG, n = 9) or control (CG, n = 7) group. TG conducted a training program (3 days/week), which consisted of 4-s maximal voluntary contractions of elbow flexors and extensors by simultaneously contracting both muscle groups at 90° of the elbow joint, followed by 4-s muscle relaxation (10 repetitions/set, 5 sets/day) for 12 weeks. In addition to the muscle thicknesses of elbow flexors and extensors, the torque and electromyograms (EMGs) of the two muscle groups during isometric maximal voluntary contraction (MVC) were determined before (Pre), after 4 weeks, and 12 weeks of intervention.

Results

After intervention, CG showed no significant changes in all measured variables. In TG, MVC torque significantly increased in both elbow flexors (+13 % at 4 weeks and +15 % at 12 weeks) and extensors (+27 % at 4 weeks and +46 % at 12 weeks) from Pre. Muscle thickness also significantly increased in both elbow flexors (+4 %) and extensors (+4 %) at 12 weeks. Agonist EMG activities during MVC significantly increased in both elbow flexors (+31 % at 4 weeks and +44 % at 12 weeks) and extensors (+27 % at 4 weeks and +40 % at 12 weeks), without changes in antagonist involuntary coactivation level in both muscle groups.

Conclusion

These results indicate that maximal voluntary co-contraction is applicable as a training modality for increasing the size and strength of antagonistic muscle pairs without increasing involuntary coactivation level.     

Prepubescent males are less susceptible to neuromuscular fatigue following resistance exercise

Purpose

To determine if prepubescent and adult males have similar fatigue profiles following high and lower intensity knee extensions.

Methods

Ten male children and ten adults completed two sessions of three sets of high repetition (17 typical muscle endurance training) high repetition (High RM) or low repetition (seven typical strength training) maximum (Low RM) dynamic knee extensions. Voluntary and evoked contractile properties, heart rate (HR), and rating of perceived exertion (RPE) were assessed before and after each knee extension RM.

Results

Knee extension RM measures revealed that boys performed more (children set 2, 6.7 ± 0.5; set 3, 5.7 ± 0.5 vs. adult set 2, 5.2 ± 0.4; set 3, 3.5 ± 0.5; P < 0.001) repetitions, had a faster (children 39.9 ± 8.6 vs. adult 9.4 ± 3.7 bpm; P < 0.001) HR recovery and lower (6.4 ± 0.43; P < 0.001) RPE compared to adults (8.0 ± 0.4). Post-knee extension measures also revealed a smaller MVC force decrement (P < 0.001) with boys (94.3 % ±6.1 vs. 76.3 % ±4.1). Unlike adults, there were no significant decrements to children’s evoked contractile properties or EMG. The greater boys’ antagonist activation (children 125.7 % ±9.2 vs. adult: 103.5 % ±6.7; P < 0.001) post-knee extension would suggest muscle coordination changes as a primary mechanism for MVC force decrements. The lower RPE and similar agonist EMG activity may also indicate an inability of boys to perceive or produce a maximal effort.

Conclusion

Independent of High or Low RM knee extensions, boys had greater neuromuscular fatigue resistance and recovered faster than adults.     

Influence of simulated microgravity on mechanical properties in the human triceps surae muscle in vivo. I: Effect of 120 days of bed-rest without physical training on human muscle musculo-tendinous stiffness and contractile properties in young women

Purpose

The aim of this study was to investigate the effect of a 120-day 5° head-down tilt (HDT) bed-rest on the mechanical properties of the human triceps surae muscle in healthy young women subjects.

Methods

Measurements included examination of the properties of maximal voluntary contractions (MVC), twitch contractions (P _t) and tetanic contractions (P _o). The difference between P _o and MVC expressed as a percentage of P _o and referred to as force deficiency (P _d), was calculated. Electromyographic (EMG) activity in the soleus muscle, electromechanical delay (EMD) and total reaction time (TRT) were also calculated. EMD was the time interval between the change in EMG and the onset of muscle tension. Premotor time (PMT) was taken to be the time interval from the delivery of the signal to change in EMG.

Results

After HDT P _t, MVC and P _o had decreased by 11.5, 36.1, 24.4 %, respectively, P _d had increased by 38.8 %. Time-to-peak tension had increased by 13.6 %, but half-relaxation time had decreased by 19.2 %. The rate of rise in isometric voluntary tension development had reduced, but no changes were observed in the electrically evoked contraction. EMD had increased by 27.4 %; PMT and TRT decreased by 21.4, and 13.7 %, respectively.

Conclusion

The experimental findings indicated that neural as well as muscle adaptation occurred in response to HDT. EMD is a simple and quick method for evaluation of muscle stiffness changes and can serve as an indicator of the functional condition of the neuromuscular system.     

Motor unit activity and surface electromyogram power spectrum during increasing force of contraction

Summary Twelve male subjects were tested to determine the relationship between motor unit (MU) activities and surface electromyogram (EMG) power spectral parameters with contractions increasing linearly from zero to 80% of maximal voluntary contraction (MVC). Intramuscular spike and surface EMG signals recorded simultaneously from biceps brachii were analyzed by means of a computer-aided intramuscular MU spike amplitude-frequency (ISAF) histogram and an EMG frequency power spectral analysis. All measurements were made in triplicate and averaged. Results indicate that there were highly significant increases in surface EMG amplitude (71±31.3 to 505±188 V,p<0.01) and mean power frequency (89±13.3 to 123±23.5 Hz,p<0.01) with increasing force. These changes were accompanied by progressive increases in the firing frequency of MU's initially recruited, and of newly recruited MU's with relatively larger spike ampltitudes. The group data in the ISAF histograms revealed significant increases in mean spike amplitude (412±79 to 972±117 V,p<0.01) and mean firing frequency (17.8±5.4 to 24.7±4.1 Hz,p<0.01). These data suggest that surface EMG spectral analysis can provide a sensitive measure of the relative changes in MU activity during increasing force output.     

Insight into motor adaptation to pain from between-leg compensation

Purpose

Although it appears obvious that we change movement behaviors to unload the painful region, non-systematic motor adaptations observed in simple experimental tasks with pain question this theory. We investigated the effect of unilateral pain on performance of a bilateral plantarflexion task. This experimental task clearly allowed for stress on painful tissue to be reduced by modification of load sharing between legs.

Methods

Fourteen participants performed a bilateral plantarflexion at 10, 30, 50 and 70 % of their MVC during 5 conditions (Baseline, Saline-1, Washout-1, Saline-2, Washout-2). For Saline-1 and -2, either isotonic saline (Iso) or hypertonic saline (Pain) was injected into the left soleus.

Results

The force produced by the painful leg was less during Pain than Baseline (range ?52.6 % at 10 % of MVC to ?20.1 % at 70 % of MVC; P < 0.003). This was compensated by more force produced by the non-painful leg (range 18.4 % at 70 % of MVC to 70.2 % at 10 % of MVC; P < 0.001). The reduction in plantarflexion force was not accompanied by a significant decrease in soleus electromyographic activity at 10 and 30 % of MVC. Further, no significant linear relationship was found between changes in soleus electromyographic activity and change in plantarflexion force for the painful leg (with the exception of a weak relationship at 10 % of MVC, i.e., R ² = 0.31).

Conclusion

These results show that when the nervous system is presented with an obvious solution to decrease stress on irritated tissue, this option is selected. However, this was not strongly related to a decrease in soleus (painful muscle) activity level.     

Comparison of the power spectral changes of the voluntary surface electromyogram and M wave during intermittent maximal voluntary contractions

Introduction

To compare the power spectral changes of the voluntary surface electromyogram (sEMG) and of the compound action potential (M wave) in the vastus medialis and vastus lateralis muscles during fatiguing contractions.

Methods

Interference sEMG and force were recorded during 48 intermittent 3-s isometric maximal voluntary contractions (MVC) from 13 young, healthy subjects. M waves and twitches were evoked using supramaximal femoral nerve stimulation between the successive MVCs. Mean frequency (F _mean), and median frequency were calculated from the sEMG and M waves. Muscle fiber conduction velocity (MFCV) was computed by cross-correlation.

Results

The power spectral shift to lower frequencies was significantly greater for the voluntary sEMG than for the M waves (P < 0.05). Over the fatiguing protocol, the overall average decrease in MFCV (~25 %) was comparable to that of sEMG F _mean (~22 %), but significantly greater than that of M-wave F _mean (~9 %) (P < 0.001). The mean decline in MFCV was highly correlated with the mean decreases in both sEMG and M-wave F _mean.

Conclusions

The present findings indicated that, as fatigue progressed, central mechanisms could enhance the relative weight of the low-frequency components of the voluntary sEMG power spectrum, and/or the end-of-fiber (non-propagating) components could reduce the sensitivity of the M-wave spectrum to changes in conduction velocity.     

Muscle activation during low- versus high-load resistance training in well-trained men

Purpose

It has been hypothesized that lifting light loads to muscular failure will activate the full spectrum of MUs and thus bring about muscular adaptations similar to high-load training. The purpose of this study was to investigate EMG activity during low- versus high-load training during performance of a multi-joint exercise by well-trained subjects.

Methods

Employing a within-subject design, 10 young, resistance-trained men performed sets of the leg press at different intensities of load: a high-load (HL) set at 75 % of 1-RM and a low-load (LL) set at 30 % of 1-RM. The order of performance of the exercises was counterbalanced between participants, so that half of the subjects performed LL first and the other half performed HL first, separated by 15 min rest. Surface electromyography (EMG) was used to assess mean and peak muscle activation of the vastus medialis, vastus lateralis, rectus femoris, and biceps femoris.

Results

Significant main effects for trials and muscles were found (p < 0.01). Significantly greater peak EMG activity was found during the HL set (M = 177.3, SD = 89.53) compared to the LL set (M = 137.73, SD = 95.35). Significantly greater mean EMG activity was found during the HL set (M = 63.7, SD = 37.23) compared to the LL set (M = 41.63, SD = 28.03).

Conclusions

Results indicate that training with a load of 30 % 1-RM to momentary muscular failure does not maximally activate the full motor unit pool of the quadriceps femoris and hamstrings during performance of multi-joint lower body exercise.     

Active Immunological Profile Is Associated with Systemic Sj?gren��s Syndrome

Background

The aim of this paper was to study the evolution of primary Sjögren??s syndrome (pSS) immunological profile, its impact on pSS activity and long-term evolution in a bicentric cohort of French patients with pSS (n?=?445, mean age 53.6?±?14 years, mean follow-up 76.1?±?51 months).

Methods

This is a retrospective cohort study.

Results

Two hundred twelve patients were Sjögren??s syndrome A (SSA) positive, and 131 were both SSA and Sjögren??s syndrome B (SSB) positive. Sixty-eight patients (15%) had cryoglobulinemia. Active systemic profile (i.e., hypergammaglobulinemia, rheumatoid factor (RF), and anti-Sjögren??s syndrome A (anti-SSA), anti-Sjögren??s syndrome B (anti-SSB) positivity), associated with multisystemic involvement, leads to an increased utilization of corticosteroid and hydroxychloroquine. Multivariate analysis pointed out independent statistical association between hypergammaglobulinemia, anti-SSA, anti-SSB, and RF. Cryoglobulinemia is associated with multi-systemic involvement, lymphoma, and pSS-related death.

Conclusion

The subset of patients with active immunological profile is characterized by systemic complications leading to immunosuppressive drug utilization and polyclonal B-cell activation profile.     

Beneficial effects of serial contractions on muscle performance after a brief period of rest

Purpose

The study investigated potential mechanisms underlying the beneficial effects of performing serial contractions for increasing muscle performance.

Methods

Thirteen men performed maximal isometric dorsiflexion contractions to a limited amount of fatigue. The recovery pattern was monitored over 15 min. Force, surface electromyography (sEMG) of the agonist and antagonist, and skin temperature were observed. Evoked potentials were elicited.

Results

Force decreased to 206 ± 40 N and recovered to 243 ± 55 N (102 % of initial value). While full recovery is noteworthy, it was not significant (p = 0.24). Surface EMG root-mean-square (RMS) amplitude did not fully recover: tibialis anterior (219 ± 46 versus 242 ± 63 μV, 91 %) and soleus (13 ± 5 versus 16 ± 6 μV, 82 %). The result was an 11 % decrease in the co-activation ratio during recovery. Tibialis anterior and soleus mean power frequency (MPF) “over-recovered” to 117 % (144 ± 25 versus 124 ± 27 Hz) and 118 % (80 ± 14 versus 68 ± 11 Hz) of initial value, respectively. Peak rate of force development (RFD) during recovery was 146 % of initial (743 ± 246 versus 509 ± 271 N). Skin temperature increased 0.6 °C over the test session. Potentiation was not present during recovery, as assessed by twitch force.

Conclusions

The return of force to initial values, rather than a persistent decrement was a result of several mechanisms operating simultaneously. The sEMG data indicate an increase in conduction velocity, while an increase in peak RFD suggests contraction-induced facilitation of ATPase pump activity. However, alterations in muscle coordination were observed as a reduction in antagonist co-activity.     

Assessment of calf muscle fatigue during submaximal exercise using transcranial magnetic stimulation versus transcutaneous motor nerve stimulation

Purpose

Few studies have assessed the time-dependent response of fatigue (i.e., loss of force) during submaximal exercise without the use of maximum contractions. There is unexplored potential in the use of the superimposed muscle twitch (SIT), evoked by transcranial magnetic stimulation (TMS) or motor nerve stimulation (MNS), to assess fatigue during voluntary submaximal contractions. For the human triceps surae muscles, there are also no data on TMS-evoked twitches.

Methods

To optimise the TMS stimulus for assessment of fatigue, we first tested the effects of TMS power (40, 55, 70, 85, 100 % max) on SIT force during contractions (0–100 % MVC in 10 % increments) in six subjects. Then, we compared SIT responses (TMS and MNS) during submaximal contractions and MVCs (all at 60 s intervals) during a continuous protocol of intermittent contractions (30 % MVC) consisting of consecutive 5 min periods of baseline, fatigue (ischaemia) and recovery.

Results

For TMS, SIT force increased as a diminishing function of TMS power (P < 0.05), the relationships between SIT force and the force of voluntary contraction at all TMS powers were parabolic, and SIT force was maximised at ~20–40 % MVC. During intermittent contractions, MVC and SIT forces were stable during baseline, decreased similarly during ischaemia by 40–50 % (P < 0.05), and recovered similarly to baseline values (P > 0.05) before the end of the protocol.

Conclusion

TMS can be used to evoke twitches during submaximal contractions of the human calf muscle and, along with MNS, used to assess fatigue during submaximal exercise.     

Metabolic and cardiovascular responses during voluntary pedaling exercise with electrical muscle stimulation

Purpose

We aimed to test the effect of additional electrical muscle stimulation (EMS) during moderate-intensity voluntary pedaling exercise on metabolic and cardiovascular responses.

Methods

Eleven healthy male subjects performed moderate-intensity pedaling exercise at a constant workload (80 % of ventilatory threshold) for 20 min while EMS was applied to thigh muscles from 5 to 10 min and from 15 to 20 min during the exercise.

Results

A significantly higher oxygen uptake (VO₂), heart rate, and respiratory gas exchange ratio were observed during the exercise periods with EMS despite the constant workload. These changes were accompanied by an elevated blood lactate concentration, suggesting the existence of additional fast-twitch motor unit (MU) recruitment during the exercise with EMS.

Conclusion

Our data suggest that the use of intermittent EMS during a constant load exercise mimics the high-intensity interval training, possibly due to additional fast-twitch MU recruitment and co-contractions of the quadriceps and hamstrings muscles, leading to higher anaerobic metabolism and a lower mechanical efficiency.     

Is Borg’s perceived exertion scale a useful indicator of muscular and cardiovascular load in blue-collar workers with lifting tasks? A cross-sectional workplace study

Purpose

To investigate associations between perceived exertion and objectively assessed muscular and cardiovascular load during a full working day among workers with manual lifting tasks.

Methods

A total of 159 men and 41 women from 14 workplaces with manual lifting tasks participated. Participants reported perceived exertion (BORG-CR10) at midday and after work. Surface electromyography of the thigh, lower back and neck muscles were normalized to isometric voluntary contractions (MVC) to express relative muscle load during the day. Cardiovascular load was measured with electrocardiography and calculated as the average percentage of the heart rate reserve capacity (((heart rate during work – resting heart rate) / (maximum heart rate ? resting heart rate)) * 100) during the day.

Results

Using linear regression, significant but weak associations (β < 0.23) were observed between perceived exertion and (1) high muscle activity (>60 % of MVC) of the neck muscles and (2) inactivity (<1 % of MVC) of the thigh muscles and (3) cardiovascular load, respectively. Using logistic regression, perceived exertion ≥4 (high exertion), referencing <4 (low-to-moderate exertion), was related to high activity of the trapezius muscle [OR 18 (95 % CI 2–143)], i.e., the odds for experiencing high exertion during work increased 18-fold for each percentage increase in time above 60 % MVC.

Conclusions

During a full working day among blue-collar workers with lifting tasks, high neck muscle activity increases the odds for experiencing high perceived physical exertion. Perceived exertion of at least 4 on the BORG CR10 scale appears to be a good indicator that high muscular loading occurs.     

Changes in corticospinal excitability during an acute bout of resistance exercise in the elbow flexors

Purpose

Hypertrophic resistance exercise (HRE) induces central and peripheral fatigue. However, more detailed information about changes in corticospinal excitability remains to be elucidated.

Methods

Eleven volunteers participated in the upper arm HRE which included one repetition maximum (1 RM) control contractions and three sets of 13 RM (SET1–3). Transcranial magnetic stimulation (TMS) was applied during maximal isometric voluntary contraction (MVC) at the end of each set and during control contractions to study changes in corticospinal excitability. Electrical stimulation was used in order to measure peripheral changes.

Results

MVC decreased after each set when compared to control contractions. Motor evoked potential (MEP) were 138.7 ± 52.7 % (p < 0.05), 130.4 ± 44.7 and 113.1 ± 31.4 % after SET1, SET2 and SET3, respectively, when compared to pre-exercise value. A significant reduction in MEP area between SET1 and SET3 (p < 0.05) was observed while silent period (SP) duration increased (~151–165 ms, p < 0.05) simultaneously between these sets. TMS-evoked twitch force during MVC increased significantly following each set when compared to pre-exercise value. Simultaneously, a significant reduction was observed in resting twitch force over the sets.

Conclusions

The results of this study clearly support the existence of both central and peripheral fatigue during HRE of elbow flexors. However, changes in the MEP area and SP suggest that during HRE of the elbow flexors, the corticospinal excitability increases first, until at some point, supraspinal fatigue takes over.     

Effect of neuromuscular electrical stimulation intensity over the tibial nerve trunk on triceps surae muscle fatigue

Purpose

This study was designed to investigate whether the intensity modulation of a neuromuscular electrical stimulation (NMES) protocol delivered over the nerve trunk of the plantar flexors would lead to differential peripheral and central contributions of muscle fatigue.

Methods

Three fatiguing isometric protocols of the plantar flexors matched for the same amount of isometric torque-time integral (TTI) were randomly performed including a volitional protocol at 20 % of the maximal voluntary contraction (MVC) and two NMES protocols (one at constant intensity, CST; the other at intensity level progressively adjusted to maintain 20 % of MVC, PROG).

Results

No time x protocol interaction was found for any of the variables. The MVC decreased similarly (≈12 %, p < 0.001) after all protocols, so did the potentiated twitch responses (p = 0.001). Although voluntary activation of the plantar flexors did not change, maximal H-reflex to M-wave ratio of the soleus (SOL) and the gastrocnemius medialis (GM) muscles showed an overall increase (SOL: p = 0.037, GM: p = 0.041), while it remained stable for the gastrocnemius lateralis muscle (p = 0.221). A main time effect was observed only for the SOL maximal V-wave to the superimposed M-wave ratio (p = 0.024) and to the superimposed H-reflex (p = 0.008). While similar central and peripheral adaptations were observed after the three fatiguing protocols, the individual contribution of the three different triceps surae muscles was different.

Conclusion

Whether the current intensity was increased or not, the adaptations after a NMES protocol yield to similar muscle fatigue adaptations as voluntary contractions likely through similar pathways matching a similar TTI.     

Pacing strategies during repeated maximal voluntary contractions

Purpose

Pacing strategies have been reported to occur during continuous cyclical exercises. However, currently no studies have examined if pacing takes place during repeated maximal voluntary muscle contractions (MVCs). Accordingly, the purpose of this study was to examine if informing subjects on the number of MVCs they would perform would affect force and root mean squared electromyography (EMG), during similar fatiguing protocols.

Methods

Thirty well-trained male subjects completed three fatiguing protocols in a randomized order. In the control condition participants were informed they would perform 12 MVCs, and then completed all 12. In the unknown condition they were not told how many MVCs they would perform, but were stopped after 12. Lastly, in the deception condition they were initially told they would perform only 6 MVCs, but after the 6 contractions they were asked to perform a few more repetitions and were stopped after 12.

Results

Compared to the unknown condition, subjects demonstrated greater forces (p < 0.05, ES = 0.35–1.14, 2–7.5 %) and biceps EMG (p < 0.05, ES = 0.6, 6 %) in the deception condition during the first six MVCs. Additionally, under all conditions subjects applied greater forces in the last repetition (#12) relative to the previous one (#11) (p < 0.06, ES = 0.36–0.5, 2.8–3.8 %).

Conclusions

The anticipation of performing a certain number of MVCs led the subjects to utilize different pacing strategies. The results also question the assumption that subjects followed the instruction to exert maximal effort during repeated MVCs.     

Glucocorticoids improve high-intensity exercise performance in humans

Purpose

It was investigated whether oral dexamethasone (DEX) administration improves exercise performance by reducing the initial rate of muscle fatigue development during dynamic exercise.

Methods

Using a double-blinded placebo controlled randomized crossover design, subjects ingested either 2 × 2 mg of DEX or placebo for five consecutive days. Muscle function was investigated using one-legged kicking exercise and whole body performance was evaluated using a 20-m shuttle run and a 30-m sprint test.

Results

One-legged dynamic knee-extensor exercise time to exhaustion was 29 ± 35 % (mean ± SD) longer (P < 0.05) in DEX compared to Placebo. Likewise, total running distance in the shuttle run test was 19 ± 23 % longer (P < 0.05), whereas 30-m sprint performance was unaltered. During the initial 75 s of dynamic leg extensions, peak force and rate of force development determined from an electrically evoked twitch declined in a similar way in DEX and placebo. Similarly, the EMG root mean square was similar with DEX and placebo treatment.

Conclusion

Short-term dexamethasone administration increases high-intensity one-legged kicking time to exhaustion and 20-m shuttle run performance, although sprint ability and the initial loss of muscular force generating capacity are similar after DEX and placebo.     

Maximal and explosive strength training elicit distinct neuromuscular adaptations, specific to the training stimulus

Purpose

To compare the effects of short-term maximal (MST) vs. explosive (EST) strength training on maximal and explosive force production, and assess the neural adaptations underpinning any training-specific functional changes.

Methods

Male participants completed either MST (n = 9) or EST (n = 10) for 4 weeks. In training participants were instructed to: contract as fast and hard as possible for ~1 s (EST); or contract progressively up to 75 % maximal voluntary force (MVF) and hold for 3 s (MST). Pre- and post-training measurements included recording MVF during maximal voluntary contractions and explosive force at 50-ms intervals from force onset during explosive contractions. Neuromuscular activation was assessed by recording EMG RMS amplitude, normalised to a maximal M-wave and averaged across the three superficial heads of the quadriceps, at MVF and between 0–50, 0–100 and 0–150 ms during the explosive contractions.

Results

Improvements in MVF were significantly greater (P < 0.001) following MST (+21 ± 12 %) than EST (+11 ± 7 %), which appeared due to a twofold greater increase in EMG at MVF following MST. In contrast, early phase explosive force (at 100 ms) increased following EST (+16 ± 14 %), but not MST, resulting in a time × group interaction effect (P = 0.03), which appeared due to a greater increase in EMG during the early phase (first 50 ms) of explosive contractions following EST (P = 0.052).

Conclusions

These results provide evidence for distinct neuromuscular adaptations after MST vs. EST that are specific to the training stimulus, and demonstrate the independent adaptability of maximal and explosive strength.     

Prolonged infrapatellar tendon vibration does not influence quadriceps maximal or explosive isometric force production in man

Purpose

The influence of muscle/tendon vibration on maximal muscle performance is unclear. This study examined the effect of a prolonged tendon vibration stimulus on maximum voluntary contraction (MVC) and explosive voluntary contraction (EVC) performance.

Methods

Eighteen young healthy males (nine strength trained and nine untrained) completed a series of isometric unilateral knee extensions (EVCs, electrically evoked octet responses, MVCs, ramp contractions) pre and post two separate 30-min intervention trials; infrapatellar tendon vibration (80 Hz), and quiet sitting (control). H _max and M _max were measured at the start and end of each series of contractions, both pre- and post-intervention (i.e., at four time points). Knee extensor force and both quadriceps and hamstrings EMG were measured throughout each series of contractions.

Results

Vibration had no effect on either maximum force (ANOVA, trial × time interaction P = 0.92), explosive force (P ≥ 0.36), or the associated agonist EMG amplitude during these tasks (P ≥ 0.23). Octet responses were also unaffected by vibration (P ≥ 0.39). Conversely, post-intervention H _max/M _max was 60 % lower in the vibration trial vs. control, and remained 38 % lower at the end of the post-intervention measurements (t test, both P < 0.01). Individual H _max/M _max depression did not correlate to changes in either maximum or explosive force (Spearman’s Rank, P ≥ 0.54), and training status had no influence on the effect of vibration.

Conclusion

Prolonged infrapatellar tendon vibration depressed H-reflex amplitude, but did not affect either maximal or explosive isometric force production of the quadriceps.     

The influence of acetaminophen on repeated sprint cycling performance

Introduction

The aim of this study was to investigate the effect of acetaminophen on repeated sprint cycling performance.

Methods

Nine recreationally active male participants completed a graded exercise test, a familiarisation set of Wingate Anaerobic Tests (WAnTs) and two experimental sets of WAnTs (8 × 30 s sprints, 2 min active rest intervals). In the experimental WAnTs, participants ingested either 1.5 g acetaminophen or a placebo in a double-blind, randomised, crossover design. During the WAnT trials, participants provided ratings of perceived pain 20 s into each sprint. Mean and peak power output and heart rate were recorded immediately following each sprint, and percentage decrement in mean power output was subsequently calculated.

Results

Participants cycled at a significantly greater mean power output over the course of 8 WAnTs (p < 0.05) following the ingestion of acetaminophen (391 ± 74 vs. 372 ± 90 W), due to a significantly greater mean power output during sprints 6, 7 and 8 (p < 0.05). Percentage decrements in mean power output were also significantly reduced (p < 0.05) following acetaminophen ingestion (17 ± 14 vs. 24 ± 17 %). No significant differences in peak power output, perceived pain or heart rate were observed between conditions.

Conclusion

Acetaminophen may have improved performance through the reduction of pain for a given work rate, thereby enabling participants to exercise closer to a true physiological limit. These results suggest that exercise may be regulated by pain perception, and that an increased pain tolerance can improve exercise performance.