Physiological Responses to Acute Cycling With Blood Flow Restriction.

Aerobic exercise with blood flow restriction (BFR) can improve muscular function and aerobic capacity. However, the extent to which cuff pressure influences acute physiological responses to aerobic exercise with BFR is not well documented. We compared blood flow, tissue oxygenation, and neuromuscular responses to acute cycling with and without BFR. Ten participants completed four intermittent cycling (6 × 2 min) conditions: low-load cycling (LL), low-load cycling with BFR at 60% of limb occlusion pressure (BFR60), low-load cycling with BFR at 80% of limb occlusion pressure (BFR80), and high-load cycling (HL). Tissue oxygenation, cardiorespiratory, metabolic, and perceptual responses were assessed during cycling and blood flow was measured during recovery periods. Pre- to post-exercise changes in knee extensor function were also assessed. BFR60 and BFR80 reduced blood flow (~33 and ~ 50%, respectively) and tissue saturation index (~5 and ~15%, respectively) when compared to LL (all p < 0.05). BFR60 resulted in lower VO2, heart rate, ventilation, and perceived exertion compared to HL (all p < 0.05), whereas BFR80 resulted in similar heart rates and exertion to HL (both p > 0.05). BFR60 and BFR80 elicited greater pain compared to LL and HL (all p < 0.05). After exercise, knee extensor torque decreased by ~18 and 40% for BFR60 and BFR80, respectively (both p < 0.05), and was compromised mostly through peripheral mechanisms. Cycling with BFR increased metabolic stress, decreased blood flow, and impaired neuromuscular function. However, only BFR60 did so without causing very severe pain (>8 on pain intensity scale). Cycling with BFR at moderate pressure may serve as a potential alternative to traditional high-intensity aerobic exercise.

Neural and muscular alterations of the plantar flexors in middle-aged women.

INTRODUCTION: Considering the large population of middle-aged adults, it is important to understand the age-related change in lower limb muscles and the possible mechanisms before old age (> 65 years old). The purpose of this study was to investigate age-related neural and muscular alterations of the plantar flexors in young and middle-aged women. METHODS: Twenty two middle-aged (54.0 ± 5.8 yrs) and 17 young (21.8 ± 1.4 yrs) recreationally active women performed rapid maximal voluntary isometric contractions (MVIC) of the plantar flexors. Absolute and normalized rate of torque development (RTD) and electromyography (EMG) were examined. Electrical stimulation was used to examine voluntary activation and contractile properties of the muscle. Ultrasonography was used to examine medial and lateral gastrocnemius muscle thickness and pennation angle. A 6-minute walk and sit to stand task were also performed by all participants. RESULTS: The middle-aged women had significantly lower MVIC torque (141 ± 49 vs. 109 ± 30 Nm, P = 0.031), absolute RTD (753.0 ± 313.6 vs. 423.0 ± 156.1 Nm/s, P = 0.001), and normalized peak RTD (554.0 ± 191.0 vs. 388.0 ± 91.9% MVIC/s, P = 0.001). Normalized early RTD0-50 and late RTD100-200, voluntary activation, and EMG were similar between groups. Resting twitch data showed that time to peak (124.0 ± 20.4 vs. 143.0 ± 16.7 ms, P = 0.002) and half relaxation time (73.1 ± 15.2 vs. 107.0 ± 28.2 ms, P < 0.001) was significantly faster for the young women. Thickness was greater in the lateral gastrocnemius (1.6 ± 0.2 vs. 1.4 ± 0.2 cm, P = 0.006) for the young women. Pennation angle of both muscles were greater for the young women (15.8 ± 3.9 vs. 13.1 ± 2.7 degrees, P < 0.05). Performance of the 6-minute walk was similar between groups, however, the young women performed more repetitions during the sit to stand task (25.6 ± 6.7 vs. 18.3 ± 4.7 reps, P < 0.001). CONCLUSION: Compared to young women, middle-age women were shown to have lower MVIC torque, peak RTD, and functional performance. Muscle architecture and contractile properties are affected by aging.

Choline-Based Multi-Ingredient Supplementation Can Improve Explosive Strength during a Fatiguing Task.

Various choline-based multi-ingredient supplementations (CMS) have been suggested in the current market, but the research is limited. The purpose of this study was to investigate the acute effect of a CMS on physical performance. Fourteen male college football players (20.4 ± 1.0 years) participated in a randomized double-blind crossover experiment separated by 7 days. Subjects were given a CMS or a placebo 60 min before physical performance testing measures, including maximum vertical jumps, maximum voluntary isometric contractions (MVIC), maximal voluntary concentric contractions (MVCC), and fatiguing contractions. Four MVICs and seven sets of two MVCCs at various loads (1 N·m to 60% MVIC torque) were performed with the knee extensor muscles while seated on a dynamometer before and after the fatiguing tasks. During the fatiguing tasks, 120 MVCCs (4 sets × 30 reps) were performed with a load equivalent to 20% MVIC. Twitch interpolation technique was used to assess muscle contractile properties and voluntary activation. No significant differences were seen at baseline between sessions for all testing measures including vertical jump height, strength, power, muscle contractile properties and voluntary activation. Rate of torque development and impulse was higher in supplemental session compared to control session throughout the fatiguing contractions (p = 0.018, p < 0.001, respectively). Acute CMS can improve explosive strength by delaying the onset of fatigue.

The Influence of Thermal Alterations on Prefrontal Cortex Activation and Neuromuscular Function during a Fatiguing Task.

The purpose of this study was to examine prefrontal cortex (PFC) activation, neuromuscular function, and perceptual measures in response to a fatiguing task, following thermal alterations of an exercising arm. Nineteen healthy adults completed three experimental sessions. At baseline, participants performed maximum voluntary isometric contractions (MVIC) of the elbow flexors. Next, participants submerged their right arm in a water bath for 15 min. Cold (C), neutral (N), and hot (H) water temperatures were maintained at 8, 33, and 44 °C, respectively. Following water immersion, participants performed an isometric elbow flexion contraction, at 20% of their MVIC, for 5 min. Ratings of perceived exertion (RPE), muscular discomfort, and task demands were assessed. Functional near-infrared spectroscopy was used to measure activation (oxygenation) of the PFC during the fatiguing task. Reductions in MVIC torque at the end of the fatiguing task were greater for the H (25.7 ± 8.4%) and N (22.2 ± 9.6%) conditions, compared to the C condition (17.5 ± 8.9%, p < 0.05). The increase in oxygenation of the PFC was greater for the H (13.3 ± 4.9 µmol/L) and N (12.4 ± 4.4 µmol/L) conditions, compared to the C condition (10.3 ± 3.8 µmol/L, p < 0.001) at the end of the fatiguing task. The increase in RPE, muscular discomfort, and task demands were greater in the H condition compared to the N and C conditions (p < 0.01). These results indicate that precooling an exercising arm attenuates the rise in PFC activation, muscle fatigue, and psychological rating during a fatiguing task.

Sex Differences in Mechanisms of Recovery after Isometric and Dynamic Fatiguing Tasks.

PURPOSE: The purpose of this study was to determine whether supraspinal mechanisms contribute to the sex difference in fatigability during and recovery from a dynamic and isometric fatiguing task with the knee extensors. METHODS: Transcranial magnetic stimulation and electrical stimulation were used to determine voluntary activation and contractile properties of the knee extensors in 14 men and 17 women (20.8 ± 1.9 yr) after a 1) 60-s sustained, maximal voluntary isometric contraction (MVIC), and 2) dynamic fatiguing task involving 120 maximal voluntary concentric contractions with a 20% MVIC load. RESULTS: There were no differences between men and women in the reduction of maximal torque during the sustained MVIC (54.4% ± 18.9% vs 55.9% ± 11.2%, P = 0.49) or in the decrease in power during the dynamic fatiguing task (14.7% ± 20.1% vs 14.2% ± 18.5%, P = 0.92). However, MVIC torque recovered more quickly for women than men after the sustained MVIC and the dynamic task (P < 0.05). The transcranial magnetic stimulation-elicited superimposed twitch was larger for men than for women during the sustained MVIC and in recovery (immediately post, R0.1: 4.7% ± 3.3% vs 2.4% ± 1.9% MVIC; P = 0.02), with no sex difference after the dynamic task (P = 0.35). The reduction in resting twitch amplitude was larger for men than for women immediately after the dynamic task (37% ± 22% vs 23% ± 18%; P = 0.016) with no sex difference after the sustained MVIC (64% ± 16% vs 67% ± 11%; P = 0.46). CONCLUSIONS: Supraspinal fatigue contributed to fatigability of the knee extensors more for men than for women after a maximal isometric task, whereas contractile mechanisms explained the sex difference in torque recovery after the fast-velocity dynamic task. The mechanisms for the sex difference in fatigability are task dependent.

The effect of cold ambient temperatures on climbing-specific finger flexor performance.

Different ambient temperatures are known to affect muscular performance based on the type of contraction. The effect of cold (10°C) and thermoneutral (TN) (24°C) ambient temperatures on finger flexor performance was examined in 12 rock climbers. After 30 min of seated rest in the designated temperature condition, participants completed maximal voluntary contractions (MVC) on a climbing-specific finger flexor assessment device equipped with a crimp grip hold. Participants then completed an intermittent fatiguing task until failure. The fatiguing task consisted of 10-s contractions at 40% MVC followed by a 3-s of rest. MVC recovery was assessed immediately, 5, 10, and 15 min post-task failure. Estimated muscle temperature and subjective thermal ratings were significantly lower throughout testing in the cold condition (P < .001). Finger flexor MVC strength was similar between conditions at baseline and throughout recovery. Time to task failure was significantly longer (364 ± 135 vs. 251 ± 97 s, P = .003) and force time integral was greater (53,715 ± 19,988 vs. 40,243 ± 15,360 Ns, P = .001) during the cold condition. No significant differences were found between conditions for force variability or electromyography (EMG) at the start and end of the fatiguing task. However, the rate of increase in EMG for the TN condition was significantly faster (P = .03). These results suggest important implications for researchers when examining climbing performance, especially in outdoor settings where temperatures may vary from day to day. Inconsistencies in testing temperatures might significantly affect muscular endurance.

Sex differences in neuromuscular function after repeated eccentric contractions of the knee extensor muscles.

PURPOSE: This study examined the mechanisms for force and power reduction during and up to 48 h after maximal eccentric contractions of the knee extensor muscles in young men and women. METHODS: 13 men (22.8 ± 2.6 years) and 13 women (21.6 ± 2.2 years) performed 150 maximal effort eccentric contractions (5 sets of 30) with the knee extensor muscles at 60° s-1. Maximal voluntary isometric contractions (MVIC) and maximal voluntary concentric contractions (MVCC) were performed before and after the 150 eccentric contractions. The MVCCs involved a set of two isokinetic contractions at 60° s-1 and sets of isotonic contractions performed at seven different resistance loads (1 N m, 10, 20, 30, 40, 50, and 60% MVIC). Electrical stimulation was used during the MVICs and at rest to determine changes in voluntary activation and contractile properties. RESULTS: At baseline, men were stronger than women (MVIC: 276 ± 48 vs. 133 ± 37 N m) and more powerful (MVCC: 649 ± 77 vs. 346 ± 78 W). At termination of the eccentric contractions, voluntary activation, resting twitch amplitude, and peak power during concentric contractions at the seven loads and at 60° s-1 decreased (P < 0.05) similarly in the men and women. At 48 h post-exercise, the MVIC torque, power (for loads ≥20-60% MVIC), and voluntary activation remained depressed (P < 0.05), but the resting twitch had returned to baseline (P > 0.05) with no sex differences. CONCLUSION: Central mechanisms were primarily responsible for the depressed maximal force production up to 48 h after repeated eccentric contractions of the knee extensors and these mechanisms were similar in men and women.

Age differences in dynamic fatigability and variability of arm and leg muscles: Associations with physical function.

INTRODUCTION: It is not known whether the age-related increase in fatigability of fast dynamic contractions in lower limb muscles also occurs in upper limb muscles. We compared age-related fatigability and variability of maximal-effort repeated dynamic contractions in the knee extensor and elbow flexor muscles; and determined associations between fatigability, variability of velocity between contractions and functional performance. METHODS: 35 young (16 males; 21.0±2.6years) and 32 old (18 males; 71.3±6.2years) adults performed a dynamic fatiguing task involving 90 maximal-effort, fast, concentric, isotonic contractions (1 contraction/3s) with a load equivalent to 20% maximal voluntary isometric contraction (MVIC) torque with the elbow flexor and knee extensor muscles on separate days. Old adults also performed tests of balance and walking endurance. RESULTS: Old adults had greater fatigue-related reductions in peak velocity compared with young adults for both the elbow flexor and knee extensor muscles (P<0.05) with no sex differences (P>0.05). Old adults had greater variability of peak velocity during the knee extensor, but not during the elbow flexor fatiguing task. The age difference in fatigability was greater for the knee extensor muscles (35.9%) compared with elbow flexor muscles (9.7%, P<0.05). Less fatigability of the knee extensor muscles was associated with greater walking endurance (r=-0.34, P=0.048) and balance (r=-0.41, P=0.014) among old adults. CONCLUSIONS: An age-related increase in fatigability of a dynamic fatiguing task was greater for the knee extensor compared with the elbow flexor muscles in males and females, and greater fatigability was associated with lesser walking endurance and balance.

Sex differences with aging in the fatigability of dynamic contractions.

This study determined the sex difference with aging in fatigability of the elbow flexor muscles during a dynamic fatiguing task, and explored the associated mechanisms. We compared fatigability of the elbow flexor muscles in 18 young (20.2 ± 1 years: 9 men) and 36 old adults (73.5 ± 1 years: 16 men) during and in recovery from repeated dynamic contractions (~60°/s) with a load equivalent to 20% of maximal voluntary isometric contraction (MVIC) torque until failure. Transcranial magnetic stimulation (TMS) was used to assess supraspinal fatigue (an increase in the superimposed twitch, SIT) and the peak rate of muscle relaxation. Time to failure was briefer for the men than the women (6.1 ± 2.1 vs. 9.7 ± 5.5 min, respectively; P=0.02) with no difference between young and old adults (7.2 ± 2.9 vs. 8.4 ± 5.2 min, respectively, P=0.45) and no interaction (P>0.05). The relative decline in peak relaxation rate with fatigability was similar for young and old adults (P=0.11), but greater for men than women (P=0.046). Supraspinal fatigue increased for all groups and was associated with the time to failure (P<0.05). Regression analysis however, indicated that the time to failure was best predicted by the peak relaxation rate (baseline values and slowing with fatigability) (r(2)=0.55). Rate-limiting contractile mechanisms (e.g. excitation-contraction coupling) were responsible for the increased fatigability of the elbow flexors of men compared with women for a dynamic fatiguing task of slow angular velocity, and this sex difference was maintained with aging. The age difference in fatigability for the dynamic task was diminished for both sexes relative to what is typically observed with isometric fatiguing contractions.

Sex Differences in Arm Muscle Fatigability With Cognitive Demand in Older Adults.

BACKGROUND: Muscle fatigability can increase when a stressful, cognitively demanding task is imposed during a low-force fatiguing contraction with the arm muscles, especially in women. Whether this occurs among older adults (>60 years) is currently unknown. QUESTIONS/PURPOSES: We aimed to determine if higher cognitive demands, stratified by sex, increased fatigability in older adults (>60 years). Secondarily, we assessed if varying cognitive demand resulted in decreased steadiness and was explained by anxiety or cortisol levels. METHODS: Seventeen older women (70±6 years) and 13 older men (71±5 years) performed a sustained, isometric, fatiguing contraction at 20% of maximal voluntary contraction until task failure during three sessions: high cognitive demand (high CD=mental subtraction by 13); low cognitive demand (low CD=mental subtraction by 1); and control (no subtraction). RESULTS: Fatigability was greater when high and low CD were performed during the fatiguing contraction for the women but not for the men. In women, time to failure with high CD was 16±8 minutes and with low CD was 17±4 minutes, both of which were shorter than time to failure in control contractions (21±7 minutes; high CD mean difference: 5 minutes [95% confidence interval {CI}, 0.78-9.89], p=0.02; low CD mean difference: 4 minutes [95% CI, 0.57-7.31], p=0.03). However, in men, no differences were detected in time to failure with cognitive demand (control: 13±5 minutes; high CD mean difference: -0.09 minutes [95% CI, -2.8 to 2.7], p=1.00; low CD mean difference: 0.75 minutes [95% CI, -1.1 to 2.6], p=0.85). Steadiness decreased (force fluctuations increased) more during high CD than control. Elevated anxiety, mean arterial pressure, and salivary cortisol levels in both men and women did not explain the greater fatigability during high CD. CONCLUSIONS: Older women but not men showed marked increases in fatigability when low or high CD was imposed during sustained static contractions with the elbow flexor muscles and contrasts with previous findings for the lower limb. Steadiness decreased in both sexes when high CD was imposed. CLINICAL RELEVANCE: Older women are susceptible to greater fatigability of the upper limb with heightened mental activity during sustained postural contractions, which are the foundation of many work-related tasks.

Age and sex differences in steadiness of elbow flexor muscles with imposed cognitive demand.

PURPOSE: These studies determined (1) age- and sex-related differences in steadiness of isometric contractions when high cognitive demand was imposed across a range of forces with the elbow flexor muscles (study 1) and; (2) sex differences in steadiness among older adults when low cognitive demand was imposed (study 2). METHODS: 36 young adults (18-25 years; 18 women) and 30 older adults (60-82 years; 17 women) performed isometric contractions at 5, 30 and 40 % of maximum voluntary contraction (MVC). Study 1 involved a high-cognitive demand session (serial subtractions by 13 during the contraction) and a control session (no mental math). Study 2 (older adults only) involved a low-cognitive demand session (subtracting by 1s). RESULTS: Older individuals exhibited greater increases in force fluctuations (coefficient of variation of force, CV) with high cognitive demand than young adults, with the largest age difference at 5 % MVC (P = 0.01). Older adults had greater agonist EMG activity with high-cognitive demand and women had greater coactivation than men (P < 0.05). In study 2, CV of force increased with low cognitive demand for the older women but not for the older men (P = 0.03). CONCLUSION: Older adults had reduced steadiness and increased muscle activation when high cognitive demand was imposed while low cognitive demand induced increased force fluctuations in older women but not older men. These findings have implications for daily and work-related tasks that involve cognitive demand performed simultaneously during submaximal isometric contractions in an aging workforce.

Motor Variability during Sustained Contractions Increases with Cognitive Demand in Older Adults.

To expose cortical involvement in age-related changes in motor performance, we compared steadiness (force fluctuations) and fatigability of submaximal isometric contractions with the ankle dorsiflexor muscles in older and young adults and with varying levels of cognitive demand imposed. Sixteen young (20.4 ± 2.1 year: 8 men, 9 women) and 17 older adults (68.8 ± 4.4 years: 9 men, 8 women) attended three sessions and performed a 40 s isometric contraction at 5% maximal voluntary contraction (MVC) force followed by an isometric contraction at 30% MVC until task failure. The cognitive demand required during the submaximal contractions in each session differed as follows: (1) high-cognitive demand session where difficult mental math was imposed (counting backward by 13 from a 4-digit number); (2) low-cognitive demand session which involved simple mental math (counting backward by 1); and (3) control session with no mental math. Anxiety was elevated during the high-cognitive demand session compared with other sessions for both age groups but more so for the older adults than young adults (p < 0.05). Older adults had larger force fluctuations than young adults during: (1) the 5% MVC task as cognitive demand increased (p = 0.007), and (2) the fatiguing contraction for all sessions (p = 0.002). Time to task failure did not differ between sessions or age groups (p > 0.05), but the variability between sessions (standard deviation of three sessions) was greater for older adults than young (2.02 ± 1.05 vs. 1.25 ± 0.51 min, p < 0.05). Thus, variability in lower limb motor performance for low- and moderate-force isometric tasks increased with age and was exacerbated when cognitive demand was imposed, and may be related to modulation of synergist and antagonist muscles and an altered neural strategy with age originating from central sources. These data have significant implications for cognitively demanding low-force motor tasks that are relevant to functional and ergonomic in an aging workforce.

Brain areas associated with force steadiness and intensity during isometric ankle dorsiflexion in men and women.

Although maintenance of steady contractions is required for many daily tasks, there is little understanding of brain areas that modulate lower limb force accuracy. Functional magnetic resonance imaging was used to determine brain areas associated with steadiness and force during static (isometric) lower limb target-matching contractions at low and high intensities. Fourteen young adults (6 men and 8 women; 27.1 ± 9.1 years) performed three sets of 16-s isometric contractions with the ankle dorsiflexor muscles at 10, 30, 50, and 70 % of maximal voluntary contraction (MVC). Percent signal changes (PSCs, %) of the blood oxygenation level-dependent response were extracted for each contraction using region of interest analysis. Mean PSC increased with contraction intensity in the contralateral primary motor area (M1), supplementary motor area, putamen, pallidum cingulate cortex, and ipsilateral cerebellum (p < 0.05). The amplitude of force fluctuations (standard deviation, SD) increased from 10 to 70 % MVC but relative to the mean force (coefficient of variation, CV %) was greatest at 10 % MVC. The CV of force was associated with PSC in the ipsilateral parietal lobule (r = -0.28), putamen (r = -0.29), insula (r = -0.33), and contralateral superior frontal gyrus (r = -0.33, p < 0.05). There were minimal sex differences in brain activation across the isometric motor tasks indicating men and women were similarly motivated and able to activate cortical motor centers during static tasks. Control of steady lower limb contractions involves cortical and subcortical motor areas in both men and women and provides insight into key areas for potential cortical plasticity with impaired or enhanced leg function.

Stressor-induced increase in muscle fatigability of young men and women is predicted by strength but not voluntary activation.

This study investigated mechanisms for the stressor-induced changes in muscle fatigability in men and women. Participants performed an isometric-fatiguing contraction at 20% maximal voluntary contraction (MVC) until failure with the elbow flexor muscles. Study one (n = 55; 29 women) involved two experimental sessions: 1) a high-stressor session that required a difficult mental-math task before and during a fatiguing contraction and 2) a control session with no mental math. For some participants (n = 28; 14 women), cortical stimulation was used to examine mechanisms that contributed to muscle fatigability during the high-stressor and control sessions. Study two (n = 23; nine women) determined the influence of a low stressor, i.e., a simple mental-math task, on muscle fatigability. In study one, the time-to-task failure was less for the high-stressor session than control (P < 0.05) for women (19.4%) and men (9.5%): the sex difference response disappeared when covaried for initial strength (MVC). MVC force, voluntary activation, and peak-twitch amplitude decreased similarly for the control and high-stressor sessions (P < 0.05). In study two, the time-to-task failure of men or women was not influenced by the low stressor (P > 0.05). The greater fatigability, when exposed to a high stressor during a low-force task, was not exclusive to women but involved a strength-related mechanism in both weaker men and women that accelerated declines in voluntary activation and slowing of contractile properties.

Corticomotor excitability during a noxious stimulus before and after exercise in women with fibromyalgia.

The purposes of this study were to assess corticomotor excitability in people with fibromyalgia during a noxious stimulus before and after fatiguing exercise and examine associations with pain perception. Fifteen women with fibromyalgia completed three sessions: one familiarization and two experimental. The experimental sessions were randomized and involved measurement of pain perception and motor evoked potentials before and after (1) quiet rest and (2) isometric contraction of the elbow flexor muscles. Motor evoked potential amplitude of brachioradialis muscle was measured following transcranial magnetic stimulation delivered before, during, and after a noxious mechanical stimulus. After quiet rest, there was no change in pain perception. After the submaximal contraction, there was considerable variability in the pain response. Based on the changes in the experimental pain, subjects were divided into three groups (increase, decrease, and no change in pain). There was an interaction between pain response and the pain-induced change in motor evoked potentials. Those individuals who had an increase in motor evoked potentials during the pain test had an increase in pain after exercise. Thus, women with fibromyalgia were classified based on their pain response to exercise, and this response was associated with the change in corticomotor excitability during the application of a noxious stimulus.

Fatigue and recovery from dynamic contractions in men and women differ for arm and leg muscles.

INTRODUCTION: Whether there is a gender difference in fatigue and recovery from maximal velocity fatiguing contractions and across muscles is not understood. METHODS: Sixteen men and 19 women performed 90 isotonic contractions at maximal voluntary shortening velocity (maximal velocity concentric contractions, MVCC) with the elbow flexor and knee extensor muscles (separate days) at a load equivalent to 20% maximal voluntary isometric contraction (MVIC). RESULTS: Power (from MVCCs) decreased similarly for men and women for both muscles (P > 0.05). Men and women had similar declines in MVIC of elbow flexors, but men had greater reductions in knee extensor MVIC force and MVIC electromyogram activity than women (P < 0.05). The decline in MVIC and power was greater, and force recovery was slower for the elbow flexors compared with knee extensors. CONCLUSIONS: The gender difference in muscle fatigue often observed during isometric tasks was diminished during fast dynamic contractions for upper and lower limb muscles.

Fatigability and recovery of arm muscles with advanced age for dynamic and isometric contractions.

This study determined whether age-related mechanisms can increase fatigue of arm muscles during maximal velocity dynamic contractions, as it occurs in the lower limb. We compared elbow flexor fatigue of young (n=10, 20.8±2.7 years) and old men (n=16, 73.8±6.1 years) during and in recovery from a dynamic and an isometric postural fatiguing task. Each task was maintained until failure while supporting a load equivalent to 20% of maximal voluntary isometric contraction (MVIC) torque. Transcranial magnetic stimulation (TMS) was used to assess supraspinal fatigue (superimposed twitch, SIT) and muscle relaxation. Time to failure was longer for the old men than for the young men for the isometric task (9.5±3.1 vs. 17.2±7.0 min, P=0.01) but similar for the dynamic task (6.3±2.4 min vs. 6.0±2.0 min, P=0.73). Initial peak rate of relaxation was slower for the old men than for the young men, and was associated with a longer time to failure for both tasks (P<0.05). Low initial power during elbow flexion was associated with the greatest difference (reduction) in time to failure between the isometric task and the dynamic task (r=-0.54, P=0.015). SIT declined after both fatigue tasks similarly with age, although the recovery of SIT was associated with MVIC recovery for the old (both sessions) but not for the young men. Biceps brachii and brachioradialis EMG activity (% MVIC) of the old men were greater than that of the young men during the dynamic fatiguing task (P<0.05), but were similar during the isometric task. Muscular mechanisms and greater relative muscle activity (EMG activity) explain the greater fatigue during the dynamic task for the old men compared with the young men in the elbow flexor muscles. Recovery of MVC torque however relies more on the recovery of supraspinal fatigue among the old men than among the young men.

Supraspinal fatigue impedes recovery from a low-intensity sustained contraction in old adults.

This study determined the contribution of supraspinal fatigue and contractile properties to the age difference in neuromuscular fatigue during and recovery from a low-intensity sustained contraction. Cortical stimulation was used to evoke measures of voluntary activation and muscle relaxation during and after a contraction sustained at 20% of maximal voluntary contraction (MVC) until task failure with elbow flexor muscles in 14 young adults (20.9 ± 3.6 yr, 7 men) and 14 old adults (71.6 ± 5.4 yr, 7 men). Old adults exhibited a longer time to task failure than the young adults (23.8 ± 9.0 vs. 11.5 ± 3.9 min, respectively, P < 0.001). The time to failure was associated with initial peak rates of relaxation of muscle fibers and pressor response (P < 0.05). Increments in torque (superimposed twitch; SIT) generated by transcranial magnetic stimulation (TMS) during brief MVCs, increased during the fatiguing contraction (P < 0.001) and then decreased during recovery (P = 0.02). The increase in the SIT was greater for the old adults than the young adults during the fatiguing contraction and recovery (P < 0.05). Recovery of MVC torque was less for old than young adults at 10 min post-fatiguing contraction (75.1 ± 8.7 vs. 83.6 ± 7.8% of control MVC, respectively, P = 0.01) and was associated with the recovery of the SIT (r = -0.59, r(2) = 0.35, P < 0.001). Motor evoked potential (MEP) amplitude and the silent period elicited during the fatiguing contraction increased less for old adults than young adults (P < 0.05). The greater fatigue resistance with age during a low-intensity sustained contraction was attributable to mechanisms located within the muscle. Recovery of maximal strength after the low-intensity fatiguing contraction however, was impeded more for old adults than young because of greater supraspinal fatigue. Recovery of strength could be an important variable to consider in exercise prescription of old populations.

Supraspinal fatigue is similar in men and women for a low-force fatiguing contraction.

PURPOSE: This study determined the contribution of supraspinal fatigue to the sex difference in neuromuscular fatigue for a low-intensity fatiguing contraction. Because women have greater motor responses to arousal than men, we also examined whether cortical and motor nerve stimulation, techniques used to quantify central fatigue, would alter the sex difference in muscle fatigue. METHODS: In study 1, cortical stimulation was elicited during maximal voluntary contractions (MVC) before and after a submaximal isometric contraction at 20% MVC with the elbow flexor muscles in 29 young adults (20 ± 2.6 yr, 14 men). In study 2, 10 men and 10 women (19.1 ± 2.9 yr) performed a fatiguing contraction in the presence and absence of cortical and motor nerve stimulation. RESULTS: Study 1: Men had a briefer time to task failure than women (P = 0.009). Voluntary activation was reduced after the fatiguing contraction (P < 0.001) similarly for men and women. Motor-evoked potential area and the EMG silent period increased similarly with fatigue for both sexes. Peak relaxation rates, however, were greater for men than women and were associated with time to task failure (P < 0.05). Force fluctuations, RPE, HR, and mean arterial pressure increased at a greater rate for men than for women during the fatiguing contraction (P < 0.05). Study 2: Time to task failure, force fluctuations, and all other physiological variables assessed were similar for the control session and stimulation session (P > 0.05) for both men and women. CONCLUSIONS: Supraspinal fatigue was similar for men and women after the low-force fatiguing contraction, and the sex difference in muscle fatigue was associated with peripheral mechanisms. Furthermore, supraspinal fatigue can be quantified in both men and women without influencing motor performance.

Age and load compliance alter time to task failure for a submaximal fatiguing contraction with the lower leg.

The purpose of this study was to compare the time to failure and muscle activation of young and old adults for a sustained isometric submaximal contraction with the dorsiflexor muscles when the foot was restrained to a force transducer (force-control task) compared with supporting an equivalent inertial load unrestrained in the sagittal plane (position-control task). Seventeen young (23.6+/-6.5 yr) and 12 old (70.0+/-5.0 yr) adults performed the force-control and position-control tasks at 30% maximal voluntary contraction (MVC) until task failure on separate days. Despite the similar load torque for each task, time to failure was longer for the force-control than position-control task (10.4+/-4.5 vs. 8.6+/-3.4 min, P=0.03) for the young and old adults. The old adults, however, had a longer time to task failure than the young adults for both tasks (11.4+/-4.4 vs. 8.1+/-2.1 min, P=0.01), with no interaction of age and task (P=0.83). The rate of increase in agonist and antagonist root-mean-square EMG, agonist EMG bursting activity, mean arterial pressure, and heart rate during the fatiguing contraction was greater for the position-control than force-control task for the young and old adults. The old adults had a less rapid rate of increase in EMG activity, fluctuations in motor output, and cardiovascular measures than the young adults for both tasks. Development of fatigue can be manipulated in young and old adults by providing greater support to the foot and less ankle compliance during daily and ergonomic tasks that require prolonged activation of the lower leg. Minimizing load compliance to one degree of freedom during a position-control task maintained the greater fatigue resistance with age for an isometric contraction.