Intramuscular pressure and tissue oxygenation during low-force static contraction do not underlie muscle fatigue

Myoelectric, Myo-Oxygenation, and Myotonometry Changes during Robot-Assisted Bilateral Arm Exercises with Varying Resistances

Robot-assisted bilateral arm training has demonstrated its effectiveness in improving motor function in individuals post-stroke, showing significant enhancements with increased repetitions. However, prolonged training sessions may lead to both mental and muscle fatigue. We conducted two types of robot-assisted bimanual wrist exercises on 16 healthy adults, separated by one week: long-duration, low-resistance workouts and short-duration, high-resistance exercises. Various measures, including surface electromyograms, near-infrared spectroscopy, heart rate, and the Borg Rating of Perceived Exertion scale, were employed to assess fatigue levels and the impacts of exercise intensity. High-resistance exercise resulted in a more pronounced decline in electromyogram median frequency and recruited a greater amount of hemoglobin, indicating increased muscle fatigue and a higher metabolic demand to cope with the intensified workload. Additionally, high-resistance exercise led to increased sympathetic activation and a greater sense of exertion. Conversely, engaging in low-resistance exercises proved beneficial for reducing post-exercise muscle stiffness and enhancing muscle elasticity. Choosing a low-resistance setting for robot-assisted wrist movements offers advantages by alleviating mental and physiological loads. The reduced training intensity can be further optimized by enabling extended exercise periods while maintaining an approximate dosage compared to high-resistance exercises.

Effects of compression garment on muscular efficacy, proprioception, and recovery after exercise-induced muscle fatigue onset for people who exercise regularly

Fatigue is a major cause of exercise-induced muscle damage (EIMD). Compression garments (CGs) can aid post-exercise recovery, therefore, this study explored the effects of CGs on muscular efficacy, proprioception, and recovery after exercise-induced muscle fatigue in people who exercise regularly. Twelve healthy participants who exercised regularly were enrolled in this study. Each participant completed an exercise-induced muscle fatigue test while wearing a randomly assigned lower-body CG or sports pants (SP); after at least 7 days, the participant repeated the test while wearing the other garment. The dependent variables were muscle efficacy, proprioception (displacements of center of pressure/COP, and absolute error), and fatigue recovery (muscle oxygen saturation/SmO2, deoxygenation and reoxygenation rate, and subjective muscle soreness). A two-way repeated measure analysis of variance was conducted to determine the effect of garment type. The results indicated that relative to SP use, CG use can promote muscle efficacy, proprioception in ML displacement of COP, and fatigue recovery. Higher deoxygenation and reoxygenation rates were observed with CG use than with SP use. For CG use, SmO2 quickly returned to baseline value after 10 min of rest and was maintained at a high level until after 1 h of rest, whereas for SP use, SmO2 increased with time after fatigue onset. ML displacement of COP quickly returned to baseline value after 10 min of rest and subsequently decreased until after 1 hour of rest. Relative to SP use, CG use was associated with a significantly lower ML displacement after 20 min of rest. In conclusion, proprioception and SmO2 recovery was achieved after 10 min of rest; however, at least 24 h may be required for recovery pertaining to muscle efficacy and soreness regardless of CG or SP use.

Self-Regulated Force and Neuromuscular Responses During Fatiguing Isometric Leg Extensions Anchored to a Rating of Perceived Exertion

The purpose of the study was to examine the fatigue-related patterns of responses for electromyography (EMG), mechanomyography (MMG), and force during a sustained isometric muscle action anchored to RPE = 5. Ten men (22.9 ± 2.0 year) performed maximal voluntary isometric contractions (MVIC) prior to and following an isometric leg extension muscle action, which was sustained for a maximal time-limit of 5 min or until it could not be maintained at RPE = 5 (actual time-limit). EMG amplitude (AMP), EMG mean power-frequency (MPF), MMG AMP, MMG MPF, and force values were determined every 5% of the actual time-limit. Regression analyses were used to examine the neuromuscular parameters and force responses, and a t test was used to examine MVIC. The pretest MVIC (62.4 ± 14.3 kg) was significantly (p < 0.001; d = 1.07) greater than posttest (47.9 ± 12.8 kg). The percent decline in force during the sustained isometric muscle action was 47.5 ± 19.6%, and there was a significant, negative force versus time relationship (p < 0.001; R = - 0.980). There was a significant, negative EMG AMP versus time relationship (p < 0.001; R = -0.789), but no significant (p > 0.05) relationships for EMG MPF, MMG AMP, or MMG MPF versus time. The findings indicated that it was necessary to reduce force and EMG AMP to maintain RPE = 5. We hypothesize that the maintenance of RPE = 5 was initially accomplished by an anticipatory feedforward mechanism and then continuous integrations of afferent feedback, which resulted in reductions of EMG AMP and force, due to reductions in neural drive, to attenuate the impact of metabolic byproducts.

Segmenting Mechanomyography Measures of Muscle Activity Phases Using Inertial Data

Electromyography (EMG) is the standard technology for monitoring muscle activity in laboratory environments, either using surface electrodes or fine wire electrodes inserted into the muscle. Due to limitations such as cost, complexity, and technical factors, including skin impedance with surface EMG and the invasive nature of fine wire electrodes, EMG is impractical for use outside of a laboratory environment. Mechanomyography (MMG) is an alternative to EMG, which shows promise in pervasive applications. The present study used an exerting squat-based task to induce muscle fatigue. MMG and EMG amplitude and frequency were compared before, during, and after the squatting task. Combining MMG with inertial measurement unit (IMU) data enabled segmentation of muscle activity at specific points: entering, holding, and exiting the squat. Results show MMG measures of muscle activity were similar to EMG in timing, duration, and magnitude during the fatigue task. The size, cost, unobtrusive nature, and usability of the MMG/IMU technology used, paired with the similar results compared to EMG, suggest that such a system could be suitable in uncontrolled natural environments such as within the home.

Task and sex differences in muscle oxygenation during handgrip fatigue development

The purpose of this study was to examine task and sex differences in forearm muscle oxygenation, measured using near infrared spectroscopy, during sustained submaximal handgrip exercises. Forty-eight adults (50% males) performed fatiguing handgrip exercises at 20, 40, 60 and 80% of their maximum handgrip strength. While males and females exhibited similar levels of relative fatigability, forearm oxygenation was found to be task (i.e. contraction intensity and phase of fatigue development) and sex dependent. Higher contraction intensities were associated with greater desaturation over time. Compared to females, males exhibited greater desaturation as fatigue progressed and this was augmented at higher contraction intensities. These may be likely affected by sex differences in muscle mass, morphology and strength differences during exercises at relative intensities. Future work that explores sex differences in muscle oxygenation during absolute force intensities are needed, which may have implications for muscle fatigue development and potential fatigue mitigation strategies. Practitioner Summary: Muscle oxygenation impacts fatigue development that can in turn affect worker health and productivity. Males exhibit greater forearm desaturation than females at higher relative work intensities, despite similar fatigue levels. Females may be predisposed to greater muscle delivery and oxygenation challenges that can increase their fatigability during work at absolute load levels.

Different ways to balance the spine in sitting: Muscle activity in specific postures differs between individuals with and without a history of back pain in sitting

BACKGROUND

Previous research explored muscle activity in four distinct sitting postures with fine-wire electromyography, and found that lumbar multifidus muscle activity increased incrementally between sitting with flat thoracolumbar and lumbar regions, long thoracolumbar lordosis, or short lordosis confined to the lumbar region. This study used similar methods to explore whether people with a history of low back pain provoked by prolonged sitting used different patterns of trunk muscle activity in specific postures.

METHODS

Fine-wire electromyography electrodes were inserted into the right lumbar multifidus (deep and superficial), iliocostalis (lateral and medial), longissimus thoracis and transversus abdominis muscles. Superficial abdominal muscle activity was recorded with surface or fine-wire electrodes. Electromyography amplitude was compared between postures for the back pain group and observations were contrasted with the changes previously reported for pain-free controls. For comparison between groups normalised and non-normalised electromyography amplitudes were compared.

FINDINGS

Individuals with a history of back pain demonstrated greater activity of the longissimus thoracis muscle in the long lordosis compared with the flat posture [mean difference (95% CI): 46.6 (17.5-75.7)%, normalised to sitting posture peak activity], but pain-free participants did not [mean difference: 7.7 (minus 12-27.6)%]. Pain-free participants modulated lumbar multifidus activity with changes in lumbar curve, but people with a history of pain in prolonged sitting did not change multifidus activity between the long and short lordotic postures.

INTERPRETATION

In clinical ergonomic interventions that modify spinal curves and sagittal balance in sitting, the muscle activity used in those postures may differ between people with and without a history of back pain.

Unilateral fatiguing exercise and its effect on ipsilateral and contralateral resting mechanomyographic mean frequency between aerobic populations

The purpose of this investigation was to establish a better understanding of contralateral training and its effects between homologous muscles following unilateral fatiguing aerobic exercise during variable resting postural positions, and to determine if any observable disparities could be attributed to the differences between the training ages of the participants. Furthermore, we hypothesized that we would observe a contralateral cross-over effect for both groups, with the novice trained group having the higher mechanomyographic mean frequency values in both limbs, across all resting postural positions. Twenty healthy male subjects exercised on an upright cycle ergometer, using only their dominate limb, for 30 min at 60% of their VO2 peak. Resting electromyographic and mechanomyographic signals were measured prior to and following fatiguing aerobic exercise. We found that there were resting mechanomyographic mean frequency differences of approximately 1.9 ± 0.8% and 0.9 ± 0.7%; 9.1 ± 0.3% and 10.2 ± 3.7%; 2 ± 1.8% and 3 ± 1.4%; and 0.9 ± 0.6% and 0.2 ± 1.3% between the novice and advanced trained groups (for the upright sitting position with legs extended 180°; upright sitting position with legs bent 90°; lying supine position with legs extended 180°; and lying supine with legs bent 90°, respectively), from the dominant and nondominant limbs, respectively. We have concluded that despite the relative matching of exercise intensity between groups, acute responses to contralateral training become less accentuated as one progresses in training age. Additionally, our results lend support to the notion that there are multiple, overlapping neural and mechanical mechanisms concurrently contributing to the contralateral cross-over effects observed across the postexercise resting time course.

Exploratory study of electromyographic behavior of the vastus medialis and vastus lateralis at neuromuscular fatigue onset

This study aimed to determine and analyze the neuromuscular fatigue onset by median frequency (MDF) and the root mean square (RMS) behavior of an electromyographic signal (EMG). Eighteen healthy men with no prior knee problems initially performed three maximum voluntary isometric contractions (MVIC). After two days of MVIC test, participants performed a fatiguing protocol in which they performed submaximal knee-extension contractions at 20% and 70% MVIC held to exhaustion. The MDF and RMS values from the EMG signals were recorded from the vastus medialis (VM) and the vastus lateralis (VL). Analysis of the MDF and RMS behavior enabled identification of neuromuscular fatigue onset for VM and VL muscles in 20% and 70% loads. Alterations between the VM and VL in the neuromuscular fatigue onset, at 20% and 70% MVIC, were not significant. These findings suggest that the methodology proposal was capable of indicating minute differences sensible to alterations in the EMG signals, allowing identification of the moment when the MDF and the RMS showed significant changes in behavior. The methodology used was also a viable one for describing and identifying the neuromuscular fatigue onset by means of the analysis of EMG signals.

Skeletal muscle fatigue

Skeletal muscle fatigue is defined as the fall of force or power in response to contractile activity. Both the mechanisms of fatigue and the modes used to elicit it vary tremendously. Conceptual and technological advances allow the examination of fatigue from the level of the single molecule to the intact organism. Evaluation of muscle fatigue in a wide range of disease states builds on our understanding of basic function by revealing the sources of dysfunction in response to disease.

A critical review on physical factors and functional characteristics that may explain a sex/gender difference in work-related neck/shoulder disorders

The objective of this paper is to critically review recent literature on physical and functional sex/gender (s/g) differences, with focus on physical determinants associated with neck/shoulder musculoskeletal injuries. It is well known that there are s/g differences in anthropometrical and functional body characteristics (e.g. size and strength). However, s/g differences may be wrongly attributed if data analysis does not include appropriate corrections (e.g. by strength for endurance). Recent literature on motor control shows that there may indeed be s/g differences in muscle coordination and movement strategies during upper limb tasks that are not currently explained by methodological inadequacies. Moreover, recent studies have shown differences between men and women in sensory hypersensitivity characteristics associated with neck/shoulder injuries. Taken together, the literature points to the importance of accounting for possible s/g differences at all levels of the biopsychosocial system in order to better understand sex- and gender-specific issues relevant to workplace health.

PRACTITIONER SUMMARY

This article critically reviews recent literature and a conceptual model highlighting s/g differences in physical and functional characteristics related to neck/shoulder musculoskeletal disorders (NSMSD). Findings have implications on understanding how personal factors may affect NSMSD risk. With better understanding, practitioners can make more appropriate decisions to prevent work-related NSMSD.

Biofeedback effectiveness to reduce upper limb muscle activity during computer work is muscle specific and time pressure dependent

Continuous electromyographic (EMG) activity level is considered a risk factor in developing muscle disorders. EMG biofeedback is known to be useful in reducing EMG activity in working muscles during computer work. The purpose was to test the following hypotheses: (1) unilateral biofeedback from trapezius (TRA) can reduce bilateral TRA activity but not extensor digitorum communis (EDC) activity; (2) biofeedback from EDC can reduce activity in EDC but not in TRA; (3) biofeedback is more effective in no time constraint than in the time constraint working condition. Eleven healthy women performed computer work during two different working conditions (time constraint/no time constraint) while receiving biofeedback. Biofeedback was given from right TRA or EDC through two modes (visual/auditory) by the use of EMG or mechanomyography as biofeedback source. During control sessions (no biofeedback), EMG activity was (mean ± SD): 2.4 ± 1.1, 2.5 ± 2.1, and 9.1 ± 3.1%max-EMGrms for right and left TRA and EDC, respectively. During biofeedback from TRA, activity was reduced in right TRA (1.7 ± 1.6%max-EMGrms) and left TRA (1.2 ± 2.0%max-EMGrms) compared to control. During biofeedback from EDC, activity in EDC was reduced (8.3 ± 3.3%max-EMGrms) compared with control. During time constraint, activity was reduced in right TRA (1.9 ± 1.3%max-EMGrms), left TRA (1.5 ± 1.5%max-EMGrms), and EDC (8.4 ± 3.2%max-EMGrms), during biofeedback compared to control.

CONCLUSION

biofeedback reduced muscle activity in TRA by ∼ 30-50% and in EDC by ∼ 10% when given from the homologous or bilateral muscle but not from the remote muscle, and was significant in the time constraint condition; while feedback source and presentation mode showed only minor differences in the effect on reducing homologous muscle activity. This implies that biofeedback should be given from the most affected muscle in the occupational setting for targeting relief and prevention of muscle pain most effectively.

An integrative model of motor unit activity during sustained submaximal contractions

The purpose of the study was to expand a model of motor unit recruitment and rate coding (30) to simulate the adjustments that occur during a fatiguing contraction. The major new components of the model were the introduction of time-varying parameters for motor unit twitch force, recruitment, discharge rate, and discharge variability, and a control algorithm that estimates the net excitation needed by the motoneuron pool to maintain a prescribed target force. The fatigue-induced changes in motor unit activity in the expanded model are a function of changes in the metabolite concentrations that were computed with a compartment model of the intra- and extracellular spaces. The model was validated by comparing the simulation results with data available from the literature and experimentally recorded in the present study during isometric contractions of the first dorsal interosseus muscle. The output of the model was able to replicate a number of experimental findings, including the time to task failure for a range of target forces, the changes in motor unit discharge rates, the skewness and kurtosis of the interspike interval distributions, discharge variability, and the discharge characteristics of newly recruited motor units. The model output provides an integrative perspective of the adjustments during fatiguing contractions that are difficult to measure experimentally.

Characteristics of power spectrum density function of EMG during muscle contraction below 30%MVC

The aim of the study was to quantify changes in PSDF frequency bands of the EMG signal and EMG parameters such as MF, MPF and zero crossing, with an increase in the level of muscle contractions in the range from 0.5% to 30% RMS(max) and to determine the frequency bands with the lowest dependency on RMS level so that this could be used in investigating muscle fatigue. Sixteen men, aged from 23 to 33 years old (mean 26.1), who participated in the study performed two force exertion tests. Fragments of EMG which corresponded to the levels of muscle contraction of 0.5%, 1%, 2.5%, 5%, 10%, 15%, 20%, 25%, 30% RMS(max) registered from left and right trapezius pars descendents (TP) and left and right extensor digitorum superficialis (ED) muscles were selected for analysis. The analysis included changes in standard parameters of the EMG signal and changes in PSDF frequency bands, which occurred across muscle contraction levels. To analyze changes in PSDF across the level of muscle contraction, the spectrum was divided into six frequency bandwidths. The analysis of parameters focused on the differences in those parameters between the analyzed muscles, at different levels of muscle contraction. The study revealed that, at muscle contraction levels below 5% RMSmax, contraction level influences standard parameters of the EMG signal and that at such levels of muscle contraction every change in muscle contraction level (recruitment of additional MUs) is reflected in PSDF. The frequency band with the lowest dependency on contraction level was 76-140 Hz for which in both muscles no contraction level effect was detected for contraction levels above 5% RMS(max). The reproducibility of the results was very high, since the observations in of the left and right muscles were almost equal. The other factor, which strongly influences PSDF of the EMG signal, is probably the examined muscle structure (muscle morphology, size, function, subcutaneous layer, cross talk). It seems that low frequency bands up to 25 Hz are especially feasible for type of muscle.

Low-level activity of the trunk extensor muscles causes electromyographic manifestations of fatigue in absence of decreased oxygenation

This study was designed to determine whether trunk extensor fatigue occurs during low-level activity and whether this is associated with a drop in muscle tissue oxygenation. Electromyography (EMG) feedback was used to impose constant activity in a part of the trunk extensor muscles. We hypothesized that electromyographic manifestations of fatigue and decreased oxygenation would be observed at the feedback site and that EMG activity at other sites would be more variable without fatigue manifestations. Twelve volunteers performed 30-min contractions at 2% and 5% of the maximum EMG amplitude (EMGmax) at the feedback site. EMG was recorded from six sites over the lumbar extensor muscles and near-infrared spectroscopy was used to measure changes in oxygenation at the feedback site (left L3 level, 3 cm paravertebral). In both conditions, mean EMG activity was not significantly different between electrode sites, whereas the coefficient of variation was lower at the feedback site compared to other recording sites. The EMG mean power frequency (MPF) decreased consistently at the feedback site only. At 5% EMGmax, the decrease in MPF was significant at the group level at all sites ipsilateral to the feedback site. These results suggest that the limited variability of muscle activity at the EMG feedback site and at ipsilateral locations enhances fatigue development. No decreases in tissue oxygenation were detected. In conclusion, even at mean activity levels as low as 2% EMGmax, fatigue manifestations were found in the trunk extensors. These occurred in absence of changes in oxygenation of the muscle tissue.

A comparison of central aspects of fatigue in submaximal and maximal voluntary contractions

Magnetic and electrical stimulation at different levels of the neuraxis show that supraspinal and spinal factors limit force production in maximal isometric efforts ("central fatigue"). In sustained maximal contractions, motoneurons become less responsive to synaptic input and descending drive becomes suboptimal. Exercise-induced activity in group III and IV muscle afferents acts supraspinally to limit motor cortical output but does not alter motor cortical responses to transcranial magnetic stimulation. "Central" and "peripheral" fatigue develop more slowly during submaximal exercise. In sustained submaximal contractions, central fatigue occurs in brief maximal efforts even with a weak ongoing contraction (<15% maximum). The presence of central fatigue when much of the available motor pathway is not engaged suggests that afferent inputs contribute to reduce voluntary activation. Small-diameter muscle afferents are likely to be activated by local activity even in sustained weak contractions. During such contractions, it is difficult to measure central fatigue, which is best demonstrated in maximal efforts. To show central fatigue in submaximal contractions, changes in motor unit firing and force output need to be characterized simultaneously. Increasing central drive recruits new motor units, but the way this occurs is likely to depend on properties of the motoneurons and the inputs they receive in the task. It is unclear whether such factors impair force production for a set level of descending drive and thus represent central fatigue. The best indication that central fatigue is important during submaximal tasks is the disproportionate increase in subjects' perceived effort when maintaining a low target force.

Near-infrared spectroscopy/imaging for monitoring muscle oxygenation and oxidative metabolism in healthy and diseased humans

Near-infrared spectroscopy (NIRS) was initiated in 1977 by Jobsis as a simple, noninvasive method for measuring the presence of oxygen in muscle and other tissues in vivo. This review honoring Jobsis highlights the progress that has been made in developing and adapting NIRS and NIR imaging (NIRI) technologies for evaluating skeletal muscle O(2) dynamics and oxidative energy metabolism. Development of NIRS/NIRI technologies has included novel approaches to quantification of the signal, as well as the addition of multiple source detector pairs for imaging. Adaptation of NIRS technology has focused on the validity and reliability of NIRS measurements. NIRS measurements have been extended to resting, ischemic, localized exercise, and whole body exercise conditions. In addition, NIRS technology has been applied to the study of a number of chronic health conditions, including patients with chronic heart failure, peripheral vascular disease, chronic obstructive pulmonary disease, varying muscle diseases, spinal cord injury, and renal failure. As NIRS technology continues to evolve, the study of skeletal muscle function with NIRS first illuminated by Jobsis continues to be bright.

Sustained contraction at very low forces produces prominent supraspinal fatigue in human elbow flexor muscles

During sustained maximal voluntary contractions (MVCs), most fatigue occurs within the muscle, but some occurs because voluntary activation of the muscle declines (central fatigue), and some of this reflects suboptimal output from the motor cortex (supraspinal fatigue). This study examines whether supraspinal fatigue occurs during a sustained submaximal contraction of 5% MVC. Eight subjects sustained an isometric elbow flexion of 5% MVC for 70 min. Brief MVCs were performed every 3 min, with stimulation of the motor point, motor cortex, and brachial plexus. Perceived effort and pain, elbow flexion torque, and surface EMGs from biceps and brachioradialis were recorded. During the sustained 5% contraction, perceived effort increased from 0.5 to 3.9 (out of 10), and elbow flexor EMG increased steadily by approximately 60-80%. Torque during brief MVCs fell to 72% of control values, while both the resting twitch and EMG declined progressively. Thus the sustained weak contraction caused fatigue, some of which was due to peripheral mechanisms. Voluntary activation measured by motor point and motor cortex stimulation methods fell to 90% and 80%, respectively. Thus some of the fatigue was central. Calculations based on the fall in voluntary activation measured with cortical stimulation indicate that about two-thirds of the fatigue was due to supraspinal mechanisms. Therefore, sustained performance of a very low-force contraction produces a progressive inability to drive the motor cortex optimally during brief MVCs. The effect of central fatigue on performance of the weak contraction is less clear, but it may contribute to the increase in perceived effort.

Electromyogram and perceived fatigue changes in the trapezius muscle during typewriting and recovery

The purpose of the present study was to investigate the development and recovery of muscle fatigue in the upper trapezius muscle by analyzing electromyographic signals. Six male subjects performed a simulated typewriting task for four 25-min sessions. During fatigue and the following rest periods, subjective fatigue and surface electromyography (EMG) from the trapezius muscle during isometric contraction at 30% maximum voluntary contraction (MVC) were periodically measured in the interval. We detected a significant decrease in muscle fiber conduction velocity (MFCV) (P = 0.008) and median frequency (MDF) (P = 0.026) as well as an increase in root mean square (RMS) (P = 0.039) and subjective fatigue (P = 0.0004) during the fatigue period. During the recovery period, subjective fatigue decreased drastically and significantly (P = 0.0004), however, the EMG parameters did not recover completely. Thus, physiological muscle fatigue in the trapezius developed in accordance with subjective muscle fatigue during typewriting. On the other hand, differences between the physiological and subjective parameters were found during recovery. Further studies should be necessary to reveal the discrepancy could be a major factor of a transition from temporal phenomena to serious chronic muscle fatigue and to identify the necessity of some guidelines to prevent VDT work-related chronic muscle fatigue in the trapezius.

Effects of 30 versus 60 min of low-load work on intramuscular lactate, pyruvate, glutamate, prostaglandin E(2) and oxygenation in the trapezius muscle of healthy females

The aim of this study was to investigate the effects of duration of low-load repetitive work on intramuscular lactate, pyruvate, glutamate and prostaglandin E(2) (PGE(2)), and oxygen saturation in the trapezius muscle. Twenty healthy females were studied during baseline rest, during low-load repetitive work for either 30 (REP 30) or 60 min (REP 60) and 60 min recovery. Intramuscular microdialysate (IMMD) samples were obtained, and local muscle tissue oxygenation (%StO(2)) assessed with near-infrared spectroscopy (NIRS). Subjects rated their perceived exertion (Borg CR-10 scale) and capillary blood was sampled for lactate analysis. The results showed a significant increase in IMMD lactate in response to both REP 30 and REP 60 (P < 0.05 and P < 0.01, respectively) and glutamate (P < 0.0001), but no progressive increase with increasing work duration. Both IMMD pyruvate and lactate tended to be significantly increased during the recovery period. No corresponding increase in blood lactate was found. Local muscle %StO(2) did not change significantly in response to work and was not correlated to the IMMD lactate concentration. The ratings of perceived exertion increased in response to work, and remained increased after recovery for REP 60. In conclusion, the results of this study show significantly increased IMMD lactate and, glutamate concentrations in the trapezius muscle of healthy females in response to low-load work, but no progressive increase with increased work duration. Further, they do not indicate that the increased IMMD lactate concentration was caused by a locally decreased or insufficient muscle tissue oxygenation.

Oxygenation, EMG and position sense during computer mouse work. Impact of active versus passive pauses

We investigated the effects of active versus passive pauses implemented during computer mouse work on muscle oxygenation and EMG of the forearm extensor carpi radialis muscle, and on wrist position sense. Fifteen healthy female subjects (age: 19-24 years) performed a 60-min mouse-operated computer task, divided into three 20 min periods, on two occasions separated by 3-6 days. On one occasion a passive pause (subjects resting) was implemented at the end of each 20-min period, and on another occasion an active pause (subjects performed a number of high intensity extensions of the forearm) was implemented. Also at the end of each 20-min period, test contractions were conducted and subjective ratings of fatigue and stress were obtained. Another parameter of interest was total haemoglobin calculated as the summation of oxy-and deoxy-haemoglobin, since it reflects blood volume changes. The most interesting findings were an overall increasing trend in total haemoglobin throughout the mouse work (P<0.001), and that this trend was greater for the active pause as compared to the passive pause (P<0.01). These data were accompanied by an overall increase in oxygen saturation (P<0.001), with a tendency, albeit not significant, toward a higher increase for the active pause (P=0.13). EMG amplitude and median frequency tended to decrease (P=0.08 and 0.05, respectively) during the mouse work but was not different between pause types. Borg ratings of forearm fatigue showed an overall increase during the activity (P<0.001), but the perceptions of stress did not change. Position sense did not change due to the mouse work for either pause type. While increasing trends were found for both pause types, the present study lends support to the hypothesis of an enhancement in oxygenation and blood volume for computer mouse work implemented with active pauses. However, a presumption of an association between this enhancement and attenuated fatigue during the mouse work was not supported.