Human Muscle Fatigue Research Articles

Exercise-induced abdominal muscle fatigue in healthy humans

The abdominal muscles have been shown to fatigue in response to voluntary isocapnic hyperpnea using direct nerve stimulation techniques. We investigated whether the abdominal muscles fatigue in response to dynamic lower limb exercise using such techniques. Eleven male subjects [peak oxygen uptake (VO2 peak) = 50.0 +/- 1.9 (SE) ml.kg(-1).min(-1)] cycled at >90% VO2 peak to exhaustion (14.2 +/- 4.2 min). Abdominal muscle function was assessed before and up to 30 min after exercise by measuring the changes in gastric pressure (Pga) after the nerve roots supplying the abdominal muscles were magnetically stimulated at 1-25 Hz. Immediately after exercise there was a decrease in Pga at all stimulation frequencies (mean -25 +/- 4%; P < 0.001) that persisted up to 30 min postexercise (-12 +/- 4%; P = 0.001). These reductions were unlikely due to changes in membrane excitability because amplitude, duration, and area of the rectus abdominis M wave were unaffected. Declines in the Pga response to maximal voluntary expiratory efforts occurred after exercise (158 +/- 13 before vs. 145 +/- 10 cmH2O after exercise; P = 0.005). Voluntary activation, assessed using twitch interpolation, did not change (67 +/- 6 before vs. 64 +/- 2% after exercise; P = 0.20), and electromyographic activity of the rectus abdominis and external oblique increased during these volitional maneuvers. These data provide new evidence that the abdominal muscles fatigue after sustained, high-intensity exercise and that the fatigue is primarily due to peripheral mechanisms.

Contralateral muscle fatigue in human quadriceps muscle: evidence for a centrally mediated fatigue response and cross-over effect

The purpose of this investigation was to examine the effects of voluntary muscular fatigue in one lower limb and determine whether a 'cross-over' of fatigue is evident in the contralateral limb. Twenty-eight subjects (13 males and 15 females) performed a series of voluntary and evoked isometric contractions of both the dominant (exercised) and non-dominant (non-exercised) leg extensor muscles, prior to and after a fatigue protocol consisting of a 100-s sustained maximal isometric contraction (MVC) performed by the dominant limb only. Force values and surface electromyography (EMG) from the vastus lateralis muscle were obtained allowing for the determination of twitch and compound action potential (M-wave) values. Maximal twitch tension and peak-to-peak amplitude were significantly decreased after the fatigue test in the dominant limb, as was maximal voluntary force (approximately 65 N reduction), EMG activity (approximately 0.1 mV decrease) and voluntary activation (approximately 17% decline). However, no significant changes were observed in the non-dominant limb with respect to twitch and M-wave properties nor in MVC force. The voluntary activation of the non-dominant limb decreased significantly by 8.7% after the fatigue test, which was performed only on the dominant limb. The results of the present study suggest that the decrease in force production in the exercised limb was primarily related to peripheral fatigue mechanisms, with central fatigue making a lesser contribution. Centrally mediated mechanisms appear to be the sole contributor to fatigue in the non-exercised limb suggesting an anticipatory fatigue response and a 'cross-over' of central fatigue between the exercised and non-exercised contralateral limb.

Effects of N‐acetylcysteine on glutathione oxidation and fatigue during handgrip exercise

Fatigue of hand and forearm muscle groups can limit task performance by astronauts wearing space suits. Countermeasures to delay fatigue would therefore be useful to the space program. N-acetylcysteine (NAC) has been shown to inhibit fatigue during other tasks so we tested its effects during handgrip exercise. Volunteers practiced isometric handgrip maneuvers until performance was reproducible over three successive sessions (baseline). Performance then was retested after ingesting NAC (150 mg.kg(-1)) or saline. Drug administration increased NAC and cysteine blood levels (P < 0.001). Performance of sustained maximal efforts was unaffected. During repetitive submaximal efforts, NAC delayed fatigue (130% baseline) and inhibited glutathione oxidation. Saline did not alter glutathione status or performance of sustained maneuvers; repetitive task performance was increased by 15% (P < 0.05), a placebo effect. These data indicate that NAC supports glutathione homeostasis in exercising humans and may delay muscle fatigue during repetitive handgrip exercise. Our findings support oxidative stress as a causal factor in human muscle fatigue and argue for larger translational studies to define NAC effects on human performance.

Effect of exercise-induced arterial hypoxemia on quadriceps muscle fatigue in healthy humans

The effect of exercise-induced arterial hypoxemia (EIAH) on quadriceps muscle fatigue was assessed in 11 male endurance-trained subjects [peak O2 uptake (VO2 peak) = 56.4 +/- 2.8 ml x kg(-1) x min(-1); mean +/- SE]. Subjects exercised on a cycle ergometer at >or=90% VO2 peak) to exhaustion (13.2 +/- 0.8 min), during which time arterial O2 saturation (Sa(O2)) fell from 97.7 +/- 0.1% at rest to 91.9 +/- 0.9% (range 84-94%) at end exercise, primarily because of changes in blood pH (7.183 +/- 0.017) and body temperature (38.9 +/- 0.2 degrees C). On a separate occasion, subjects repeated the exercise, for the same duration and at the same power output as before, but breathed gas mixtures [inspired O2 fraction (Fi(O2)) = 0.25-0.31] that prevented EIAH (Sa(O2) = 97-99%). Quadriceps muscle fatigue was assessed via supramaximal paired magnetic stimuli of the femoral nerve (1-100 Hz). Immediately after exercise at Fi(O2) 0.21, the mean force response across 1-100 Hz decreased 33 +/- 5% compared with only 15 +/- 5% when EIAH was prevented (P < 0.05). In a subgroup of four less fit subjects, who showed minimal EIAH at Fi(O2) 0.21 (Sa(O2) = 95.3 +/- 0.7%), the decrease in evoked force was exacerbated by 35% (P < 0.05) in response to further desaturation induced via Fi(O2) 0.17 (Sa(O2) = 87.8 +/- 0.5%) for the same duration and intensity of exercise. We conclude that the arterial O2 desaturation that occurs in fit subjects during high-intensity exercise in normoxia (-6 +/- 1% DeltaSa(O2) from rest) contributes significantly toward quadriceps muscle fatigue via a peripheral mechanism.

Doublets and low‐frequency fatigue in potentiated human muscle

To test the hypothesis that doublets compensate for low-frequency fatigue. Doublets increase force output from muscles stimulated at low frequencies. Low-frequency fatigue is a decline in the force elicited by low-frequency stimulation. Human flexor carpi radialis muscles were stimulated with 20 Hz trains with and without an initial doublet and with and without low-frequency fatigue and the resulting force response measured. An initial doublet caused an increase in the maximum rate of force rise of 179.6 +/- 27.9% in rested and 242.9 +/- 37.7% in muscles with low-frequency fatigue, and a substantial enhancement in force in the first three inter-pulse intervals after the extra pulse. The magnitude and time course of the early doublet enhancement were very similar regardless of low-frequency fatigue, consistent with current theories regarding the mechanisms of the doublet effect and of low-frequency fatigue. By the end of the 1 s stimulus train, force enhancement was insignificant in rested muscles and was small and subject-dependent in muscles with low-frequency fatigue (17.3 +/- 8.1% of force without a doublet), reducing the force deficit by 25.2 +/- 5.5%. The time course of doublet force enhancement implies that an initial doublet may effectively compensate for the deficit in rate of force rise in muscles with low-frequency fatigue, but may not compensate for force deficits beyond the first few inter-pulse intervals.

Gender Differences In The Fatigability Of The Inspiratory Muscles

It is well documented that gender differences exist in human skeletal muscle fatigue; however, a comparison on the fatigability of the inspiratory muscles between females and males has received relatively little attention. PURPOSE To determine if gender differences are present in the fatigability of the inspiratory muscles during intermittent inspiratory muscle contractions. METHODS Inspiratory muscle strength was assessed noninvasively by measuring maximal inspiratory static pressure (PImax) at the mouth near residual volume. Peak pressures reached during the PImax maneuver were recorded and all PImax values are reported as the average of the two highest efforts within 5%.Females (n=11, age=23.5±1.5 yrs; Mean±SEM)) and males (n=11, age=24.1±1.5 yrs) performed resistive breathing on an inspiratory threshold device at 18 breaths/min, set at a target pressure of 65–75%PImax until task failure. Task failure was defined as the inability of the subject to generate sufficient inspiratory pressure to open the threshold valve. For each subject the rate of inspiratory muscle fatigue was calculated as the slope of PImax measured every 2 min during resistive breathing. Recovery of inspiratory muscle force was assessed by measuring PImax in 5 min increments until 45 min post-task failure. RESULTS Resting PImax was lower in females than males (F:-137.0±7.6 cmH2o;M:-172.5±9.8 cmH2o, p < 0.05). No gender difference was present in time to task failure (F:843.1±123.5 s; M:735.8±99.6 s) or in the drop in PImax at task failure (F:14.9±2.5 %; M:16.5±2.3 %). After 2 min of resistive breathing, PImax was lower in females than males (to 89.8±3.8% vs. 94.9±0.1%, respectively), reaching significance for only the females (p < 0.05). During resistive breathing, females were found to have a slower rate of fatigue than males (F:-1.5±0.4 cmH20·min−1; M:-2.9±0.3 cmH20·min−1; p < 0.05). Following task failure, females exhibited prolonged recovery of inspiratory muscle force that was longer than the recovery period found for males (15 min vs. 5 min post-task failure, respectively). CONCLUSION In spite of similar time to task failure, the rate of inspiratory muscle fatigue was slower for females than males. However, recovery of inspiratory muscle strength following task failure was prolonged for females as compared to males.

Age differences in human skeletal muscle fatigue during high‐intensity intermittent exercise

It has been shown at similar relative work rates that children have higher resistance to fatigue than adults during repeated bouts of high-intensity exercise. This age-related difference in fatigue resistance may be explained by factors including muscle mass, muscle morphology, energy metabolism and neuromuscular activation. During high-intensity intermittent exercise, recovery periods play an important role in limiting fatigue. Age-related differences in fatigue resistance could also be explained by differences in the rates of resynthesis of some energetic substrates and the rates of removal of various muscle metabolites.

Age differences in human skeletal muscle fatigue during high-intensity intermittent exercise

Age differences in human skeletal muscle fatigue during high-intensity intermittent exercise

Interaction of fibre type, potentiation and fatigue in human knee extensor muscles.

To examine the effect of fibre type on potentiation and fatigue. Young men (n = 4 per group) with a predominance of type I [61.4 +/- 6.9% (SD), group I (GI)] or type II [71.8 +/- 9.2%, group II (GII)] fibres in vastus lateralis, performed a fatigue protocol of sixteen 5-s maximal voluntary isometric contractions (MVCs) of the right knee extensors. Maximal twitches and corresponding muscle action potentials (M-waves) were evoked before the first MVC, during the 3-s rest period after each MVC and at intervals during the 5-min recovery period after the last MVC. Group II [49.3 +/- 2.6% (SE)] had a greater decrease in MVC force than GI (22.8 +/- 6.2%) during the fatigue protocol. Group II (126.4 +/- 13.6%) showed greater twitch force potentiation early in the fatigue protocol than GI (38.2 +/- 2.3%), but greater depression at the end (33.7 +/- 13.7% vs.17.4 +/- 3.4%). Twitch time-to-peak torque (TPT) and half relaxation time (HRT) initially decreased but then increased as the fatigue protocol progressed; GII had a greater increase in HRT. During a 5-min recovery period twitch force increased above the prefatigue level and remained so until the end of the recovery period; the pattern was similar in GI and GII. Twitch TPT and HRT remained elevated during recovery. M-wave area increased throughout the fatigue protocol and the first part of recovery before returning to baseline values in GII, whereas there were no significant changes in GI. The interaction between potentiation and fatigue was amplified in GII early in the fatigue protocol with concurrently greater twitch and M-wave potentiation, and greater MVC force decrease and HRT increase. Late in the protocol, GII had a greater decrease in twitch and MVC force combined with greater M-wave potentiation. It is concluded that fibre type distribution influences potentiation and fatigue of the twitch, and potentiation of the M-wave during fatiguing exercise.

Progressive decrease of intramyocellular accumulation of H+ and Pi in human skeletal muscle during repeated isotonic exercise.

The purpose of this study was to evaluate the hypotheses that accumulation of hydrogen ions and/or inorganic phosphate (Pi) in skeletal muscle increases with repeated bouts of isotonic exercise. (31)P-Magnetic resonance spectroscopy was used to examine the gastrocnemius muscle of seven highly aerobically trained females during four bouts of isotonic plantar flexion. The exercise bouts (EX1-4) of 3 min and 18 s were separated by 3 min and 54 s of complete rest. Muscle ATP did not change during the four bouts. Phosphocreatine (PCr) degradation during EX1 (13.3 +/- 2.4 mmol/kg wet weight) was higher (P < 0.01) compared with EX3-4 (9.7 +/- 1.6 and 9.6 +/- 1.8 mmol/kg wet weight, respectively). The intramyocellular pH at the end of EX1 (6.87 +/- 0.05) was significantly lower (P < 0.001) than those of EX2 (6.97 +/- 0.02), EX3 (7.02 +/- 0.01), and EX4 (7.02 +/- 0.02). Total Pi and diprotonated Pi were significantly higher (P < 0.001) at the end of EX1 (17.3 +/- 2.7 and 7.8 +/- 1.6 mmol/kg wet weight, respectively) compared with the values at the end of EX3 and EX4. The monoprotonated Pi at the end of EX1 (9.5 +/- 1.2 mmol/kg wet weight) was also significantly higher (P < 0.001) than that after EX4 (7.5 +/- 1.1 mmol/kg wet weight). Subjects' rating of perceived exertion increased (P < 0.001) toward exhaustion as the number of exercises progressed (7.1 +/- 0.4, EX1; 8.0 +/- 0.3, EX2; 8.5 +/- 0.3, EX3; and 9.0 +/- 0.4, EX4; scale from 0 to 10). The present results indicate that human muscle fatigue during repeated intense isotonic exercise is not due to progressive depletion of high energy phosphates nor to intracellular accumulation of hydrogen ions, total, mono-, or diprotonated Pi.

Sex differences in human skeletal muscle fatigue are eliminated under ischemic conditions.

Several studies have suggested that women may be more resistant to muscle fatigue than men (Fulco CS, Rock PB, Muza SA, Lammi E, Cymerman A, Butterfield G, Moore, LG, Braun B, and Lewis SF. Acta Physiol Scand 167: 233-239, 1999) possibly because of differences in muscle oxidative metabolism. We evaluated muscle fatigue produced by intermittent, maximal volitional isometric contractions of the dorsiflexor muscles of healthy young (21-34 yr) men (n = 8) and women (n = 8) under two conditions: free-flow (FF) circulation and ischemia. Measures of voluntary and stimulated (10- and 50-Hz) force, central activation ratio (CAR), and compound muscle action potential (CMAP) were collected in each session. The ischemic protocol induced greater fatigue than the FF protocol, in both sexes, and was associated with greater reductions in CAR, CMAP, stimulated force, and the ratio of 10- to 50-Hz force compared with the FF condition. Women fatigued less than men in FF but not during ischemia, and this difference was roughly paralleled by a difference in CAR. No sex effects on the CMAP, tetanic force, and measures of excitation-contraction coupling function were found in the FF condition, suggesting that the primary mechanism behind the difference in fatigue was a relatively greater impairment of central activation in men. The observation that ischemia eliminated the sex differences in fatigue is consistent with a number of studies (Kent-Braun JA, Ng AV, Doyle JW, and Towse TF. J Appl Physiol 93: 1813-1823, 2002) relating fatigue to muscle metabolism and might be the result of sex-based differences in metabolic pathway utilization during muscle contraction.

Postural sway under muscle vibration and muscle fatigue in humans

Separate studies have demonstrated that vibration and fatigue of ankle muscles alter postural control. The purpose of the present experiment was to investigate the effect of ankle muscle vibration on the regulation of postural sway in bipedal stance following ankle muscle fatigue. Center of foot pressure displacements were recorded using a force platform. Results showed a similar increase in postural sway under muscle fatigue as well as under muscle vibration. Interestingly, under muscle fatigue muscle vibration did not induce a further increase in postural sway. Two hypotheses could, at least, account for this observation: (1) fatigued muscles are less sensitive to muscle vibration and (2), the central nervous system relies less upon proprioceptive information originating from fatigued muscles for regulating postural sway.

SYMPOSIUM-SEX DIFFERENCES IN MUSCLE FATIGUE AND PERFORMANCE

There is a sex difference in muscle fatigability, performance, and energy metabolism when men and women perform exercise at the same relative intensity. This symposium will synthesize the most significant research from various disciplines to further understand the mechanisms for sex differences in muscle fatigue and performance as well as the impact of age on these sex differences. The presentations will address the sex differences observed during isometric and dynamic contractions by isolated muscle groups and during whole body aerobic performance in young and old adults. Specifically, Dr Hunter will present research on low-force contractions of arm muscles and discuss the mechanisms underlying sex differences in young adults during isometric fatiguing contractions. Dr Kent-Braun will examine the independent effects of aging and sex on human muscle fatigue with particular focus on the role of neural, metabolic and contractile mechanisms in the leg muscles under various fatiguing conditions. Dr Tarmopolsky will identify the hormonal etiology for the sex specific differences in substrate utilization and the metabolic response to endurance exercise. Dr Seals will present research on maximal aerobic capacity and athletic performance in events that require the contribution of aerobic energy metabolism to elucidate the potential interactive effects of age and sex on physiological functional capacity.

Pulmonary function subsequent to expiratory muscle fatigue in healthy humans.

One of the mechanisms proposed to explain the decrement in pulmonary function often seen after exercise is fatigue of the expiratory muscles. To test the hypothesis that expiratory muscle fatigue alters lung function, several indices of pulmonary function were measured before and after expiratory muscle fatigue was induced by expiratory loaded breathing. Eight subjects completed a fatigue trial (EF) in which expiratory threshold loaded breathing was performed at an initial resistance equal to 80 % of their maximal expiratory pressure (MEP), at a respiratory rate of 13 bpm, and a duty cycle (T(I)/T(Tot)) of 0.33. MEP was taken at predefined intervals throughout the loaded breathing protocol, and loaded breathing was discontinued when MEP was less than 80 % of each subject's pre EF trial MEP (T(Lim)). FVC, FEV(1.0), FEF(25 %), FEF(25-75 %), and maximal inspiratory and expiratory pressures (MIP and MEP) were taken prior to, immediately after, and at 5, 10, and 15 min post fatigue. On a separate day a control trial (CON) was performed that was identical to each subjects EF trial with the exception that no expiratory load was utilized. At T(Lim) MEP was significantly reduced (p < 0.001) by 23.5 % from the pre-expiratory loaded breathing value (183.1 +/- 39.56 to 140.13 +/- 30.45 mmHg), whereas it remained unchanged during the CON trial (191.06 +/- 44.18 to 188.06 +/- 43.50 mmHg). FVC measured prior to and immediately after T(Lim) remained unchanged following both the EF (5349.45 +/- 1130.8 to 5387.43 +/- 1139.92 mL) and CON trials (5287.75 +/- 1220.29 and 5352.78 +/- 1191.30 mL). These results suggest that any expiratory muscle fatigue developed during exercise by itself does not result in altered pulmonary function. However, any interactions between expiratory muscle fatigue and other consequences of exercise that may alter lung function cannot be ruled out.

Fatigue and pain in human jaw muscles during a sustained, low-intensity clenching task

Fatigue, pain and changes in the electromyographic (EMG) activity of the jaw-closing muscles are well documented during short, high-intensity tooth-clenching tasks but less so during sustained, low-intensity tasks. In this study, 11 healthy men clenched on a bite-force meter for 60 min at 10% of the maximal voluntary contraction (MVC) and scored the intensity of fatigue and pain on separate 10 cm visual analogue scales (VAS). Surface EMG activity from the masseter and anterior temporalis muscles was recorded in 10 s epochs every 5 min throughout the task. Pressure-pain thresholds (PPTs) in the jaw-closing muscles, unassisted maximum jaw opening and MVC were determined before and after the task. All participants reported an increasing sensation of fatigue in the jaw-closing muscles during the task (mean±SD: peak VAS=7.5±2.0 cm) but all were able to maintain the required force. Most (7/11) also reported a painful sensation (peak VAS=2.7±2.8 cm). The jaw-opening capacity (59.5±7.4 vs. 58.3±6.5 mm, P=0.031) and the MVC (777±73 vs. 652±115 N, P=0.002) were slightly, but significantly, decreased immediately after the task whereas the PPTs remained unchanged (ANOVA: P=0.612). The mean frequency of the EMG activity decreased in all muscles during the task (95.7 vs. 46.6 Hz; P<0.001), and the root mean squares increased (53.2 vs. 154 μV, P<0.001). The changes in EMG activity were more strongly correlated with the sensation of fatigue than pain. These findings demonstrate that a sustained, low-intensity clenching task can induce subjective and electrophysiological indications of fatigue.

Spinal and Supraspinal Factors in Human Muscle Fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

The effects of length on fatigue and twitch potentiation in human skeletal muscle

Fatigue is the decrease in active force that happens after repeated muscle stimulation, and post tetanic twitch potentiation (PTP) is the increase in twitch force observed after repeated muscle stimulation. This study investigated the effects of length on the interaction between fatigue and PTP, as these two forms of force regulation are length-dependent and may coexist. A total number of 14 subjects were tested in 3 days, in which fatigue and PTP were induced in the knee extensor muscles in three different knee angles (30 degrees, 60 degrees and 90 degrees; full extension = 0 degree). PTP was evaluated in rested and fatigued muscles with twitch contractions elicited before and after 10 s maximal voluntary contraction (MVC), and fatigue was evaluated with nine 50 Hz electrically elicited contractions (5 s duration, 5 s interval between contractions). Fatigue was length-dependent, with force values that were (mean +/- SEM) 59 +/- 5, 56 +/- 3 and 38 +/- 1% of maximal force at 90 degrees, 60 degrees and 30 degrees, respectively. PTP was also length-dependent. Rested muscles showed PTP of 39 +/- 4, 47 +/- 2 and 68 +/- 5% at 90 degrees, 60 degrees and 30 degrees, respectively. Fatigued muscles showed PTP of 44 +/- 3, 55 +/- 6 and 68 +/- 5%, at 90 degrees, 60 degrees and 30 degrees, respectively. This study shows that fatigue and PTP may represent independent mechanisms, as they regulate force in opposite directions and are both enhanced in short muscle lengths.

A predictive model of fatigue in human skeletal muscles.

Fatigue is a major limitation to the clinical application of functional electrical stimulation. The activation pattern used during electrical stimulation affects force and fatigue. Identifying the activation pattern that produces the greatest force and least fatigue for each patient is, therefore, of great importance. Mathematical models that predict muscle forces and fatigue produced by a wide range of stimulation patterns would facilitate the search for optimal patterns. Previously, we developed a mathematical isometric force model that successfully identified the stimulation patterns that produced the greatest forces from healthy subjects under nonfatigue and fatigue conditions. The present study introduces a four-parameter fatigue model, coupled with the force model that predicts the fatigue induced by different stimulation patterns on different days during isometric contractions. This fatigue model accounted for 90% of the variability in forces produced by different fatigue tests. The predicted forces at the end of fatigue testing differed from those observed by only 9%. This model demonstrates the potential for predicting muscle fatigue in response to a wide range of stimulation patterns.

Human Inspiratory Muscle Fatigue during Strenuous Resistive Loads under Normoxic and Hypoxic Conditions

A study involving nine healthy males was conducted to determine the developmental rate of inspiratory muscle fatigue during resistive loads under normoxic and hypoxic conditions. The subjects aged 19 to 38 years performed a bicycle exercise test of increasing power during an exhaustive inspiratory–expiratory resistive load of 40 cm water column/l s–1 inhaling air or oxygen. The volumetric and temporal respiratory parameters, the partial CO2 pressure in the alveolar gas, the total force of inspiratory muscle contractions, the electrical activity of parasternal muscles (EMG), and the initial inspiratory activity were recorded. The degree of inspiratory muscle fatigue was assessed by the “tension–time” index Pm /Pm maxTI /TT as well as by the ratio of the mean amplitudes of the EMG-signal spectrum in the high-frequency (H) range to the mean spectrum amplitudes in the low-frequency (L) range (H/L). It was established that human working capacity during increasing muscular loads against the background of highly resistive breathing was not a function of the oxygen content in the inhalation mixtures within 21 to 100%; i.e., the maximal power of the work done did not significantly differ. It was shown that pulmonary ventilation, the force generated by inspiratory muscles, the breathing effort values, the initial inspiratory activity value, and the “tension–time” index increased in parallel with the intensity of the exercise when both air and oxygen were inhaled. The Pm /Pm maxTI /TT values in the last minutes of the muscular load significantly exceeded the fatigue zone range, attaining 0.25 to 0.45 in different subjects. By the moment of refusal to continue the work, the H/L ratio had decreased by an average of 36% when air and oxygen were inhaled. The limitation of the physical working capacity of a healthy individual during an exhaustive resistive load is considered to be linked to inspiratory muscle fatigue developing at an equal rate under both normoxic and hyperoxic conditions. An impairment of the arterial blood supply to inspiratory muscles due to occlusion of the intramuscular vessels during intense muscular contractions and a considerable shortening of the time of the relaxation of the respiratory muscles in the expiratory phase is supposed to occur in forced respiration with an additional gas flow obstruction. The fact of the alternation of the electrical activity between the parasternal and other inspiratory muscles within individual inspiratory phases combined with resistive and physical loads is established, which may serve as one of the signs of inspiratory muscle fatigue.

Wavelet analysis of electromyography for back muscle fatigue detection during isokinetic constant-torque exertions.

An investigation of the effects of human trunk extensor muscle fatigue on the temporal change in frequency content of the electromyogram as quantified using the Fourier and wavelet transforms during the performance of repetitive dynamic trunk extension. To evaluate whether alterations in the Fourier and wavelet transform measures were consistent with a shift of the signal power to lower frequencies, and to determine which measures were more highly correlated with the decline in maximal trunk extension torque. Objective assessment of trunk muscle fatigue is likely to play a more important role in the rehabilitation and prevention of low back injuries, given the association between lack of trunk muscle endurance and acquisition of low back pain. Validation of new methods designed to quantify the level of fatigue using the surface electromyogram is necessary before these techniques can be used in industrial rehabilitation settings. The wavelet transform is a recent development in the signal processing of electromyograms that shows promise as a method for assessment of fatigue. Trunk muscle electromyograms obtained from study participants performing repetitive isokinetic trunk extension endurance tests were analyzed using the wavelet and the traditional Fourier methods. Trunk extension torque was controlled at 35% and 70% of the participants' maximal voluntary contraction while they exerted at 5 and 10 repetitions per minute. The decline in maximal trunk extension torque was measured once per minute. Linear regression quantified the rate of change in Fourier and wavelet measures caused by fatigue, whereas Pearson's correlation coefficient determined their association with the decline in maximum torque. Changes in the characteristics of the electromyogram were consistent with a shift to lower frequencies: The signal power at higher frequencies was reduced, whereas the power at lower frequencies was elevated. The amount of change was dependent on the task conditions (exertion level and repetition rate). The wavelet-based measures demonstrated as strong an association with the decline in maximal torque output as the Fourier-based measures. This study demonstrates that assessment of trunk muscle fatigue during isokinetic movementis possible using both Fourier and wavelet measurements. However, the methods were not as likely to change significantly during lower rates of exertion. These methods, when implemented in a controlled setting, may be used to document the rehabilitation process and guide preventive exercise training.