Fatigue-related modulation of low-frequency common drive to motor units

Hwang, Ing Shiou; Lin, Yen Ting; Huang, Chien Chun; Chen, Yi Ching

doi:10.1007/s00421-020-04363-z

Cited by 8 publications

(4 citation statements)

References 48 publications

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“…In reference to baseline force fluctuations, both groups exhibited postexercise loss of force precision control, as indicated by the comparably greater FF-RMS T1/T0 (Table 2). The increase in force fluctuations did not vary by group, indicating a tendency of neuromuscular fatigue (42) in relation to increases in discharge variability for shifts in low-frequency common drive to MUs (43). Compared with the baseline EMG, both groups also demonstrated a comparably decreasing trend of EMG amplitude (EMG-RMS T1/T0 ) in the postcontraction stage (Table 3), concomitant with a decline in gross nervous system output.…”

Section: Discussionmentioning

confidence: 92%

“…It is known that the central nervous system lowers the recruitment threshold of MU in compensation for diminution of the force contributions of individual MU so that muscle force output can be sustained via increasing recruitment of MU (46,47). Hence, the scenario implies that the central nervous system relatively raises the recruitment threshold of MU when the twitch force of MU is potentiated with the photobiomodulation effect for rapid ATP synthesis in the after-recovery stage (32,43). Probably because the regulation of the recruitment threshold and rate coding of MU were independent processes (48), we noted an insignificant increase in discharge rate in the LLL group during the recovery stage relative to that of the sham group ( P = 0.094; Table 4).…”

Section: Discussionmentioning

confidence: 99%

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Low-Level Laser Therapy Facilitates Postcontraction Recovery with Ischemic Preconditioning

Chen

Lin

et al. 2023

Medicine &Amp; Science in Sports &Amp; Exercise

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PurposeDespite early development of muscle fatigue, ischemic preconditioning is gaining popularity for strength training combined with low-load resistance exercise. This study investigated the effect of low-level laser (LLL) on postcontraction recovery with ischemic preconditioning.MethodsForty healthy adults (22.9 ± 3.5 yr) were allocated into sham (11 men, 9 women) and LLL (11 men, 9 women) groups. With ischemic preconditioning, they were trained with three bouts of intermittent wrist extension of 40% maximal voluntary contraction (MVC). During the recovery period, the LLL group received LLL (wavelength of 808 nm, 60 J) on the working muscle, whereas the sham group received no sham therapy. MVC, force fluctuations, and discharge variables of motor units (MU) for a trapezoidal contraction were compared between groups at baseline (T0), postcontraction (T1), and after-recovery (T2).ResultsAt T2, the LLL group exhibited a higher normalized MVC (T2/T0; 86.22% ± 12.59%) than that of the sham group (71.70% ± 13.56%; P = 0.001). The LLL group had smaller normalized force fluctuations (LLL, 94.76% ± 21.95%; sham, 121.37% ± 29.02%; P = 0.002) with greater normalized electromyography amplitude (LLL, 94.33% ± 14.69%; sham, 73.57% ± 14.94%; P < 0.001) during trapezoidal contraction. In the LLL group, the smaller force fluctuations were associated with lower coefficients of variation of interspike intervals of MUs (LLL, 0.202 ± 0.053; sham, 0.208 ± 0.048; P = 0.004) with higher recruitment thresholds (LLL, 11.61 ± 12.68 %MVC; sham, 10.27 ± 12.73 %MVC; P = 0.003).ConclusionsLLL expedites postcontraction recovery with ischemic preconditioning, manifesting as superior force generation capacity and force precision control for activation of MU with a higher recruitment threshold and lower discharge variability.

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Section: Discussionmentioning

confidence: 92%

Section: Discussionmentioning

confidence: 99%

Low-Level Laser Therapy Facilitates Postcontraction Recovery with Ischemic Preconditioning

Chen

Lin

et al. 2023

Medicine &Amp; Science in Sports &Amp; Exercise

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“…As the level of force that was reached in each trapezoidal trajectory did not differ across conditions, it could be argued that increase in MU DR after acute exercises might be related to muscle fatigue ( Macaluso et al, 2000 ), assuming that active MUs would require to compensate with greater DR to achieve equivalent force outputs. However, several authors reported that in presence of fatigue, particularly following submaximal tasks (including 20% MVIC), the mean discharge rate of MUs significantly decreased ( McManus et al, 2015 ; Hwang et al, 2020 ). It has been suggested that in presence of fatigue, earlier-recruited MUs are selectively inhibited and decrease their DR, while the concurrent increase in the excitation to the motoneuron pool leads to the recruitment of new MUs to sustain the force output ( Enoka et al, 1989 ; Stewart et al, 2011 ; McManus et al, 2015 ).…”

Section: Discussionmentioning

confidence: 99%

Adjustments in the motor unit discharge behavior following neuromuscular electrical stimulation compared to voluntary contractions

et al. 2023

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Introduction: The application of neuromuscular electrical stimulation superimposed on voluntary muscle contractions (NMES+) has demonstrated a considerable potential to enhance or restore muscle function in both healthy and individuals with neurological or orthopedic disorders. Improvements in muscle strength and power have been commonly associated with specific neural adaptations. In this study, we investigated changes in the discharge characteristics of the tibialis anterior motor units, following three acute exercises consisting of NMES+, passive NMES and voluntary isometric contractions alone.Methods: Seventeen young participants participated in the study. High-density surface electromyography was used to record myoelectric activity in the tibialis anterior muscle during trapezoidal force trajectories involving isometric contractions of ankle dorsi flexors with target forces set at 35, 50% and 70% of maximal voluntary isometric contraction (MVIC). From decomposition of the electromyographic signal, motor unit discharge rate, recruitment and derecruitment thresholds were extracted and the input-output gain of the motoneuron pool was estimated.Results: Global discharge rate increased following the isometric condition compared to baseline at 35% MVIC while it increased after all experimental conditions at 50% MVIC target force. Interestingly, at 70% MVIC target force, only NMES + led to greater discharge rate compared to baseline. Recruitment threshold decreased after the isometric condition, although only at 50% MVIC. Input-output gain of the motoneurons of the tibialis anterior muscle was unaltered after the experimental conditions.Discussion: These results indicated that acute exercise involving NMES + induces an increase in motor unit discharge rate, particularly when higher forces are required. This reflects an enhanced neural drive to the muscle and might be strongly related to the distinctive motor fiber recruitment characterizing NMES+.

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“…These patients rely on an alternative compensation strategy, which involves enhancing the acceleration rate of progressively shortening inter-spike intervals in the remaining low-threshold motor units to generate force ( Figure 5 ). This compensatory rate coding may facilitate the fusion of twitch forces ( Hwang et al, 2020 ), but force precision control of increasing force without ample recruitment of high-threshold motor units is still inferior compared to their healthy counterparts ( Table 3 ).…”

Section: Discussionmentioning

confidence: 99%

Deficits in neuromuscular control of increasing force in patients with chronic lateral epicondylitis

Chen

Hong

et al. 2023

Front. Physiol.

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Objective: This study investigated the neuromuscular control of increasing and releasing force in patients with chronic lateral epicondylitis (CLE).Methods: Fifteen patients with CLE (10 males, 5 females, 46.5 ± 6.3 years) and fifteen healthy participants (9 males, 6 females, 45.3 ± 2.5 years) participated in this study. In addition to power grip and maximal voluntary contraction (MVC) of wrist extension, force fluctuation dynamics and characteristics of inter-spike intervals (ISI) of motor units (MUs) with various recruitment thresholds in the extensor carpi radialis brevis (ECRB) and extensor carpi radialis longus (ECRL) during a designated force-tracking task with a trapezoidal target (0%–75%–0% MVC) were assessed.Results: Besides a smaller MVC of wrist extension, the patients exhibited significantly greater task errors (p = 0.007) and force fluctuations (p = 0.001) during force increment than the healthy counterparts. Nevertheless, no force variables significantly differed between groups during force release (p > 0.05). During force increment, the amplitudes of the motor unit action potential of the ECRB and ECRL muscles of the patients were smaller than those of the heathy counterparts (p < 0.001). The patient group also exhibited a higher percentage of motor units (MU) with lower recruitment threshold (<5% MVC) in the ECRL/ECRB muscles and a lower percentage of MU with higher recruitment threshold (>40% MVC) in the ECRB muscle, compared to the healthy group. During force increment, the patient group exhibited a higher rate of decrease in inter-spike intervals (ISIs) of motor units with lower recruitment thresholds (<10% MVC) in the ECRB and ECRL muscles, compared to the control group (p < 0.005).Conclusion: The patients with CLE exhibited more pronounced impairment in increasing force than in releasing force. This impairment in increasing force is attributed to deficits in tendon structure and degenerative changes in the larger motor units of the wrist extensors. To compensate for the neuromuscular deficits, the rate of progressive increase in discharge rate of the remaining smaller motor units (MUs) is enhanced to generate force.Significance: The deficits in neuromuscular control observed in CLE with degenerative changes cannot be fully explained by the experimental pain model, which predicts pain-related inhibition on low-threshold motor units.

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Fatigue-related modulation of low-frequency common drive to motor units

Cited by 8 publications

References 48 publications

Low-Level Laser Therapy Facilitates Postcontraction Recovery with Ischemic Preconditioning

Low-Level Laser Therapy Facilitates Postcontraction Recovery with Ischemic Preconditioning

Adjustments in the motor unit discharge behavior following neuromuscular electrical stimulation compared to voluntary contractions

Deficits in neuromuscular control of increasing force in patients with chronic lateral epicondylitis

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