Maximal Voluntary Contraction Force Research Articles

Trunk Muscle Fatigue and Activation are Associated with Drop Jump Performance

Although the importance of core muscles on human motions has been recognized, little is known about the effect of trunk muscle fatigue and activation on drop jump (DJ) performance. PURPOSE: To examine how trunk muscle fatigue and activation relate to DJ performance. METHODS: The study included 7 healthy, well-trained males (age 20.8±1.4 years, height 168.4±5.7 cm, weight 67.1±8.0 kg). Subjects held vertical trunk position against a wire with one end attached to the posterior thorax, pulling the trunk posteriorly, with the other end attached to a weight corresponding to 25% of maximal voluntary isometric contraction force. Before and after this fatiguing task, maximum voluntary isometric trunk flexor contraction force (MVIC), and double- and single-leg DJ height (DJH), contact time (CT), and DJ index (DJI; DJH/CT) were measured. Surface electromyography was recorded from the dominant side of the anterior and posterior trunk musculature during DJs. Mean differences before and after the fatiguing task were examined using paired-sample t-tests. Simple linear regression analyses tested the relation of relative changes before and after the fatiguing task in the jump performance index and trunk muscle activation during preactivation, braking, and push off phase of each DJ. RESULTS: After the fatiguing task, MVIC was significantly decreased to 68.8±11.5% (p<0.01). DJI (Pre vs. Post fatigue for double- and single-leg DJ: 1.18±0.31 vs. 0.94±0.36, 0.52±0.13 vs. 0.41±0.15) and DJH (Pre vs. Post fatigue for double- and single-leg DJ: 22.12±5.09 cm vs. 20.01±5.24 cm, 13.21±3.04 cm vs. 11.07±3.64 cm) were significantly decreased (p<0.01), and CT (0.19±0.02 s vs. 0.22±0.03 s) was significantly extended (p<0.01). Regression analyses revealed that greater ensemble trunk muscle preactivation reduction rate was associated with CT lengthening rate in double-leg DJ (R2=0.582, B=-0.257, p=0.046), and DJI reductions rate in single-leg DJ (R2=0.910, B=0.309, p=0.001) and that greater rectus abdominus (R2=0.779, B=0.138, p=0.008) and external oblique activations reduction rate during breaking phase (R2=0.703, B=0.276, p=0.018) were associated with greater DJI reduction rate. CONCLUSIONS: Trunk muscle fatigue and decreased trunk flexor muscle activation during DJ have negative effects on both double- and single-leg DJ.

Effect of hypnotic suggestion on knee extensor neuromuscular properties in resting and fatigued states.

PurposeThe aim of this study was to investigate whether hypnotic suggestions can alter knee extensor neuromuscular function at rest and during exercise.MethodsThirteen healthy volunteers (8 men and 5 women, 27 ± 3 years old) took part in this counterbalanced, crossover study including two experimental (hypnosis and control) sessions. Knee extensor neuromuscular function was tested before and after hypnosis suggestion by using a combination of voluntary contraction, transcutaneous femoral nerve electrical stimulation and transcranial magnetic stimulation (TMS). A fatiguing exercise (sustained submaximal contraction at 20% maximal voluntary contraction (MVC) force) was also performed to evaluate the potential influence of hypnosis on the extent and origin of neuromuscular adjustments.ResultsHypnosis did not (p>0.05) alter MVC force or knee extensor neural properties. Corticospinal excitability, assessed with the amplitude of knee extensor motor evoked potentials, was also unchanged (p>0.05), as was the level of intracortical inhibition assessed with paired pulse TMS (short-interval intracortical inhibition, SICI). Time to task failure (~300 s) was not different (p>0.05) between the two sessions; accordingly, hypnosis did not influence neuromuscular adjustments measured during exercise and at task failure (p>0.05).ConclusionHypnotic suggestions did not alter neuromuscular properties of the knee extensor muscles under resting condition or during/after exercise, suggesting that hypnosis-induced improvement in exercise performance and enhanced corticospinal excitability might be limited to highly susceptible participants.

The effect of cognitive fatigue and hypoxia on repeated arm bike sprint performance: A combined and individual stressors approach

High‐altitude ascent can expose individuals to both physical and cognitive stress in combination. Despite this, limited research has investigated the combined influence of low oxygen concentrations and cognitive fatigue on high‐intensity exercise performance. The aim of the present study was to investigate the combined effects of cognitive fatigue and normobaric hypoxia on repeated arm bike sprint ability. Six active healthy males completed four experimental conditions in a balanced order (mean ± SD; age: 25.2 ± 2.9 years, body fat percentage: 14.4 ± 1.6%, max power during 5‐s arm bike sprint: 380 ± 77 W). All conditions were completed at 17°C environmental temperature, 50% relative humidity, with a wind speed 0.4 m.s−1. The four experimental conditions were: 1) normobaric normoxia (0.209 FiO2) with no prior cognitive fatigue (CON), 2) normobaric normoxia (0.209 FiO2) with cognitive fatigue induced using an individualised 16‐min dual‐task cognitive test prior to performance test (CF), 3) normobaric hypoxia (0.12 FiO2) with no prior cognitive fatigue (HYP), 4) normobaric hypoxia (0.12 FiO2) with cognitive fatigue induced as before (HYP+CF). To assess physical performance for each condition, participants completed 10 × 20‐s ‘all‐out’ arm sprints, followed by a 60‐s isometric bicep maximal voluntary contraction (MVC) at 100% maximal voluntary strength. Central and peripheral fatigue were assessed using Supramaximal Nerve Stimulation (400 V square wave, 10 ms spaced doublets, 37 ± 5 mA), with stimulations superimposed every 10‐s during the 60‐s MVC using the twitch interpolation method. Voluntary activation percentage (VA) and resting potentiated twitch force (Qtw,pot) were calculated. Near Infra‐Red Spectroscopy (NIRS), heart rate, oxygen consumption and subjective ratings of perceived exertion, physical discomfort and thermal sensation, were also measured. Cognitive fatigue was successfully induced prior to the performance test as verified (p≤0.001) using the Brunel Mood Scale (BRUMS), Stanford Sleepiness Scale (SSS) and the Visual Analogue Scale of Fatigue (VASF). High‐altitude resulted in decreased (p=0.002) NIRS muscle oxygenation by 9.7 ± 3.8%. Individually, cognitive fatigue (p=0.032) and hypoxia (p=0.037) lowered power output across all ten sprints by 7.3 ± 6.4 W and 10.5 ± 8.8 W respectively, when compared to control conditions (CON: 188 ± 22 W, HYP: 182 ± 16 W, CF: 185 ± 18 W, HYP+CF: 170 ± 16 W). Exposure to both stressors in combination produced a greater decline in average power output, and the effect was additive for the two stressors (no interaction, p=0.127). Post‐exercise perceptual and subjective sensations, as well as MVC force, VA and Qtw,pot were not different between trials (p≥0.126). In conclusion, separate exposure to cognitive fatigue and hypoxia significantly impaired self‐paced physical performance during repeated arm bike sprints. When combined, exposure to cognitive fatigue and hypoxia diminished performance additively for the two stressors.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Augmented exercise pressor response during static handgrip in women with PTSD

Post‐traumatic stress disorder (PTSD) is a psychiatric illness that is more prevalent in women than men, and there is growing evidence suggesting a clear link between PTSD and future development of cardiovascular disease. The underlying mechanisms are unclear, but enhanced sympatheticreactivity to daily stressors may be involved. We hypothesized that PTSD women would have an augmented vasomotor sympathetic response to static exercise. We measured heart rate (HR), blood pressure (BP), and muscle sympathetic nerve activity (MSNA) in 12 female patients with PTSD and 8 healthy women (controls) during static handgrip (SHG) exercise to fatigue at 40% of maximal voluntary contraction (MVC) force followed by 2 min of postexercise circulatory arrest (PECA) and 3 min of recovery. The total SHG time was divided evenly into five stages. Age, body mass index, and body surface area were not different between groups. MVC and the time to fatigue during SHG did not differ between patients and controls (P&gt;0.05). MSNA burst frequency (BF) in response to SHG was greater in patients than controls (timexgroup interaction P=0.03, time P=0.0001, group P=0.04), but not during PECA. During the early stages of SHG, PTSD patients showed greater MSNA BF compared to controls [stage 1: 40±5 (SE) vs.19±7 bursts/min; P=0.02, stage 2: 45±6 vs.25±7; P=0.03, stage 3: 55±6 vs.30±7; P=0.02). MSNA BF responses during stage 4 and 5 were not different between groups (P=0.09 and P=0.1, respectively). Similarly, PTSD patients showed greater systolic blood pressure (SBP) compared to controls during early stages of SHG (stage 1: 144±4 vs.129±5 mmHg; P=0.02, stage 2: 149±4 vs.132±5; P=0.03, stage 3: 159±6 vs.138±7; P=0.03, stage 4: 166±6 vs.143±7; P=0.04) but not in the last stage (at fatigue; P=0.2). SBP was not different between groups during PECA and recovery. Diastolic BP and HR were progressively increased during SHG, reached peak at the last stage, and were comparable between groups. These results suggest that women with PTSD have an augmented exercise pressor response, which may be attributed to enhanced mechano‐ rather than metabo‐reflexes. However, the actual role of the muscle reflex in cardiovascular risk in PTSD remains to be elucidated.Support or Funding InformationThis study is supported by Harry S. Moss Heart Trust research funding STU 082016‐057This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Is perceived force an accurate method of regulating exercise load for the neck?

ABSTRACTObjective: Patient perceived level of force is commonly used to regulate exercise load. This study determined the accuracy of perceived force as a method of regulating exercise load for the cervical flexors and extensors. Methods: Thirty healthy individuals performed submaximal isometric cervical flexion and extension exercise at perceived intensities (25%, 50%, and 75%) of their maximal voluntary contraction (MVC) with no feedback in a neck dynamometer. Measurements of “actual” versus “perceived” force levels were compared before and after the performance of an MVC. Data were analyzed descriptively and the relationship between performance error, test, and participant factors examined. Results: With the exception of the 25% MVC extension trials post-MVC, an underestimation of force was observed over all conditions that worsened as the target force increased (50% and 75%) with only some improvement following the experience of an MVC (p < 0.001). Linear mixed modeling indicated participants familiar with exercise may have better accuracy (p = 0.07). Conclusion: Regulating isometric resistance exercise load of the neck based on a perceived proportion of MVC force may only be accurate at low intensities (25% MVC) but not for moderate to high (50–75%) intensities.

Women show similar central and peripheral fatigue to men after half-marathon

Women are known to be less fatigable than men in single-joint exercises, but fatigue induced by running has not been well understood. Here we investigated sex differences in central and peripheral fatigue and in rate of force development (RFD) in the knee extensors after a half-marathon run. Ten male and eight female amateur runners (aged 25–50 years) were evaluated before and immediately after a half-marathon race. Knee extensors forces were obtained under voluntary and electrically evoked isometric contractions. Maximal voluntary isometric contraction (MVC) force and peak RFD were recorded. Electrically doublet stimuli were delivered during the MVC and at rest to calculate the level of voluntary activation and the resting doublet twitch. After the race, decreases in MVC force (males: −11%, effect size [ES] 0.52; females: −11% ES 0.33), voluntary activation (males: −6%, ES 0.87; females: −4%, ES 0.72), and resting doublet twitch (males: −6%, ES 0.34; females: −8%, ES 0.30) were found to be similar between males and females. The decrease in peak RFD was found to be similar between males and females (males: −14%, ES 0.43; females: −15%, ES 0.14). Half-marathon run induced both central and peripheral fatigue, without any difference between men and women. The maximal and explosive strength loss was found similar between sexes. Together, these findings do not support the need of sex-specific training interventions to increase the tolerance to neuromuscular fatigue in half-marathoners.

Kinesio Taping Improves Perceptions of Pain and Function of Patients With Knee Osteoarthritis: A Randomized, Controlled Trial

Context: Although increasingly used for therapeutic treatment, only limited evidence exists regarding the effects of kinesio taping on patients with knee osteoarthritis (OA). Objective: To determine the effects of kinesio taping on pain, function, gait, and neuromuscular control concerning patients with knee OA. Design: Randomized sham-controlled trial. Setting: University laboratory. Participants: A total of 141 patients (65.1 [7.0] y) with a clinical and radiographic diagnosis of knee OA. Intervention: Kinesio tape, sham tape, or no tape for 3 consecutive days. Main Outcome Measures: Self-reported pain, stiffness, and function were measured by the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Further tests included the Balance Error Scoring System, 10-m walk test, the maximum voluntary isometric contraction force of the quadriceps femoris, and knee active range of motion. Results: At baseline, there were no differences in all outcomes between groups except for knee flexion. Significant effects were found for WOMAC pain (tape vs sham, P = .05; tape vs control, P = .047), stiffness (tape vs sham, P = .01; tape vs control, P ≤ .001), and physical function (tape vs sham, P = .03; tape vs control P = .004). No interactions were found for balance, muscle strength, walking speed, or active range of motion. Conclusion: Wearing kinesio tape for 3 consecutive days had beneficial effects regarding self-reported clinical outcomes of pain, joint stiffness, and function. This emphasizes that kinesio taping might be an adequate conservative treatment for the symptoms of knee OA.

Examination of muscle composition and motor unit behavior of the first dorsal interosseous of normal and overweight children.

We examined differences between normal weight (NW) and overweight (OW) children aged 8-10 yr in strength, muscle composition, and motor unit (MU) behavior of the first dorsal interosseous. Ultrasonography was used to determine muscle cross-sectional area (CSA), subcutaneous fat (sFAT), and echo intensity (EI). MU behavior was assessed during isometric muscle actions at 20% and 50% of maximal voluntary contraction (MVC) by analyzing electromyography amplitude (EMGRMS) and relationships between mean firing rates (MFR), recruitment thresholds (RT), and MU action potential amplitudes (MUAPsize) and durations (MUAPtime). The OW group had significantly greater EI than the NW group ( P = 0.002; NW, 47.99 ± 6.01 AU; OW, 58.90 ± 10.63 AU, where AU is arbitrary units) with no differences between groups for CSA ( P = 0.688) or MVC force ( P = 0.790). MUAPsize was larger for NW than OW in relation to RT ( P = 0.002) and for MUs expressing similar MFRs ( P = 0.011). There were no significant differences ( P = 0.279-0.969) between groups for slopes or y-intercepts from the MFR vs. RT relationships. MUAPtime was larger in OW ( P = 0.015) and EMGRMS was attenuated in OW compared with NW ( P = 0.034); however, there were no significant correlations ( P = 0.133-0.164, r = 0.270-0.291) between sFAT and EMGRMS. In a muscle that does not support body mass, the OW children had smaller MUAPsize as well as greater EI, although anatomical CSA was similar. This contradicts previous studies examining larger limb muscles. Despite evidence of smaller MUs, the OW children had similar isometric strength compared with NW children. NEW & NOTEWORTHY Ultrasound data and motor unit action potential sizes suggest that overweight children have poorer muscle composition and smaller motor units in the first dorsal interosseous than normal weight children. Evidence is presented that suggests differences in action potential size cannot be explained by differences in subcutaneous fat alone.

An Innovative Ergometer to Measure Neuromuscular Fatigue Immediately after Cycling.

When assessing neuromuscular fatigue (NMF) from dynamic exercise using large muscle mass (e.g., cycling), most studies have delayed measurement for 1 to 3 min after task failure. This study aimed to determine the reliability of an innovative cycling ergometer permitting the start of fatigue measurement within 1 s after cycling. Twelve subjects participated in two experimental sessions. Knee-extensor NMF was assessed by electrical nerve and transcranial magnetic stimulation with both a traditional chair setup (PRE- and POST-Chair, 2 min postexercise) and the new cycling ergometer (PRE, every 3 min during incremental exercise and POST-Bike, at task failure). The reduction in maximal voluntary contraction force POST-Bike (63% ± 12% PRE; P < 0.001) was not different between sessions and there was excellent reliability at PRE-Bike (intraclass correlation coefficient [ICC], 0.97; coefficients of variation [CV], 3.2%) and POST-Bike. Twitch (Tw) and high-frequency paired-pulse (Db100) forces decreased to 53% ± 14% and 62% ± 9% PRE, respectively (P < 0.001). Both were reliable at PRE-Bike (Tw: ICC, 0.97; CV, 5.2%; Db100: ICC, 0.90; CV, 7.3%) and POST-Bike (Tw: ICC, 0.88; CV, 11.9; Db100: ICC, 0.62; CV, 9.0%). Voluntary activation did not change during the cycling protocol (P > 0.05). Vastus lateralis and rectus femoris M-wave and motor-evoked potential areas showed fair to excellent reliability (ICC, 0.45-0.88). The reduction in maximal voluntary contraction and Db100 was greater on the cycling ergometer than the isometric chair. The innovative cycling ergometer is a reliable tool to assess NMF during and immediately postexercise. This will allow fatigue etiology during dynamic exercise with large muscle mass to be revisited in various populations and environmental conditions.

The effects of a single session of spinal manipulation on strength and cortical drive in athletes

PurposeThe primary purpose of this study was to investigate whether a single session of spinal manipulation (SM) increases strength and cortical drive in the lower limb (soleus muscle) of elite Taekwondo athletes.MethodsSoleus-evoked V-waves, H-reflex and maximum voluntary contraction (MVC) of the plantar flexors were recorded from 11 elite Taekwondo athletes using a randomized controlled crossover design. Interventions were either SM or passive movement control. Outcomes were assessed at pre-intervention and at three post-intervention time periods (immediate post, post 30 min and post 60 min). A multifactorial repeated measures ANOVA was conducted to assess within and between group differences. Time and session were used as factors. A post hoc analysis was carried out, when an interactive effect was present. Significance was set at p ≤ 0.05.ResultsSM increased MVC force [F(3,30) = 5.95, p < 0.01], and V-waves [F(3,30) = 4.25, p = 0.01] over time compared to the control intervention. Between group differences were significant for all time periods (p < 0.05) except for the post60 force measurements (p = 0.07).ConclusionA single session of SM increased muscle strength and corticospinal excitability to ankle plantar flexor muscles in elite Taekwondo athletes. The increased MVC force lasted for 30 min and the corticospinal excitability increase persisted for at least 60 min.

Effects of microgravity on characteristics of the accuracy control of movements

Effects of microgravity on characteristics of the accuracy control of movements

Environmental temperature and exercise modality independently impact central and muscle fatigue among people with multiple sclerosis.

BackgroundHeat sensitivity and fatigue limit the ability of multiple sclerosis patients to participate in exercise.ObjectiveThe purpose of this study was to determine the optimal aerobic exercise parameters (environmental temperature and exercise modality) to limit exercise-induced central and muscle fatigue among people with multiple sclerosis.MethodsFourteen people with multiple sclerosis with varying levels of disability completed four randomized exercise sessions at 65% of the maximal volume of oxygen: body-weight supported treadmill cool (16°C), body-weight supported treadmill room (21°C), total-body recumbent stepper cool and total-body recumbent stepper room. Maximum voluntary contraction, electromyography, and evoked contractile properties were collected from the more affected plantar flexors along with subjective levels of fatigue, body temperature and perceived level of exertion.ResultsExercise in cooler room temperature increased maximum voluntary contraction force (p = 0.010) and stabilized body temperature (p = 0.011) compared to standard room temperature. People with multiple sclerosis experienced greater peak twitch torque (p = 0.047), shorter time to peak twitch (p = 0.035) and a longer half relaxation time (p = 0.046) after total-body recumbent stepper suggestive of less muscle fatigue.ConclusionCooling the exercise environment limits the negative effects of central fatigue during aerobic exercise and using total-body recumbent stepper (work distributed among four limbs) rather than body-weight supported treadmill lessens muscular fatigue. Therapists can titrate these two variables to help people with multiple sclerosis achieve sufficient exercise workloads.

Manipulation of sensory input can improve stretching outcomes

The primary purpose of our study was to assess the influence of modulating sensory input with either transcutaneous electrical nerve stimulation (TENS) or self-massage with therapy balls on the maximal range of motion (ROM) about the ankle joint when stretching the calf muscles. We also investigated the influence of these two conditions on the force capacity and force control of plantar flexor muscles. Twenty healthy adults (25 ± 3 yr) performed three sessions of ankle plantar flexor stretching (three stretches of 30 s each): stretching alone (SS), stretching with concurrent TENS (TENS), and stretching after self-massage using therapy balls (SM). TENS was applied for 60 s prior to and during each stretch, and SM was performed for 60 s prior to each of the three stretches. Maximal voluntary contraction (MVC) torque and force steadiness at 20% MVC were recorded before and at 15 min after the final stretch. Ankle dorsiflexion ROM was assessed before, after, and at 5, 10, and 15 min after the last stretch. The increase in ROM was greater after SM (24%) than after SS (13%) and TENS (9%; p < .001). Maximal discomfort level (0–10 VAS) during stretching was similar for all conditions. MVC torque increased after SM only (p < .001, Cohen’s D = 1.5): SM, 16%; SS, –1%; TENS, –3%. Force steadiness did not change. The sensory fibres that contribute to stretch tolerance were engaged by self-massage but not by TENS, resulting in greater increases in flexibility and MVC torque after self-massage.

Neuromuscular Fatigue After Repeated Jumping With Concomitant Electrical Stimulation.

To evaluate the etiology and extent of neuromuscular fatigue induced by 50 squat jumps performed with and without neuromuscular electrical stimulation (NMES) of the knee extensors. Nine healthy, recreationally active men (24 ± 2y) took part in 2 experiments. These consisted of 50 squat jumps performed with stimulation (NMES) or without (CON). Maximal voluntary contraction (MVC) force, maximal voluntary activation level (VAL), and forces evoked by single and double (10 and 100Hz) stimulations were recorded before and after the 50 jumps. NMES was delivered at the maximal tolerated intensity. Despite average jump height being ∼16% lower in the NMES than in the CON session, a reduction over time in jump height was only found in the NMES condition (-6%). After the 50 jumps, MVC force was reduced to a greater extent in NMES than in CON (-25% ± 11% vs -11% ± 12%). Similarly, forces evoked by single stimulations, as well as by 10-Hz and 100-Hz paired stimulations, were reduced to a greater extent in NMES (-33% ± 12%, -42% ± 15%, and -25% ± 13%) than in CON (-21% ± 6%, -30% ± 9%, and -14% ± 11%). VAL was not significantly altered by either condition. Performing repeated squat jumps with concomitant NMES induced a greater fatigue than squat jumps performed alone and might potentially represent a stronger training stimulus.

Etiology and Recovery of Neuromuscular Fatigue following Competitive Soccer Match-Play.

Aim: Previous research into the etiology of neuromuscular fatigue following competitive soccer match-play has primarily focused on peripheral perturbations, with limited research assessing central nervous system function in the days post-match. The aim of the present study was to examine the contribution and time-course of recovery of central and peripheral factors toward neuromuscular fatigue following competitive soccer match-play.Methods: Sixteen male semi-professional soccer players completed a 90-min soccer match. Pre-, post- and at 24, 48, and 72 h participants completed a battery of neuromuscular, physical, and perceptual tests. Maximal voluntary contraction force (MVC) and twitch responses to electrical (femoral nerve) and transcranial magnetic stimulation (TMS) of the motor cortex during isometric knee-extension and at rest were measured to assess central nervous system (voluntary activation, VA) and muscle contractile (potentiated twitch force, Qtw, pot) function. Electromyography responses of the rectus femoris to single- and paired-pulse TMS were used to assess corticospinal excitability and short-interval intracortical inhibition (SICI), respectively. Fatigue and perceptions of muscle soreness were assessed via visual analog scales, and physical function was assessed through measures of jump (countermovement jump height and reactive strength index) and sprint performance.Results: Competitive match-play elicited significant post-match declines in MVC force (−14%, P < 0.001) that persisted for 48 h (−4%, P = 0.01), before recovering by 72 h post-exercise. VA (motor point stimulation) was reduced immediately post-match (−8%, P < 0.001), and remained depressed at 24 h (−5%, P = 0.01) before recovering by 48 h post-exercise. Qtw,pot was reduced post-match (−14%, P < 0.001), remained depressed at 24 h (−6%, P = 0.01), before recovering by 48 h post-exercise. No changes were evident in corticospinal excitability or SICI. Jump performance took 48 h to recover, while perceptions of fatigue persisted at 72 h.Conclusion: Competitive soccer match-play elicits substantial impairments in central nervous system and muscle function, requiring up to 48 h to resolve. The results of the study could have important implications for fixture scheduling, the optimal management of the training process, squad rotation during congested competitive schedules, and the implementation of appropriate recovery interventions.

An Acute Exposure to Muscle Vibration Decreases Knee Extensors Force Production and Modulates Associated Central Nervous System Excitability.

Local vibration (LV) has been recently validated as an efficient training method to improve muscle strength. Understanding the acute effects may help elucidate the mechanism(s). This study aimed to investigate the effects of a single bout of prolonged LV on knee extensor force production and corticospinal responsiveness of vastus lateralis (VL) and rectus femoris (RF) muscles in healthy young and old adults. Across two visits, 23 adult subjects (20–75 years old) performed pre- and post-test measurements, separated by 30-min of either rest (control; CON) or LV. Maximal voluntary contraction (MVC) force was assessed and transcranial magnetic stimulation (TMS) was used to evaluate cortical voluntary activation (VATMS) as well as the motor evoked potential (MEP) and silent period (SP). In 11 young adults, thoracic electrical stimulation was used to assess the thoracic motor evoked potential (TMEP). Although MVC decreased after both CON (−6.3 ± 4.4%, p = 0.01) and LV (−12.9 ± 7.7%, p < 0.001), the MVC loss was greater after LV (p = 0.001). Normalized maximal electromyographic (EMG) activity decreased after LV for both VL (−25.1 ± 10.7%) and RF (−20.9 ± 16.5%; p < 0.001), while it was unchanged after CON (p = 0.32). For RF, the TMEP and MEP/TMEP ratio decreased (p = 0.01) and increased (p = 0.01) after LV, respectively. Both measures were unchanged for VL (p = 0.27 and p = 0.15, respectively). No changes were reported for TMS-related parameters. These results confirm our hypothesis that modulations within the central nervous system would accompany the significant reduction of maximal voluntary force. A reduced motoneuron excitability seems to explain the decreased MVC after prolonged LV, as suggested by reductions in maximal EMG (all subjects) and TMEP area (data from 11 young subjects). A concomitant increased cortical excitability seems to compensate for lower excitability at the spinal level.

Sex differences in neuromuscular fatigability in response to load carriage in the field in British Army recruits

ObjectivesWomen are resistant to neuromuscular fatigue compared to men in response to a range of exercise tasks. The sex differences in the neuromuscular responses to load carriage have yet to be investigated. DesignProspective cohort study. MethodsTwenty-three male and 19 female British Army recruits completed a 9.7km loaded march within 90min, with the weight carried dependent on military trade (16±2kg for men and 15±1kg for women). Isometric maximal voluntary contraction (MVC) force of the knee extensors and vertical jump (VJ) height were examined pre- and post-loaded march to examine neuromuscular fatigue. Heart rate (HR) was recorded throughout and ratings of perceived exertion (RPE) was recorded following the march. ResultsHR was higher for women (173±9bmin−1, 83±6% heart rate reserve) than men (158±8bmin−1, 72±6% heart rate reserve) (p≤0.001). RPE following the march was also higher for women than men (6±2 vs 4±2, respectively, p<0.001). The loss in MVC force was greater for men than women (−12±9% vs −9±13%, respectively, p=0.031), however VJ height was impaired to a similar extent (−5±11% vs −5±6%, respectively, p=0.582). ConclusionsThe greater physiological stress during load carriage for women compared to men did not translate to a greater severity of knee extensor muscle fatigue, with women demonstrating fatigue resistance.

Maximal Voluntary Activation of the Elbow Flexors Is under Predicted by Transcranial Magnetic Stimulation Compared to Motor Point Stimulation Prior to and Following Muscle Fatigue.

Transcranial magnetic (TMS) and motor point stimulation have been used to determine voluntary activation (VA). However, very few studies have directly compared the two stimulation techniques for assessing VA of the elbow flexors. The purpose of this study was to compare TMS and motor point stimulation for assessing VA in non-fatigued and fatigued elbow flexors. Participants performed a fatigue protocol that included twelve, 15 s isometric elbow flexor contractions. Participants completed a set of isometric elbow flexion contractions at 100, 75, 50, and 25% of maximum voluntary contraction (MVC) prior to and following fatigue contractions 3, 6, 9, and 12 and 5 and 10 min post-fatigue. Force and EMG of the bicep and triceps brachii were measured for each contraction. Force responses to TMS and motor point stimulation and EMG responses to TMS (motor evoked potentials, MEPs) and Erb's point stimulation (maximal M-waves, Mmax) were also recorded. VA was estimated using the equation: VA% = (1−SITforce/PTforce) × 100. The resting twitch was measured directly for motor point stimulation and estimated for both motor point stimulation and TMS by extrapolation of the linear regression between the superimposed twitch force and voluntary force. MVC force, potentiated twitch force and VA significantly (p < 0.05) decreased throughout the elbow flexor fatigue protocol and partially recovered 10 min post fatigue. VA was significantly (p < 0.05) underestimated when using TMS compared to motor point stimulation in non-fatigued and fatigued elbow flexors. Motor point stimulation compared to TMS superimposed twitch forces were significantly (p < 0.05) higher at 50% MVC but similar at 75 and 100% MVC. The linear relationship between TMS superimposed twitch force and voluntary force significantly (p < 0.05) decreased with fatigue. There was no change in triceps/biceps electromyography, biceps/triceps MEP amplitudes, or bicep MEP amplitudes throughout the fatigue protocol at 100% MVC. In conclusion, motor point stimulation as opposed to TMS led to a higher estimation of VA in non-fatigued and fatigued elbow flexors. The decreased linear relationship between TMS superimposed twitch force and voluntary force led to an underestimation of the estimated resting twitch force and thus, a reduced VA.

Effects of Brake Pedal Horizontal Location of Hospital Beds on Force Exertions and Work Efficiency

The brake pedal on hospital beds is critical during bed maneuvering, however, substantial force and awkward postures are usually required during pedal engagement tasks. Nine professional caregivers were recruited to investigate how brake pedal horizontal location affected maximal voluntary contraction (MVC) force, acceptable force to engage the pedal (AFE), force efficiency and task completion time. The results demonstrated reduced MVC, AFE and force efficiency whereas increased task completion time with greater pedal depths. Pedal depth was significantly correlated with MVC, force efficiency and task completion time and these correlations are moderate (0.25≤r&lt;0.50) or good (0.50≤r&lt;075). These findings provide important information for hospital bed design.

Effect of Social Stress on Motor Function in Older Adults: an fNIRS Investigation

Stress has adverse impacts on mental and physical health and quality of life, especially in older adults. Stress can impair cognitive function including short and long-term memory, and this functional declines can further be associated with decreased neuromuscular performance (Mehta &amp; Parasuraman, 2014) and increased fatigability (Keller-Ross et al., 2014). Since older adults are more susceptible to the effect of stress because their limited mobility caused by aging can worsen under stress (Noven at al., 2014), it is important to examine the effect of acute stress on neuromuscular function in older adults. In the present study, we tested the effect of social stress on neuromuscular function of both upper and lower extremity in older adults before and after a short bout of social stress. Thirty participants (15 males, 15 females, mean age: 73.3 (5.6) yrs) performed ten trials of submaximal voluntary contraction at 30% of their maximum voluntary contraction force level before and after the Trier Social Stress Test (TSST) session. Handgrip and knee extension motor performance was measured on separate days. TSST consisted of five minutes speech and five minutes serial arithmetic subtraction tasks. We measured force steadiness and electromyography (EMG) of working muscles to evaluate motor function. Additionally, electrocardiogram (ECG), the Visual Analogue Scale of Stress (VAS), and salivary cortisol were collected to evaluate the effect of the TSST. Neural activation pattern changes of prefrontal and sensorimotor area during exercise and TSST sessions was recorded using functional Near Infrared Spectroscopy (fNIRS). To confirm whether the TSST session increased stress level in our participants, we first analysed the stress metrics. Heart rate increased during the TSST and returned to prestress level instantly after the TSST session. Perceived stress level using the VAS increased after TSST. While not significant, salivary cortisol level increased after the TSST session. Findings indicate that handgrip force steadiness improved after the TSST session, whereas knee extension force steadiness remained unchanged. On the other hand, handgrip EMG root mean square (RMS) did not change after stress while knee extension EMG RMS was found to increase. Neural activation during handgrip exercise increased at the left motor area, and neural activation during knee extension increased at the left sensory area after the TSST session. Change of heart rate and VAS indicates that participants’ stress level was increased after stress and the improved motor performance during handgrip exercise after the stress is consistent with a previous study that reported increased memory function after stress in older adults (Pulopulos et al., 2015). However, the differential effects of stress based on upper or lower extremity indicates increased sensitivity of certain motor tasks to social stress than other. While social stress is known to affect response time but not memory function (Guez et al., 2016), different spatial activation pattern between handgrip and knee extension exercises observed in the present study suggest that different neural strategies were adapted to compensate for the effects of acute social stress to maintain motor performance.