Fatigue-induced Increase Research Articles

The Neuromuscular Fatigue-Induced Loss of Muscle Force Control.

Neuromuscular fatigue is characterised not only by a reduction in the capacity to generate maximal muscle force, but also in the ability to control submaximal muscle forces, i.e., to generate task-relevant and precise levels of force. This decreased ability to control force is quantified according to a greater magnitude and lower complexity (temporal structure) of force fluctuations, which are indicative of decreased force steadiness and adaptability, respectively. The "loss of force control" is affected by the type of muscle contraction used in the fatiguing exercise, potentially differing between typical laboratory tests of fatigue (e.g., isometric contractions) and the contractions typical of everyday and sporting movements (e.g., dynamic concentric and eccentric contractions), and can be attenuated through the use of ergogenic aids. The loss of force control appears to relate to a fatigue-induced increase in common synaptic input to muscle, though the extent to which various mechanisms (afferent feedback, neuromodulatory pathways, cortical/reticulospinal pathways) contribute to this remains to be determined. Importantly, this fatigue-induced loss of force control could have important implications for task performance, as force control is correlated with performance in a range of tasks that are associated with activities of daily living, occupational duties, and sporting performance.

The Impact Of Fatiguing, Intermittent Isometric Contractions On Muscle Force Variability

PURPOSE: To evaluate the influence of fatigue on force variability of the vastus lateralis (VL) during repeated isometric exercise performed at 30% maximal voluntary isometric contraction force (MVIC). METHODS: Twelve resistance-trained males (23.8 ± 3.6 y; 87.3 ± 10.5 kg; 176.3 ± 6 cm) completed MVIC testing and then repeated isometric contractions at 30% MVIC while tracking target force trajectories repetitively until they could no longer achieve 95% target force. Force and surface electromyographic (EMG) signals from the VL were collected during exercise. One-way repeated measures ANOVAs were used to analyze average force (FAVG), coefficient of variation of force (FCV), EMG amplitude (EMGRMS), and median power frequency (MDF) during every 25% of the repetitions (REP) completed. To examine the relationship between EMGRMS and FCV across the work bout, FCV was plotted against EMGRMS, linear regression lines were fit, and correlation coefficients were calculated for each subject. RESULTS: No main effect of REP was observed for FAVG (p = 0.07) or MDF (p = 0.29), but there was for FCV and EMGRMS (p < 0.0001). FCV increased from the first to the 50-100% REP (p ≤ 0.001-0.02), 25% to the 100% REP (p < 0.001), and the 50% to the 100% REP (p < 0.001). EMGRMS increased from the first to the 25-100% REP (p = 0.002-0.024), the 25% to the 50-100% REP (p = 0.001-0.019), the 50% to the 75-100% REP (p = 0.002-0.018) and the 75% to the 100% REP (p = 0.014). Additionally, all 12 subjects displayed strong, significant, positive relationships for FCV vs. EMGRMS across the exercise bout (Table 1). CONCLUSIONS: The increase in both FCV and EMGRMS suggest that as fatigue develops during intermittent isometric exercise, force steadiness decreases while EMG amplitude increases. It is possible that the decrease in force steadiness occurred due to increased common synaptic input to the motor neuron pool secondary to fatigue-induced increases in net excitatory input.

Effects of anti-pronation shoes on lower limb kinematics and kinetics in female runners with pronated feet: The role of physical fatigue.

Physical fatigue and pronated feet constitute two risk factors for running-related lower limb injuries. Accordingly, different running shoe companies designed anti-pronation shoes with medial support to limit over pronation in runners. However, there is little evidence on the effectiveness and clinical relevance of anti-pronation shoes. This study examined lower limb kinematics and kinetics in young female runners with pronated feet during running with anti-pronation versus regular (neutral) running shoes in unfatigued and fatigued condition. Twenty-six female runners aged 24.1±5.6 years with pronated feet volunteered to participate in this study. Kinetic (3D Kistler force plate) and kinematic analyses (Vicon motion analysis system) were conducted to record participants’ ground reaction forces and joint kinematics when running with anti-pronation compared with neutral running shoes. Physical fatigue was induced through an individualized submaximal running protocol on a motorized treadmill using rate of perceived exertion and heart rate monitoring. The statistical analyses indicated significant main effects of “footwear” for peak ankle inversion, peak ankle eversion, and peak hip internal rotation angles (p<0.03; d = 0.46–0.95). Pair-wise comparisons revealed a significantly greater peak ankle inversion angle (p<0.03; d = 0.95; 2.70°) and smaller peak eversion angle (p<0.03; d = 0.46; 2.53°) when running with anti-pronation shoes compared with neutral shoes. For kinetic data, significant main effects of “footwear” were found for peak ankle dorsiflexor moment, peak knee extensor moment, peak hip flexor moment, peak hip extensor moment, peak hip abductor moment, and peak hip internal rotator moment (p<0.02; d = 1.00–1.79). For peak positive hip power in sagittal and frontal planes and peak negative hip power in horizontal plane, we observed significant main effects of “footwear” (p<0.03; d = 0.92–1.06). Pairwise comparisons revealed that peak positive hip power in sagittal plane (p<0.03; d = 0.98; 2.39 w/kg), peak positive hip power in frontal plane (p = 0.014; d = 1.06; 0.54 w/kg), and peak negative hip power in horizontal plane (p<0.03; d = 0.92; 0.43 w/kg) were greater with anti-pronation shoes. Furthermore, the statistical analyses indicated significant main effects of “Fatigue” for peak ankle inversion, peak ankle eversion, and peak knee external rotation angles. Pair-wise comparisons revealed a fatigue-induced decrease in peak ankle inversion angle (p<0.01; d = 1.23; 2.69°) and a fatigue-induced increase in peak knee external rotation angle (p<0.05; d = 0.83; 5.40°). In addition, a fatigue-related increase was found for peak ankle eversion (p<0.01; d = 1.24; 2.67°). For kinetic data, we observed a significant main effect of “Fatigue” for knee flexor moment, knee internal rotator moment, and hip extensor moment (p<0.05; d = 0.83–1.01). The statistical analyses indicated significant a main effect of “Fatigue” for peak negative ankle power in sagittal plane (p<0.01; d = 1.25). Finally, we could not detect any significant footwear by fatigue interaction effects for all measures of joint kinetics and kinematics. Running in anti-pronation compared with neutral running shoes produced lower peak moments and powers in lower limb joints and better control in rear foot eversion. Physical fatigue increased peak moments and powers in lower limb joints irrespective of the type of footwear.

Caffeine improved cycling trial performance in mentally fatigued cyclists, regardless of alterations in prefrontal cortex activation

PurposeTo verify whether caffeine (CAF) could increase the prefrontal cortex (PFC) activation and improve 20 km cycling time trial (TT20km) performance in mentally fatigued cyclists. MethodsAfter preliminary TT20km, twelve recreational cyclists (VO2MAX of 58.9 ± 6.2 mL kg min−1) performed a familiarization with a cognitive test to induce mental fatigue (MF) and psychological scales. Thereafter, they performed: 2) a baseline TT20km; 3) a mentally fatigued TT20km (MF); 4 and 5) a mentally fatigued TT20km after CAF (MF + CAF) or placebo (MF + PLA) ingestion, in a double-blind, counterbalanced design. Performance and psychological responses were obtained throughout the TT20km, while PFC electroencephalography (EEG) theta wave was obtained before and after the mental fatigue test. ResultsThe mental fatigue-induced increase in EEG theta wave (↑ ~ 4.8%) was reverted with CAF (↓ 8.8%) and PLA ingestion (↓ 4.8%). CAF improved TT20km performance in mentally fatigued cyclists by reducing time (p = .00; ↓ ~ 1.7%) and increasing WMEAN (p = .00; ↑ ~ 3.6%), when compared to MF + PLA. The RPE-power output ratio was lower (p = .01), but affect (p = .018), motivation (p = .033) and emotional arousal (p = .001) were greater throughout the TT20km in MF + CAF than in MF + PLA. ConclusionsCAF ingestion improved TT20km performance and psychological responses in mentally fatigued cyclists, despite the unaltered PFC activation.

Effects of Fatigue on Voluntary Electromechanical and Relaxation Electromechanical Delay.

The purposes of the present study were to examine: 1) the effects of fatigue on electromechanical delay from the onsets of the electromyographic signal to force production (EMDE-F), the onsets of the electromyographic to mechanomyographic signals (EMDE-M), the onsets of the mechanomyographic signal to force production (EMDM-F), as well as the cessations of the electromyographic to force production (R-EMDE-F), cessation of the electromyographic to mechanomyographic signals (R-EMDE-M), and cessations of the mechanomyographic signal to force production (R-EMDM-F); and 2) the relative contributions from EMDE-M and EMDM-F to EMDE-F as well as R-EMDE-M and R-EMDM-F to R-EMDE-F from the vastus lateralis in non-fatigued and fatigued states. The values EMDE-F, EMDE-M, EMDM-F, R-EMDE-F, R-EMDE-M and R-EMDM-F were calculated during maximal voluntary isometric contractions, before and after 70% 1-repetition maximum leg extensions to failure. There were significant pretest to posttest increases in EMDE-F (73%;p<0.01), EMDE-M (99%;p<0.01), EMDM-F (60%;p<0.01), R-EMDE-F (101%;p<0.01) and R-EMDM-F (368%;p<0.01), but no significant change in R-EMDE-M (25%;p=0.46). Fatigue-induced increase in EMDE-F indicated excitation-contraction coupling failure (EMDE-M) and increases in the compliance of the series elastic component (EMDM-F). Increases in R-EMDE-F were due to increases in relaxation time for the series elastic component (R-EMDM-F), but not changes in the reversal of excitation-contraction coupling (R-EMDE-M).

Dynamic versus isometric electromechanical delay in non-fatigued and fatigued muscle: A combined electromyographic, mechanomyographic, and force approach

This study used a combined electromyographic, mechanomyographic, and force approach to identify electromechanical delay (EMD) from the onsets of the electromyographic to force signals (EMDE-F), onsets of the electromyographic to mechanomyogrpahic signals (EMDE-M), and onsets of mechanomyographic to force signals (EMDM-F). The purposes of the current study were to examine: (1) the differences in EMDE-F, EMDE-M, and EMDM-F from the vastus lateralis during maximal, voluntary dynamic (1 repetition maximum [1-RM]) and isometric (maximal voluntary isometric contraction [MVIC]) muscle actions; and (2) the effects of fatigue on EMDE-F, EMDM-F, and EMDE-M. Ten men performed pretest and posttest 1-RM and MVIC leg extension muscle actions. The fatiguing workbout consisted of 70% 1-RM dynamic constant external resistance leg extension muscle actions to failure. The results indicated that there were no significant differences between 1-RM and MVIC EMDE-F, EMDE-M, or EMDM-F. There were, however, significant fatigue-induced increases in EMDE-F (94% and 63%), EMDE-M (107%), and EMDM-F (63%) for both the 1-RM and MVIC measurements. Therefore, these findings demonstrated the effects of fatigue on EMD measures and supported comparisons among studies which examined dynamic or isometric EMD measures from the vastus lateralis using a combined electromyographic, mechanomyographic, and force approach.

Rates of performance loss and neuromuscular activity in men and women during cycling: evidence for a common metabolic basis of muscle fatigue.

Although men and women differed considerably in their absolute cycling performances, there was no sex difference in the metabolically based exponential time constant that described the performance-duration relationship. Similarly, the fatigue-induced increases in neuromuscular activity were not different between the sexes when compared from a metabolic perspective. These data suggest that men and women have similar rate-limiting mechanisms for short-duration dynamic exercise that are determined by the extent the exercise is supported by anaerobic metabolism.

Cortical Mechanisms of Central Fatigue and Sense of Effort.

The purpose of this study was to investigate cortical mechanisms upstream to the corticospinal motor neuron that may be associated with central fatigue and sense of effort during and after a fatigue task. We used two different isometric finger abduction protocols to examine the effects of muscle activation and fatigue the right first dorsal interosseous (FDI) of 12 participants. One protocol was intended to assess the effects of muscle activation with minimal fatigue (control) and the other was intended to elicit central fatigue (fatigue). We hypothesized that high frequency repetitive transcranial magnetic stimulation (rTMS) of the supplementary motor area (SMA) would hasten recovery from central fatigue and offset a fatigue-induced increase in sense of effort by facilitating the primary motor cortex (M1). Constant force-sensation contractions were used to assess sense of effort associated with muscle contraction. Paired-pulse TMS was used to assess intracortical inhibition (ICI) and facilitation (ICF) in the active M1 and interhemispheric inhibitory (IHI) was assessed to determine if compensation occurs via the resting M1. These measures were made during and after the muscle contraction protocols. Corticospinal excitability progressively declined with fatigue in the active hemisphere. ICF increased at task failure and ICI was also reduced at task failure with no changes in IHI found. Although fatigue is associated with progressive reductions in corticospinal excitability, compensatory changes in inhibition and facilitation may act within, but not between hemispheres of the M1. rTMS of the SMA following fatigue enhanced recovery of maximal voluntary force and higher levels of ICF were associated with lower sense of effort following stimulation. rTMS of the SMA may have reduced the amount of upstream drive required to maintain motor output, thus contributing to a lower sense of effort and increased rate of recovery of maximal force.

Individual Responses for Muscle Activation, Repetitions, and Volume during Three Sets to Failure of High- (80% 1RM) versus Low-Load (30% 1RM) Forearm Flexion Resistance Exercise

This study compared electromyographic (EMG) amplitude, the number of repetitions completed, and exercise volume during three sets to failure of high- (80% 1RM) versus low-load (30% 1RM) forearm flexion resistance exercise on a subject-by-subject basis. Fifteen men were familiarized, completed forearm flexion 1RM testing. Forty-eight to 72 h later, the subjects completed three sets to failure of dumbbell forearm flexion resistance exercise with 80% (n = 8) or 30% (n = 7) 1RM. EMG amplitude was calculated for every repetition, and the number of repetitions performed and exercise volume were recorded. During sets 1, 2, and 3, one of eight subjects in the 80% 1RM group demonstrated a significant linear relationship for EMG amplitude versus repetition. For the 30% 1RM group, seven, five, and four of seven subjects demonstrated significant linear relationships during sets 1, 2, and 3, respectively. The mean EMG amplitude responses show that the fatigue-induced increases in EMG amplitude for the 30% 1RM group and no change in EMG amplitude for the 80% 1RM group resulted in similar levels of muscle activation in both groups. The numbers of repetitions completed were comparatively greater, while exercise volumes were similar in the 30% versus 80% 1RM group. Our results, in conjunction with those of previous studies in the leg extensors, suggest that there may be muscle specific differences in the responses to high- versus low-load exercise.

A-40 Free Communication/Poster - Fitness Assessment Strategies for Children and Adolescents

PURPOSE: The purposes of the present study were twofold: 1) to determine if the model used for estimating the physical working capacity at the fatigue threshold (PWCFT) during incremental cycle ergometry could be applied to treadmill running to derive a new neuromuscular fatigue threshold, and 2) to compare the running velocities associated with the PWCFT, ventilatory threshold (VT), and respiratory compensation point (RCP). METHODS: Fifteen moderately-trained males and females (mean age ± SD = 21.5 ± 1.3 yrs; 68.7 ± 10.5 kg; 175.9 ± 6.7 cm) volunteered to perform an incremental treadmill test that began at 6.0 mph (1% grade) and increased 1.0 mph every two minutes until volitional fatigue. Expired gas samples were collected for the determination of VT and RCP. During each 2-min stage of the test, six, 10-second electromyographic (EMG) samples were recorded from the vastus lateralis. The EMG amplitude (μVrms) values were calculated for each 10-second epoch and separately plotted across time for each stage (i.e. running velocity) of the test. The PWCFT was then determined by averaging the highest running velocity that resulted in a non-significant (P > 0.05) slope coefficient for the EMG amplitude versus time relationship, with the lowest running velocity that resulted in a significant (P < 0.05) positive slope coefficient. RESULTS: The one-way ANOVA with repeated-measures and post-hoc analyses indicated there were significant mean differences in running velocities between the VT (7.0 ± 0.8 mph) and PWCFT (8.7 ± 1.4 mph), and the VT and RCP (8.7 ± 1.1 mph), but not between the PWCFT and RCP. In addition, there were significant zero-order correlations for VT versus PWCFT (r = 0.70), RCP versus PWCFT (r = 0.70), and VT versus RCP (r = 0.91). CONCLUSIONS: The findings of the present study indicated that the PWCFT model used to estimate the onset of neuromuscular fatigue during cycle ergometry could be applied to incremental treadmill running. Theoretically, the PWCFT treadmill test estimates the maximal running velocity that can be sustained for an extended period of time without fatigue-induced increases in muscle activation. The present findings also indicated that the PWCFT and RCP occurred at the same exercise intensity (i.e. running velocity) and thus, may be related to a common physiological mechanism of fatigue.

A-41 Free Communication/Poster - Immunology-Supplements

Recent research suggests that neuromuscular fatigue leads to increased electromechanical delay (EMD), and that differential recovery patterns are exhibited between the leg extensor and flexor muscles. However, it is unknown whether these fatigue-induced EMD responses vary by gender. PURPOSE: The purpose of this study was to investigate the effects of a fatigue-inducing bout of submaximal intermittent isometric contractions on the EMD of the leg extensors and flexors in college-aged females and males. METHODS: Twenty (mean±SD age=21.3±1.5 yrs) females and twenty-one college-aged (age=22.0±3.5) males participated in a familiarization trial followed by 2 randomly ordered experimental fatigue sessions, separated by 7(±1) days. Experimental trials involved testing of either the leg extensor or flexor muscles only, on each respective test day. All sessions began with participants performing maximal voluntary contractions (MVCs) followed by a fatigue-inducing protocol consisting of intermittent isometric contractions of the leg extensors or flexors using a 0.6 duty cycle (6s contraction, 4s relaxation) at 50% of MVC until volitional fatigue. MVCs were again performed at 0,7,15, and 30min post fatigue. EMD was calculated as the time (ms) difference between EMG and torque onset. A three-way repeated measures ANOVA was used to analyze the EMD data. RESULTS: There was no significant three-way interaction (P=0.526), nor two way interaction for time × gender (P=0.948) or muscle × gender (P=0.636), however there was a two-way interaction for muscle × time (P=0.001). For muscle × time, the EMD was greater for the leg flexors at all post fatigue time phases compared to Pre (P=0.001-0.016), but was only greater at immediately Post (i.e., 0) for the leg extensors (P=0.001). CONCLUSIONS: These findings showed that fatigue-induced increases in EMD exhibited differential recovery patterns between muscles such that the flexor EMD values had no recovery within 30min compared to more rapid recovery for the extensors. However, no gender-related differences were observed for the fatigue-induced EMD responses. These findings may have implications for performance and injury risks following fatiguing tasks, with the duration of the effects being muscle but not gender specific.

‘SIT’ down and relax: the interpolated twitch technique is still a valid measure of central fatigue during sustained contraction tasks

‘SIT’ down and relax: the interpolated twitch technique is still a valid measure of central fatigue during sustained contraction tasks

Neuromuscular and metabolic comparisons between ramp and step incremental cycle ergometer tests

We compared peak and submaximal mean values for neuromuscular and metabolic parameters between ramp (15 W · min(-1)) and step (30 W increments every 2 min) incremental cycle ergometer tests. Thirteen healthy adults (7 men and 6 women; mean ± SD age = 23.4 ± 3.3 years) performed randomly ordered ramp or step incremental tests. Two-way repeated measures analyses of variance were used to analyze the data. The ramp incremental test resulted in lower mean EMG amplitude, O2, and HR values at the common power outputs, with no differences for MMG amplitude values. It is possible that the cumulative effect of producing an increased amount of work during the step (total work = 75.83 kJ) vs. ramp (total work = 65.60 kJ) incremental cycle ergometer tests at the common power outputs may have contributed to the greater fatigue-induced increase in muscle recruitment and/or firing rate, oxygen consumption, and heart rate.

Impact of ankle muscle fatigue and recovery on the anticipatory postural adjustments to externally initiated perturbations in dynamic postural control

The aim of this study was to determine whether and how young participants modulate their postural response to compensate for postural muscle fatigue during predictable but externally initiated continuous and oscillatory perturbations. Twelve participants performed ten postural trials before and after an ankle muscle fatigue protocol. Each postural trial was 1min long and consisted of continuous backward and forward oscillations of the platform. Fatigue was induced by intermittent, bilateral isometric contractions of the ankle plantar- and dorsiflexors until the force production was reduced to 50% of the pre-fatigue maximal voluntary contraction. Changes in the center of mass (COM) displacement, center of pressure (COP) displacement, and anterior-posterior location of the COP within the base of support were quantified as well as the activity of the tibialis anterior (TA), medial gastrocnemius (MG), quadriceps, and hamstring. All participants demonstrated postural stability post-fatigue by maintaining the displacement of their COM. Everyone also demonstrated a general forward shift in the anterior-posterior location of the COP within the base of support; however, two distinct postural modifications, corresponding to either an immediate fatigue-induced increase or decrease in the COP displacement during the backward platform translation, were recorded immediately post-fatigue. The changes in muscle onset latencies lasted beyond the recovery of the force production of the fatigued postural muscles. By 10min post-fatigue, the participants showed a decrease in the COP displacement as well as an earlier activation of the postural muscles and an increased TA/MG co-activation relative to pre-fatigue. Although different strategies were used, the participants were able to adjust to and overcome postural muscle fatigue and remain balanced during the postural perturbations regardless of the direction of the platform movement. These adjustments lasted beyond the recovery of the ankle muscle force production indicating that they may be part of a centrally mediated protective response as opposed to a peripherally induced limitation to performance.

Spinal opioid receptor-sensitive muscle afferents contribute to the fatigue-induced increase in intracortical inhibition in healthy humans

We investigated the influence of spinal opioid receptor-sensitive muscle afferents on cortical changes following fatiguing unilateral knee-extensor exercise. On separate days, seven subjects performed an identical five sets of intermittent isometric right-quadriceps contractions, each consisting of eight submaximal contractions [63 ± 7% maximal voluntary contraction (MVC)] and one MVC. The exercise was performed following either lumbar interspinous saline injection or lumbar intrathecal fentanyl injection blocking the central projection of spinal opioid receptor-sensitive lower limb muscle afferents. To quantify exercise-induced peripheral fatigue, quadriceps twitch force (Q(tw,pot)) was assessed via supramaximal magnetic femoral nerve stimulation before and after exercise. Motor evoked potentials and cortical silent periods (CSPs) were evaluated via transcranial magnetic stimulation of the motor cortex during a 3% MVC pre-activation period immediately following exercise. End-exercise quadriceps fatigue was significant and similar in both conditions (Q(tw,pot) -35 and -39% for placebo and fentanyl, respectively; P = 0.38). Immediately following exercise on both days, motor evoked potentials were similar to those obtained prior to exercise. Compared with pre-exercise baseline, CSP in the placebo trial was 21 ± 5% longer postexercise (P < 0.01). In contrast, CSP following the fentanyl trial was not significantly prolonged compared with the pre-exercise baseline (6 ± 4%). Our findings suggest that the central effects of spinal opioid receptor-sensitive muscle afferents might facilitate the fatigue-induced increase in CSP. Furthermore, since the CSP is thought to reflect inhibitory intracortical interneuron activity, which may contribute to central fatigue, our findings imply that spinal opioid receptor-sensitive muscle afferents might influence central fatigue by facilitating intracortical inhibition.

A Comparison of Fatigue Thresholds Derived from the Amplitude and Frequency Domains of the Electromyographic Signal

PURPOSE: The physical work capacity at the fatigue threshold (PWCFT) is determined from fatigue-induced increases in the amplitude of the surface electromyographic (EMG) signal, while the mean power frequency fatigue threshold (MPFFT) is estimated from the compression of the power density spectrum of the signal. The purpose of this study was to compare the mean power outputs from the PWCFT and MPFFT during incremental cycle ergometry. METHODS: Ten subjects (mean ± SD age = 23.3 ± 3.7 yrs) performed an incremental cycle ergometer test to exhaustion with bipolar surface EMG signals recorded from the vastus lateralis muscle. During each 2 min power output of the incremental test, six, 10 s EMG samples were recorded. The EMG amplitude and EMG MPF values were calculated for each of the 10 s epochs and separately plotted across time for each power output of the test. The PWCFT was determined by averaging the highest power output that resulted in a non-significant (p > 0.05; single-tailed t-test) slope coefficient for the EMG amplitude versus time relationship and the lowest power output that resulted in a significant (p < 0.05) positive slope coefficient. The MPFFT was determined by averaging the highest power output that resulted in a non-significant (p > 0.05; single-tailed t-test) slope coefficient for the EMG MPF versus time relationship and the lowest power output that resulted in a significant (p < 0.05) negative slope coefficient. RESULTS: The results of a paired t-test indicated that the mean power output for the MPFFT (177.5 ± 64.8 W) was significantly greater (p < 0.05) than that of the PWCFT (152.5 ± 48.9 W). CONCLUSIONS: The significantly greater mean power output for the fatigue threshold based on the frequency (MPFFT) of the surface EMG signal than for the amplitude (PWCFT) suggested that the MPFFT and PWCFT reflect different dimensions of neuromuscular fatigue. It is possible that the PWCFT demarcates the moderate from heavy exercise intensity domains, while the MPFFT may demarcate the heavy from severe intensity domains.

Effects of Chitooligosaccharide Lactate Salt on Sleep Deprivation-Induced Fatigue in Mice

Chitooligosaccharides (COS), oligosaccharides composed of two to seven glucosamine residues, are known to exhibit various biological activities. In this study, we investigated the effects of COS in an in vivo mouse sleep deprivation-induced fatigue model in an effort to develop a functional food with anti-fatigue efficacy. Male Balb/c mice were orally administered 500 mg (kg d)(-1) of COS lactate or COS HCl for 2 weeks, and severe fatigue was induced by sleep deprivation. To evaluate the extent of fatigue, the swimming time, representing the immobility time, was measured in a forced swim test. As a result, oral intake of COS lactate-manifested anti-fatigue effects could be observed by the attenuation of fatigue-induced body weight loss and shorter immobility period. In addition, COS lactate was shown to alleviate the fatigue-induced increase in cortisol and lipid peroxidation and a decrease in superoxide dismutase (SOD) activity. Of particular note, the oral administration of COS lactate increased the mitochondrial membrane potential and the mitochondrial number significantly, indicating that COS lactate may enhance mitochondrial function. In support of this, COS lactate increased the expression of peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) and cytochrome c (Cyt C) mRNA, indicating that it may increase mitochondrial biogenesis. These results suggest that COS lactate can be an effective anti-fatigue functional food, and this anti-fatigue effect may result from, at least in part, the enhancement of mitochondrial biogenesis and the inhibition of free radical generation.

Fatigue-Induced ACL Injury Risk Stems from a Degradation in Central Control

Fatigue contributes directly to anterior cruciate ligament (ACL) injury via promotion of high risk biomechanics. The potential for central fatigue to dominate this process, however, remains unclear. With centrally mediated movement behaviors being trainable, establishing this link seems critical for improved injury prevention. We thus determined whether fatigue-induced landing biomechanics were governed by a centrally fatiguing mechanism. Twenty female NCAA athletes had initial contact (IC) and peak stance (PS) three-dimensional hip and knee biomechanics quantified during anticipated and unanticipated single-leg landings, before and during unilateral fatigue accumulation. To induce fatigue, subjects performed repetitive (n = 3) single-leg squats and randomly ordered landings, until squats were no longer possible. Subject-based dependent factors were calculated across prefatigue trials and for those denoting 100%, 75%, 50%, and 25% fatigue and were submitted to three-way mixed-design analyses of covariance to test for decision, fatigue time, and limb effects. Fatigue produced significant (P < 0.01) decreases in IC knee flexion angle and PS knee flexion moment and increases in PS hip internal rotation and knee abduction angles and moments, with differences maintained from 50% fatigue through to maximum. Fatigue-induced increases in PS hip internal rotation angles and PS knee abduction angles and loads were also significantly (P < 0.01) greater during unanticipated landings. Apart from PS hip moments, significant limb differences in fatigued landing biomechanics were not observed. Unilateral fatigue induces a fatigue crossover to the contralateral limb during single-leg landings. Central fatigue thus seems to be a critical component of fatigue-induced sports landing strategies. Hence, targeted training of central control processes may be necessary to counter successfully the debilitative impact of fatigue on ACL injury risk.

The Molecular Effects of Skeletal Muscle Fatigue on Myosin Mechanics and Kinetics

Skeletal muscle, during periods of exertion, experiences several different fatigue based changes in contraction including reductions in force, velocity, power output, and energy usage. The physiological bases of fatigue induced changes in contractility stem from many different factors including changes in neuronal activity, excitation-contraction coupling, and actomyosin contraction. The direct changes in the actomyosin contractile complex have previously been shown to be the result of alterations in the levels of metabolites, oxidative damage, and an increase in myosin regulatory light chain phosphorylation. Here, we measured the direct molecular effects of fatigue-like conditions on actomyosin velocity and force generation using the in vitro motility assay. We examined how changes in ATP, ADP, Pi, and pH affect the ability of the myosin to translocate actin. We show that fatigue-induced increases in metabolites decrease unloaded shortening velocity, with ADP and pH contributing the most to the observed changes in velocity. We also examined whether myosin regulatory light chain phosphorylation alters the sensitivity of the myosin to fatigue-like conditions. We show that under fatigue-like conditions, phosphorylation of the myosin regulatory light chian enhances force production and reduces actin sliding velocity, similar to the effects of phosphorylation under fatigue-like conditions observed in both muscle fiber and in vitro motility studies. Furthermore, we found that force production by dephosphorylated myosin is very sensitive to fatigue-like conditions whereas force production by phosphorylated myosin is rather fatigue insensitive. These results suggest that phosphorylation of the myosin regulatory light chain in skeletal muscle may serve as a protective mechanism against fatigue. Supported by NIH-HL077280, AHA-0435434T (J.R.M.), AHA-0815704D (to M.J.G.), and NIH-HL071778 (D.S-C.).

Oxygen Uptake, Heart Rate, and Ratings of Perceived Exertion at the PWCVO2

PURPOSE: Theoretically, the PWCVO2 (physical working capacity at the oxygen consumption threshold) is a test that provides an estimate of the maximal power output that can be maintained for an extended period of time without a fatigue-induced increase in the rate of oxygen consumption. The purpose of the present study was to examine the oxygen uptake (VO2), heart rate (HR) and ratings of perceived exertion (RPE (OMNI 0-10)) responses during continuous 1-hr cycle ergometer rides at the PWCVO2. METHODS: Eight male and female volunteers (mean age ± SD = 23 ± 3.2 years) performed an incremental (30W increase every 2 minutes) test to exhaustion on an electronically braked cycle ergometer for the determination of VO2peak and the ventilatory threshold (VT). The subjects also performed four randomly ordered workbouts to exhaustion at different power outputs (ranging from 84 to 245W) to determine the PWCVO2. Each subject then performed a continuous 1-hr cycle ergometer ride at the PWCVO2 during which VO2, HR, and RPE data were collected every 2 min. RESULTS: The PWCVO2 ( mean ± SD = 114 ± 39W) and VT (133 ± 44W) were not significantly different (p>0.05) and occurred at 56 and 63% VO2peak, respectively. Linear regression showed that the slope coefficients for the VO2, HR, and RPE vs. time relationships for the continuous 1-hr workbouts were significantly greater (p<0.05) than zero. Furthermore, a t-test about a single mean indicated that the mean slope coefficient for the HR vs. time relationship was significantly greater than 0.1bpm·min-1 (the rate of increase in HR that can be maintained for an 8-hr day). CONCLUSIONS: The results of this study indicated that PWCVO2 could be maintained for an extended period of time. However, the maximal power output associated with steady-state VO2, HR, and RPE responses was overestimated. The mean increase in VO2 during the continuous 1-hr ride was 270 ml which suggested that the PWCVO2 may demarcate the moderate from heavy exercise domains. The mean HR slope coefficient of 0.3bpm·min-1 indicated that the power output at the PWCVO2 could likely be maintained for greater than 2 hours.