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A significant increase in circulating cell-free DNA (cfDNA) occurs with physical exercise, which depends on the type of exertion and the duration. The aims of this study were as follows: (1) to investigate the time course of cfDNA and conventional markers of muscle damage from immediately after to 96 h after muscle-damaging exercise; and (2) to investigate the relationship between cfDNA and indicators of primary (low-frequency fatigue and maximal voluntary isometric contraction) and secondary (creatine kinase and delayed-onset muscle soreness) muscle damage in young healthy males. Fourteen participants (age, 22 ± 2 years; weight, 84.4 ± 11.2 kg; height, 184.0 ± 7.4 cm) performed 50 intermittent drop jumps at 20 s intervals. We measured cfDNA and creatine kinase concentrations, maximal voluntary isometric contraction torque, low-frequency fatigue and delayed-onset muscle soreness before and at several time points up to 96 h after exercise. Plasma cfDNA levels increased from immediately postexercise until 72 h postexercise (P < 0.01). Elevation of postexercise cfDNA was correlated with both more pronounced low-frequency fatigue (r = -0.52, P = 3.4 × 10 -11 ) and delayed-onset muscle soreness (r = 0.32, P = 0.00019). Levels of cfDNA change in response to severe primary and secondary muscle damage after exercise. Levels of cfDNA exhibit a stronger correlation with variables related to primary muscle damage than to secondary muscle damage, suggesting that cfDNA is a more sensitive marker of acute loss of muscle function than of secondary inflammation or damaged muscle fibres., (© 2024 The Author(s). Experimental Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)

Purpose: The aim of this study was to follow post-activation potentiation (PAP), low-frequency fatigue (LFF), metabolic-induced fatigue and post-contractile depression (PCD) in response to different isometric muscle contraction modalities., Methods: Young healthy men (N = 120) were randomly assigned to one of ten exercise modality groups which differed in contraction duration (5-60 s), activation pattern (intermittent or continuous contractions), activation mode (voluntary or stimulated), and intensity [maximal or submaximal (50%)]. Isometric maximal voluntary contraction (MVC), and electrically induced knee extension torque were measured at baseline and at regular intervals for 60 min after exercise., Results: Muscle contraction modalities involving 5 s MVC were the most effective for PAP, whereas the lowest PAP effectiveness was found after the 12 × 5-MVC modality. After all of the 5-15 s MVC and 6 × 5-MVC protocols, the potentiation of the twitch rate was significantly higher than that recorded after continuous 30-60 s protocols (P < 0.001). Tetanic maximal torque (100 Hz) potentiation occurred 5 min after 15-30 s repetitive MVC modalities and after modality involving 15 electrical stimuli (P < 0.05)., Conclusions: The findings demonstrate that post-activation potentiation was most effective after brief duration continuous and repetitive MVC protocols. To understand the resultant warm-up of motor performance, it is necessary to recognize the coexistence of muscle PAP, tetanic maximal force potentiation, rapid recovery of metabolic muscle, and central muscle activation processes, as well as prolonged LFF and prolonged PCD.

Acute whole-body hyperthermia (WBH) increases blood markers concentration of stress, impairs motor drive to exercising muscles, and decreases resistance to neuromuscular fatigability. The functional natural residual consequences of WBH on neuromuscular functions remain unclear. We aimed to investigate the effects of residual WBH on voluntary and electrically induced ankle plantar flexor contractility properties, motor drive transmission (reflexes), muscle torque steadiness, resistance to neuromuscular fatigability, and markers of stress as the body temperature recovers naturally to normothermia. WBH was induced by Finnish sauna bathing in 16 apparently healthy young (24 ± 4 years) adult men. Motor performance was monitored before and 2 h after the sauna, and immediately after submaximal exercise (120 s at 50% of maximal voluntary contraction). Markers of stress were monitored before and 2 h after the sauna. Finnish sauna exposure induced moderate to severe WBH (rectal temperature, 38.5-39.6 °C). At 2 h after the sauna, rectal temperature had recovered to the preheating level (preheating 37.11 ± 0.33 °C versus postheating 37.00 ± 0.29 °C, p > .05). Post-sauna recovery was accompanied by slowed salivary free cortisol diurnal kinetics, whereas noradrenaline, dopamine, and serotonin did not persist into the 2 h recovery after the sauna. Although recovery to normothermia after a sauna led to a greater acceleration of muscle contractility properties and decreased muscle steadiness, sustained isometric submaximal contraction did not provoke greater neuromuscular fatigability., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Healthy aging is associated with a marked decline in motor performance. The functional consequences of applying varying novel or unexpected motor stimuli during intermittent isometric prolonged (fatiguing) motor tasks for lower limb neuromuscular fatigability and steadiness, perception of effort, and blood markers of stress in healthy aged men compared with young men have not been investigated. The participants in this study were 15 young men (aged 22 ± 4 years) and 10 older men (aged 67 ± 6 years). They performed 100 intermittent isometric knee extensions under three experimental conditions involving intermittent isometric contraction tasks according to constant, predictable, and unpredictable torque target sequences. The variability in maximal voluntary contraction averaged 50%, and was 25, 50, and 75% for the three strategies. All included a 5-s contraction and 20-s rest. The main variables were measured before exercise, after 100 repetitions, and 1 h after exercise. In all experimental trials, the decreases in the maximal voluntary contraction and central activation ratio, and the increases in effort sensation and muscle temperature, were smaller in older men than in younger men. The coefficient of variation during the motor performance did not differ between age groups. However, in all three strategies, the dopamine concentration was significantly higher in older than in younger men. The prolactin concentration did not differ significantly between age groups or conditions, although its decrease during loading correlated negatively with the central activation ratio.

Background and objectives : Fatigue during physical activity occurs because of decreased neuromuscular function. The aim of this study was to evaluate the effect of three different strategies based on motor task performance on neuromuscular fatigue in healthy men and men with multiple sclerosis (MS). Materials and Methods : We studied age-matched (18⁻43 years of age) healthy men ( n = 15) and men with MS ( n = 9). The inclusion criteria for MS subjects were a Kurtzke Expanded Disability Status Score <4 and a Fatigue Severity Scale Score >5. Both groups performed one of three exercise trials (with at least a 1-week interval between them) of 100 intermittent isometric knee extensions with flexion of 60°. The three different experimental conditions (ECs) were intermittent isometric contraction tasks with constant, predictable, and unpredictable torque target sequences. The variation of maximal voluntary contraction contractions (MVCs) within the strategies was 25%, 50%, and 75%, with a set average of 50%. All of them had a 5 s contraction and a 20 s rest period. The variables were measured: before exercise, after 100 repetitions (100-Reps), and 1 h after exercise. Results : In all EC tasks, the central activation ratio values of healthy and MS subjects were significantly different; however, no significant differences were observed among the EC tasks. No significant differences were seen in electrically induced torque, MVC torque, muscle temperature, subjective sensation of effort, coefficient of variation, or constant and absolute error after 100-Reps and 1 h after exercise between the two groups and in all EC tasks. Conclusions : Men with MS experienced higher central motor fatigue than did healthy men, but this had no effect on the variability, accuracy, or force sensation of the movements performed.

This study investigated possible differences using the same stretch-shortening exercise (SSE) protocol on generally accepted monitoring markers (dependent variables: changes in creatine kinase, muscle soreness, and voluntary and electrically evoked torque) in males across three lifespan stages (childhood versus adulthood versus old age). The protocol consisted of 100 intermittent (30 s interval between jumps) drop jumps to determine the repeated bout effect (RBE) (first and second bouts performed at a 2-week interval). The results showed that indirect symptoms of exercise-induced muscle damage after SSE were more expressed in adult males than in boys and elderly males, suggesting that the muscles of boys and elderly males are more resistant to exercise-induced damage than those of adult males. RBE was more pronounced in adult males than in boys and elderly males, suggesting that the muscles of boys and elderly males are less adaptive to exercise-induced muscle damage than those of adult males.

The aim of this study was to investigate the differences in the length-dependent changes in quadriceps muscle torque during voluntary isometric and isokinetic contractions performed after severe muscle-damaging exercise. Thirteen physically active men (age = 23.8 ± 3.2 years, body weight = 77.2 ± 4.5 kg) performed stretch-shortening cycle (SSC) exercise comprising 100 drop jumps with 30-second intervals between each jump. Changes in the voluntary and electrically evoked torque in concentric and isometric conditions at different muscle lengths, muscle soreness, and plasma creatine kinase (CK) activity were assessed within 72 hours after SSC exercise. Isokinetic knee extension torque decreased significantly (p < 0.05) at all joint angles after SSC exercise. At 2 minutes and at 72 hours after SSC exercise, the changes in knee torque were significantly smaller at 80° (where 180° = full knee extension) than at 110-130°. At 2 minutes after SSC exercise, the optimal angle for isokinetic knee extension torque shifted by 9.5 ± 8.9° to a longer muscle length (p < 0.05). Electrically induced torque at low-frequency (20-Hz) stimulation decreased significantly more at a knee joint angle of 130° than at 90°. The subjects felt acute muscle pain and CK activity in the blood increased to 1,593.9 ± 536.2 IU·L⁻¹ within 72 hours after SSC exercise (p < 0.05). This study demonstrates that the effect of muscle-damaging exercise on isokinetic torque is greatest for contractions at short muscle lengths. These findings have practical importance because the movements in most physical activities are dynamic in nature, and the decrease in torque at various points in the range of motion during exercise might affect overall performance.

Objective: The aim of this study was to evaluate the effect of secondary progressive MS disease on central and peripheral fatigue in women and men during continuous 2-min maximal voluntary contractions (MVC) of the quadriceps muscle., Methods: We studied age-matched (40-50-years of age) healthy individuals (men, n=9; women, n=10) and MS patients (men, n=9; women, n=9). The inclusion criteria for MS patients were a Kurtzke Expanded Disability Status Score <4 and a Fatigue Severity Scale Score >5., Results: The electrically-induced torque, MVC, and voluntary activation (VA) values were significantly greater in healthy persons than in MS patients. At the end of exercise, the fatigue index of MVC and VA was greater in MS patients than in the healthy cohort, whereas the electrically-induced torque was significantly decreased in healthy individuals. Peripheral fatigue was directly correlated with VA in MS patients; in contrast, it was inversely correlated with VA in healthy men. The variability of torque increased significantly during exercise and did not depend on MS and gender., Conclusions: MS increased central fatigue and diminished peripheral fatigue significantly during 2-min MVC; however, only peripheral fatigue was significantly greater in healthy men than in healthy women., Significance: These results suggest the specified pattern of central and peripheral fatigue in MS patients and could be further exploited for potential therapeutic uses, for instance, to model exercise-induced less central fatigue, and this may not be differentiated between men and women., (Copyright © 2010 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.)

Unlabelled: The aim of this study was to investigate the effect of heating and cooling on time course of voluntary and electrically induced muscle force variation., Material and Methods: Ten volunteers performed 50 maximal voluntary and electrically induced contractions of the knee extensors at an angle of 120 degrees under the control conditions and after passive lower body heating and cooling in the control, heating, and cooling experiments. Peak torque, torque variation, and half-relaxation time were assessed during the exercise., Results: Passive lower body heating increased muscle and core temperatures, while cooling lowered muscle temperature, but did not affect core temperature. We observed significantly lower muscle fatigue during voluntary contraction compared with electrically induced contractions. Body heating (opposite to cooling) increased involuntarily induced muscle force, but caused greater electrically induced muscle fatigue. In the middle of the exercise, the coefficient of correlation for electrically induced muscle torque decreased significantly as compared with the beginning of the exercise, while during maximal voluntary contractions, this relation for torque remained significant until the end of the exercise., Conclusion: It was shown that time course of voluntary contraction was more stable than in electrically induced contractions.

The aim of the study was to evaluate the effect of plyometric training (PT) on central and peripheral (muscle) fatigue in prepubertal girls and boys. The boys (n = 13, age 10.3 ± 0.3 years) and girls (n = 13, age, 10.2 ± 0.3 years) performed continuous 2-min maximal voluntary contractions (MVCs) before and after 16 high-intensity PT sessions. PT comprised two training sessions per week of 30 jumps in each session with 20 s between jumps. The greatest effect of PT was on excitation-contraction coupling, (twitch force increased by 323% in boys and 21% in girls) and height of a counter-movement jump (increased by 37% in boys and 38% in girls). In contrast, the quadriceps voluntary activation index, central activation ratio, and MVC did not change significantly after PT. The thickness of the quadriceps muscle increased by 9% in boys and 14% in girls after PT. In conclusion, boys and girls demonstrated similar changes in indicators of central fatigue (50-60% decrease) and peripheral fatigue (45-55% decrease) after MVC before and after PT.

Healthy untrained men performed 10 series of 12 knee eccentric extension repetitions (EE) at 160 degrees /s. The maximal voluntary isometric contraction force of the quadriceps muscle, the maximal rate of electrically induced torque development (RTD) and relaxation (RTR), isokinetic concentric torque at 30 degrees /s, the electrostimulation-induced torque at 20 and 100Hz frequencies were established before and after EE at shorter and longer muscle lengths. Besides, voluntary activation (VA) index and central activation ratio (CAR) were tested. There was more peripheral fatigue than central after EE. We established more central fatigue as well as low frequency fatigue at a shorter muscle length compared to the longer muscle length. Relative RTD as well as relative RTR, improved after EE and did not depend on the muscle length. Finally, central fatigue is inversely significantly related with the eccentric torque reduction during eccentric exercise and with the changes in muscle torque induced by low frequency stimulation., (Copyright (c) 2010 Elsevier Ltd. All rights reserved.)

We tested the hypothesis that improved physiological symptoms of passively induced heat acclimation (HA) might impair the impact of hyperthermia on central fatigue and consequently cause greater peripheral fatigue. Subjects in the experimental group (7 males and 6 females) completed seven sessions of HA involving passive heating of the lower body by immersion up to the waist in a water bath at approximately 44 degrees C (air T 23 degrees C, rh 40%) for 45 min, repeated every other day for 2 weeks. During the first and the last HA sessions, participants performed a 2-min MVC of the knee extensors. A TT100 Hz was superimposed on the contraction at about 3, 14, 29, 44, 59, 74, 89, 104, and 119 s. At about 30, 60, 90, and 120 s, the knee extensors were relaxed for 2-3 s and a control TT100 Hz was delivered. The participants in the control group (6 males and 6 females) followed the same exercise protocol on days 1 and 14 in a thermoneutral condition. Peak torque and the muscle voluntary activation and half-relaxation time were assessed during the exercise. The attainment of HA was confirmed by significant decreases in the resting and final rectal temperatures (approximately 0.3 degrees C), heart rate, and physiological stress index, and increased sweating capacity. Lower-body heating resulted in greater central and peripheral fatigue when compared with the thermoneutral condition during the 2-min MVC. Following HA, central and peripheral fatigues were unchanged. We conclude that passively induced HA for 2 weeks improved the physiological symptoms, but did not change central or peripheral fatigue during exercise in hyperthermia.

Background: We investigated the impact of 1) passive heating (PH) induced by single and intermittent/prolonged hot-water immersion (HWI) and 2) the duration of PH, on muscle contractile function under the unfatigued state, and during the development of muscle fatigue. Methods: Twelve young males volunteered for this study consisting of two phases: single phase (SP) followed by intermittent/prolonged phase (IPP), with both phases including two conditions (i.e., four trials in total) performed randomly: control passive sitting (CON) and HWI (44-45°C; water up to the waist level). SP-HWI included one continuous 45-min bath (from 15 to 60 min). IPP-HWI included an initial 45-min bath (from 15 to 60 min) followed by eight additional 15-min baths interspaced with 15-min breaks at room temperature between 75 and 300 min. Intramuscular (Tmu; measured in the vastus lateralis muscle) and rectal (Trec) temperatures were determined. Neuromuscular testing (performed in the knee extensors and flexors) was performed at baseline and 60 min later during SP, and at baseline, 60, 90, 150 and 300 min after baseline during IPP. A fatiguing protocol (100 electrical stimulations of the knee extensors) was performed after the last neuromuscular testing of each trial. Results: HWI increased Tmu and Trec to 38°C-38.5°C (p < 0.05) during both SP and IPP. Under the unfatigued state, HWI did not affect electrically induced torques at 20 Hz (P20) and 100 Hz (P100). However, it induced a shift towards a faster contractile profile during both SP and IPP, as evidenced by a decreased P20/P100 ratio (p < 0.05) and an improved muscle relaxation (i.e., reduced halfrelaxation time and increased rate of torque relaxation; p < 0.05). Despite a reduced voluntary activation (i.e., -2.63% ± 4.19% after SP-HWI and -5.73% ± 4.31% after IPP-HWI; condition effect: p < 0.001), HWI did not impair maximal isokinetic and isometric contraction torques. During the fatiguing protocol, fatigue index and the changes in muscle contractile properties were larger after HWI than CON conditions (p < 0.05). Finally, none of these parameters were significantly affected by the heating duration. Conclusion: PH induces changes in muscle contractile function which are not augmented by prolonged exposure when thermal stress is moderate. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study was to determine if aging would lead to greater decline in neuromuscular function during a fatiguing task under severe whole‐body hyperthermia conditions. Methods: Twelve young (aged 19–21 years) and 11 older (aged 65–80 years) males were enrolled in the study, which comprised a randomized control trial under a thermoneutral condition at an ambient temperature of 23°C (CON) and an experimental trial with passive lower body heating in 43°C water (HWI‐43°C). Changes in neuromuscular function and fatigability, and physical performance‐influencing factors such as psychological, thermoregulatory, neuroendocrine, and immune responses to whole‐body hyperthermia were measured. Results: A slower increase in rectal temperature, and a lower heart rate, thermal sensation, and sweating rate were observed in older males than young males in response to HWI‐43°C trial (p < 0.05). Nevertheless, prolactin increased more in response to hyperthermia in young males, while interleukin‐6 and cortisol levels increased more in older males (p < 0.05). Peripheral dopamine levels decreased in older males and increased in young males in response to hyperthermia (p < 0.05). Surprisingly, older males demonstrated greater neuromuscular fatigability resistance and faster maximal voluntary contraction (MVC) torque recovery after a 2‐min sustained isometric MVC task under thermoneutral and severe hyperthermic conditions (p < 0.05). Conclusion: Neuromuscular performance during fatigue‐provoking sustained isometric exercise under severe whole‐body hyperthermia conditions appears to decline in both age groups, but a lower relative decline in torque production for older males may relate to lower psychological and thermophysiological strain along with a diminished dopamine response and prolactin release. [ABSTRACT FROM AUTHOR]

Prolonged low‐frequency force depression (PLFFD) induced by fatiguing exercise is characterized by a persistent depression in submaximal contractile force during the recovery period. Muscle glycogen depletion is known to limit physical performance during prolonged low‐ and moderate‐intensity exercise, and accelerating glycogen resynthesis with post‐exercise carbohydrate intake can facilitate recovery and improve repeated bout exercise performance. Short‐term, high‐intensity exercise, however, can cause PLFFD without any marked decrease in glycogen. Here, we studied whether recovery from PLFFD was accelerated by carbohydrate ingestion after 60 minutes of moderate‐intensity glycogen‐depleting cycling exercise followed by six 30‐seconds all‐out cycling sprints. We used a randomized crossover study design where nine recreationally active males drank a beverage containing either carbohydrate or placebo after exercise. Blood glucose and muscle glycogen concentrations were determined at baseline, immediately post‐exercise, and during the 3‐hours recovery period. Transcutaneous electrical stimulation of the quadriceps muscle was performed to determine the extent of PLFFD by eliciting low‐frequency (20 Hz) and high‐frequency (100 Hz) stimulations. Muscle glycogen was severely depleted after exercise, with a significantly higher rate of muscle glycogen resynthesis during the 3‐hours recovery period in the carbohydrate than in the placebo trials (13.7 and 5.4 mmol glucosyl units/kg wet weight/h, respectively). Torque at 20 Hz was significantly more depressed than 100 Hz torque during the recovery period in both conditions, and the extent of PLFFD (20/100 Hz ratio) was not different between the two trials. In conclusion, carbohydrate supplementation enhances glycogen resynthesis after glycogen‐depleting exercise but does not improve force recovery when the exercise also involves all‐out cycling sprints. [ABSTRACT FROM AUTHOR]

We investigated the acute neuromuscular and stress responses to three different high-intensity interval training sessions in young (age 19.5±1.3 years) and older (age 65.7±2.8 years) women. Cycling exercise comprised either 6 × 5 s or 3 × 30 s all-out, or 3 × 60 s submaximal, efforts each performed 5 weeks apart in randomized order. Peak and average power was higher in young than in older women and was largest during the 6 × 5 s strategy in both groups (p<0.05). The decrease in the ratio of torques evoked by 20 and 100 Hz electrical stimulation, representing low-frequency fatigue, was more evident after the 3 × 30 and 3 × 60 s than the 6 × 5 s bout in both groups and was larger in young than in older women (p<0.05). Both groups preferred 6 × 5 s cycling for further training. In conclusion, in young women, very low volume (6 × 5 s) all-out exercise induces significant physiological stress and seems to be an effective means of training. For older women, longer exercise sessions (3 × 60 s) are more stressful than shorter ones but are still tolerable psychologically. [ABSTRACT FROM AUTHOR]

Introduction: The aim of this study was to follow circulating brain-derived neurotrophic factor (BDNF) and interleukin-6 (IL-6) levels in response to severe muscle-damaging exercise.Methods: Young healthy men (N = 10) performed a bout of mechanically demanding stretch-shortening cycle exercise consisting of 200 drop jumps. Voluntary and electrically induced knee extension torque, serum BDNF levels, and IL-6 levels were measured before and for up to 7 days after exercise.Results: Muscle force decreased by up to 40% and did not recover by 24 hours after exercise. Serum BDNF was decreased 1 hour and 24 hours after exercise, whereas IL-6 increased immediately and 1 hour after but recovered to baseline by 24 hours after exercise. IL-6 and 100-Hz stimulation torque were correlated (r = -0.64, P < 0.05) 24 hours after exercise.Discussion: In response to acute, severe muscle-damaging exercise, serum BDNF levels decrease, whereas IL-6 levels increase and are associated with peripheral fatigue. Muscle Nerve 57: E46-E51, 2018. [ABSTRACT FROM AUTHOR]

Introduction: In this study, we questioned whether local cooling of muscle or heating involving core and muscle temperatures are the main indicators for force variability. Methods: Ten volunteers performed a 2-min maximum voluntary contraction (MVC) of the knee extensors under control (CON) conditions after passive heating (HT) and cooling (CL) of the lower body. Results: HT increased muscle and rectal temperatures, whereas CL lowered muscle temperature but did not affect rectal temperature. During 2-min MVC, peak force decreased to a lower level in HT compared with CON and CL experiments. Greater central fatigue was found in the HT experiment, and there was less in the CL experiment than in the CON experiment. Conclusions: Increased core and muscle temperature increased physiological tremor and the amount and structural complexity of force variability of the exercising muscles, whereas local muscle cooling decreased all force variability variables measured. Muscle Nerve, 2012 [ABSTRACT FROM AUTHOR]

Abstract: The aim of this study was to investigate if voluntary activation and force variability during maximal voluntary contraction (MVC) depends more on muscle (local) or body (core) temperature. Ten volunteers performed a 2-min MVC of the knee extensors under the control (CON) conditions (ambient temperature (21°C), relative humidity (30%), and air velocity (∼0.1m/s)) as well as after heating (HT) and cooling (CL) of the lower body. During water manipulation procedure lower body was immersed up to the waist in a water bath at ∼44°C for 45min for HT experiment, and ∼15°C for 30min for CL experiment. Peak torque, torque variability, muscle voluntary activation and half-relaxation time were assessed during the exercise. HT increased muscle (2.8±0.2°C) and rectal (1.9±0.1°C) temperatures while CL lowered muscle (2.2±0.2°C) temperature, but did not affect rectal temperature. During 2-min MVC, peak torque decreased (P<0.05; SP>90%) and to a lower level in HT compared to CON and CL experiments (52.6±2.3% versus 69.0±2.3% and 65.6±1.9% MVC, respectively, P<0.05; SP>90%). Torque variability increased significantly during exercise and was significantly larger in HT and lower in CL compared to CON experiment. Voluntary activation of exercising muscle was more depressed in HT (i.e. greater central fatigue) and the smallest effect was found in CL compared to CON. In conclusion increased core and muscle temperature impairs voluntary activation and increases force variability of the exercising muscles while a local muscle cooling decrease force variability but has a small effect on central fatigue. [ABSTRACT FROM AUTHOR]

The aim of this study was to test the hypothesis that intraindividual variability in isokinetic knee extension/flexion relates positively to muscle fatigue, i.e., the less the muscle fatigue during exercise, the greater is the variability of torque fluctuation. Twelve healthy males performed 100 continuous knee extensions and flexions with maximal intensity at 180°/s. The main findings were: 1) the knee extension muscle exhibited more fatigue and less intraindividual variability than the knee flexion muscles; 2) intraindividual variability in knee extension (SD for torque fluctuation in 100 repetitions) relates closely to fatigue. We conclude that intraindividual variability in isokinetic knee-extension torque relates inversely to muscle fatigue. [ABSTRACT FROM AUTHOR]

The aim of this study was to test the effect of fatigue of the knee extensors muscles on bilateral force control accuracy, variability, and coordination in the presence and absence of visual feedback. Twenty-two young physically active subjects (18 males, 4 females) were divided into two groups and performed 210 submaximal sustained bilateral isometric contractions of knee extensors muscles with and without visual feedback. One group performed a symmetrical task-both legs were set at identical positions (60° knee flexion)-while the other group performed an asymmetrical task (60° and 30° knee flexion). We used the framework of the uncontrolled manifold hypothesis to quantify two variance components: one of them did not change total force (VUCM), while the other did (VORT). Performance of bilateral isometric contractions reduced voluntary and electrically induced force without changes in bilateral force control variability and accuracy. Bilateral force production stability and accuracy were higher in both tasks with visual feedback. Synergistic (anti-phase) structure of force control between the lower limbs occurred and the values of synergy index were higher only during the performance of the asymmetrical task with visual feedback. In addition, greater bilateral force control accuracy was observed during the performance of the asymmetrical task (with and without visual feedback), despite no differences in within-trial variability of both tasks. [ABSTRACT FROM AUTHOR]