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Illusionary mirror visual feedback alters interhemispheric communication and influences cross-limb interactions. Combining forceful unimanual contractions with the mirror illusion is a convenient way to provoke robust alterations within ipsilateral motor networks. It is unknown, however, if the mirror illusion affects cross-limb fatigability. We examine this concept by comparing the ipsilateral and contralateral handgrip force and electromyographic (EMG) responses following unimanual fatigue with and without illusionary mirror visual feedback. Participants underwent three experimental sessions (mirror, no-mirror, and control), performing a unimanual fatigue protocol with and without illusionary mirror visual feedback. Maximal handgrip force and EMG activity were measured before and after each session for both hands during maximal unimanual and bimanual contractions. The associated EMG activity from the inactive forearm during unimanual contraction was also examined. The novel findings demonstrate greater relative fatigability during bimanual versus unimanual contraction following unimanual fatigue (-31.8% vs. -23.4%, P < 0.01) and the mirror illusion attenuates this difference (-30.3% vs. -26.3%, P = 0.169). The results show no evidence for a cross-over effect of fatigue with (+0.62%, -2.72%) or without (+0.26%, -2.49%) the mirror illusion during unimanual or bimanual contraction. The mirror illusion resulted in significantly lower levels of associated EMG activity in the contralateral forearm. There were no sex differences for any of the measures of fatigability. These results demonstrate that the mirror illusion influences contraction-dependent fatigue during maximal handgrip contractions. Alterations in facilitatory and inhibitory transcallosal drive likely explain these findings. NEW & NOTEWORTHY Illusionary mirror visual feedback is a promising clinical tool for motor rehabilitation, yet many features of its influence on motor output are unknown. We show that maximal bimanual force output is compromised to a greater extent than unimanual force output following unimanual fatigue, yet illusionary mirror visual feedback attenuates this difference. The mirror illusion also reduces the unintended EMG activity of the inactive, contralateral forearm during unimanual contraction.

This manuscript represents the second phase of a clinical trial designed to examine the effects of knee joint immobilization and retraining on muscle strength and mass. In Phase 2, we examined sex differences in the recovery of multiple indices of muscle quality after a resistance training-based rehabilitation program. Following 1 week of immobilization, 27 participants (16 males, 11 females) exhibiting weakness underwent twice weekly resistance training sessions designed to re-strengthen their left knee. Unilateral retraining sessions utilizing leg press, extension, and curl exercises were conducted until participants could reproduce their pre-immobilization knee extension isometric maximal voluntary contraction (MVC) peak torque. Post-immobilization, both sexes demonstrated impaired MVC peak torque (males = −10.8%, females = −15.2%), specific torque (−9.8% vs. −13.1%), echo intensity of the vastus lateralis (+6.9% vs. +5.9%) and rectus femoris (+5.9% vs. +2.1), and extracellular water/intracellular water ratio (+7.8% vs. +9.0%). The number of retraining sessions for peak torque to return to baseline for males (median = 1, mean = 2.13) versus females (median = 2, mean = 2.91) was not significantly different, though the disparity in recovery times may be clinically relevant. Following retraining, specific torque was the only muscle-quality indicator that improved along with MVC peak torque (males = 20.1%, females = 22.4%). Our findings indicate that measures of muscle quality demonstrate divergent recovery rates following immobilization, with muscle mass lagging behind improvements in strength. Greater immobilization-induced strength loss among females suggests that sex-specific rehabilitation efforts may be justified. [ABSTRACT FROM AUTHOR]

Carr, JC and Ye, X. Strength and electromyographic responses of upper and lower limbs during maximal intermittent contractions in males and females. J Strength Cond Res 36(9): 2403–2409, 2022—This study examined the strength and electromyographic (EMG) responses of upper vs. lower limb muscles during intermittent maximal contractions in both sexes. Twenty subjects (n = 7 women) performed a fatiguing protocol (6, 30-second intermittent maximal isometric contractions with a 50% duty cycle) with either the elbow flexors or the knee extensors on separate visits. Bipolar surface EMG signals were detected from the biceps brachii and vastus lateralis muscles (n = 5 women retained). Women maintained more of their maximal force than men (Δforce: men vs. women = −55.0 ± 12.8% vs. −43.3 ± 9.9%, p = 0.042). Although force loss was similar between the elbow flexors and knee extensors, the EMG responses showed greater reductions for the biceps brachii than those for the vastus lateralis (Δamplitude: biceps brachii vs. vastus lateralis: −32.0 ± 22.3% vs. −18.9 ± 28.9%; Δmedian frequency: biceps brachii vs. vastus lateralis: −31.1 ± 14.5% vs. −10.3 ± 17.0%). During a series of maximal intermittent isometric contractions with 30 seconds of recovery between work bouts, women are more fatigue resistant than men. In addition, the greater electrophysiological fatigue exhibited by the biceps brachii than that by the vastus lateralis suggests that high-intensity contractions involving elbow flexion will have a greater rate of fatigue progression than those involving knee extension. [ABSTRACT FROM AUTHOR]

The purpose of this experiment was to evaluate the bilateral index in force and electromyographic (EMG) responses for the dominant and non-dominant hands during maximal handgrip contractions in males and females. Thirty-two right-handed participants (16 females) performed maximal unilateral and bilateral handgrip contractions on two separate visits. Bilateral indices were computed for maximal force, rate of force development (RFD100), EMG amplitude, and the rate of EMG rise (RER). There was a bilateral deficit for maximal force in the dominant (−4.98 ± 7.39%, p < 0.001; d = 0.674) but not the non-dominant hand (−1.57 ± 9.10%, p = 0.334; d = 0.173). No deficits were observed for rapid force. The non-dominant flexor carpi radialis showed a bilateral facilitation in EMG amplitude (+12.32 ± 19.29%, p < 0.001; d = 0.638), yet a bilateral deficit for RER (−22.10 ± 27.80%, p < 0.001; d = 0.795). No sex differences were observed for any of the bilateral indices. These data suggest that maximal but not rapid force is susceptible to a bilateral deficit during contractions of the hands. The EMG responses did not parallel the force data. We show sex does not influence the magnitude or direction of the bilateral index in this muscle group. [ABSTRACT FROM AUTHOR]

Purpose: There have been recent attempts to characterize the mechanisms associated with fatigue-induced task failure. We compared the time to failure and the corresponding changes in the surface electromyogram (EMG) during sustained maximal and submaximal isometric force tasks.Methods: EMG activity was measured from the biceps brachii of 18 male participants as they sustained either a maximal or submaximal (60 % MVC) isometric contraction of the dominant elbow flexors until force could not be maintained above 55 % MVC.Results: Intensity-dependent patterns of change were observed for EMG amplitude and mean power frequency (MNF) between the two force tasks. Interestingly, the only significant predictor of failure time was the rate of change in EMG MNF during the submaximal task (r (2) = 0.304). In addition, EMG amplitude at submaximal failure was significantly lower (p < 0.05) than the values obtained during MVC.Conclusions: The patterns of EMG response emphasize the basis of neuromuscular fatigue and task dependency. Additionally, our data suggest that the EMG MNF should be used when monitoring the progression of local muscle fatigue. [ABSTRACT FROM AUTHOR]

We evaluated the compensatory adaptations in muscle regionalization and synergist activity after fatiguing a biarticular muscle at one joint with different muscle lengths. Eleven men (mean ± SD age = 23 ± 3 years) performed 50 maximal concentric isokinetic contractions of the dominant hip flexors on two occasions. For one trial, the knee joint was fully extended. For the other, the knee joint was fixed at 70°. Maximal voluntary contractions of the knee extensors were performed immediately before and after the hip flexion fatigue protocol while bipolar surface electromyographic signals were detected from the vastus lateralis and at five points along the length of the rectus femoris. Regardless of knee joint angle during the hip flexion fatigue protocol, knee extension peak torque was unchanged following hip flexion fatigue. Electromyographic amplitude for the vastus lateralis (p = .047, η2 = .338) and rectus femoris (p < .001, η2 = .667) showed main effects for time, indicating higher and lower post-fatigue values, respectively. There was no evidence of region-specific rectus femoris adaptations during extension at the knee following fatigue of the hip flexors. These data suggest that synergistic adaptations were involved in maintaining knee extension peak torque following hip flexion fatigue. [ABSTRACT FROM AUTHOR]

The article cites a study which examined the relationship between the fatigue index from an isokinetic fatigue test and the velocityrelated decrease in peak torque for the forearm flexors. The study findings support the possibility of using a multiple-velocity isokinetic test to estimate fiber type composition, just as fatigue-based tests have been used.

PURPOSE: The purpose of this study was to examine differences in absolute and normalized RFD between sexes during maximal handgrip contractions. METHODS: During a single visit, 32 right-hand dominant participants (n = 16 males, n = 16 females) performed maximal, unilateral isometric handgrip contractions with their dominant hands. Maximal force, the RFD from 0-50ms (RFD50), 0-100ms (RFD100), and peak RFD (RFDpeak) were computed. The RFD responses were normalized to the maximal force values during each contraction (nRFD50, nRFD100, nRFDpeak). Independent samples t-tests were used for statistical analysis and the effect size for the mean comparisons were computed. RESULTS: Males were stronger than females (p < 0.01, d = 2.36) and demonstrated greater absolute RFDpeak (p < 0.01, d = 1.11) values, but there were no sex differences for RFD50 (p = 0.112, d = 0.578) and RFD100 (p = 0.088, d = 0.624) despite the moderate effect sizes for greater RFD values for the males. There were no sex differences for the normalized RFD responses. Specifically, nRFD50 (p = 0.112, d = 0.624), nRFD100 (p = 0.167, d = 0.501), and nRFDpeak (p = 0.217, d = 0.446) showed moderate effect sizes for greater normalized RFD values for females compared to males. CONCLUSION: The data shows that sex differences in RFD are influenced by the interval of measurement and normalization procedures. These results have implications for studies comparing RFD between sexes. [ABSTRACT FROM AUTHOR]

Bilateral deficit (BLD) describes a phenomenon that the force produced during maximal simultaneous bilateral contraction is lower than the sum of those produced unilaterally. The aim of this study was to examine the potential sex-related differences in BLD in upper body proximal and distal limb muscles. Ten men and eight women performed single-joint maximal contractions with their elbow flexors and index finger abductors at separate laboratory visits, during which the maximal isometric voluntary contractions (MVICs) were performed unilaterally and bilaterally with a randomized order in the designated muscle group. Surface electromyographic (EMG) signals were recorded from the prime movers of the designated muscle groups (biceps brachii and first dorsal interosseous) during the maximal contractions. Both men and women demonstrated BLD in their elbow flexors (deficit: men = -11.0 ± 6.3%; women = -10.2 ± 5.0%). Accompanied by this force deficit was the reduced EMG amplitude from the dominant biceps brachii (collapsed across sex: p = 0.045). For the index finger abductors, only men (deficit = -13.7 ± 6.1%), but not women showed BLD. Our results suggested that the BLD in the proximal muscle group is likely induced by the decreased maximal muscle activity from the dominant prime mover. The absence of BLD in women's index finger muscle is largely due to the inter-subject variability possibly related to the sex hormone flux and unique levels of interhemispheric inhibition. [ABSTRACT FROM AUTHOR]