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The force drop after transcranial magnetic stimulation (TMS) delivered to the motor cortex during voluntary muscle contractions could inform about muscle relaxation properties. Because of the physiological relation between skeletal muscle fibertype distribution and size and muscle relaxation, TMS could be a noninvasive index of muscle relaxation in humans. By combining a noninvasive technique to record muscle relaxation in vivo (TMS) with the gold standard technique for muscle tissue sampling (muscle biopsy), we investigated the relation between TMS-induced muscle relaxation in unfatigued and fatigued states, and muscle fiber-type distribution and size. Sixteen participants (7F/9M) volunteered to participate. Maximal knee-extensor voluntary isometric contractions were performed with TMS before and after a 2-min sustained maximal voluntary isometric contraction. Vastus lateralis muscle tissue was obtained separately from the participants’ dominant limb. Fiber type I distribution and relative cross-sectional area of fiber type I correlated with TMS-induced muscle relaxation at baseline (r = 0.67, adjusted P = 0.01; r = 0.74, adjusted P = 0.004, respectively) and normalized TMS-induced muscle relaxation as a percentage of baseline (r = 0.50, adjusted P = 0.049; r = 0.56, adjusted P = 0.031, respectively). The variance in the normalized peak relaxation rate at baseline (59.8%, P < 0.001) and in the fatigue resistance (23.0%, P = 0.035) were explained by the relative cross-sectional area of fiber type I to total fiber area. Fiber type I proportional area influences TMS-induced muscle relaxation, suggesting TMS as an alternative method to noninvasively inform about skeletal muscle relaxation properties. NEW & NOTEWORTHY Transcranial magnetic stimulation (TMS)-induced muscle relaxation reflects intrinsic muscle contractile properties by interrupting the drive from the central nervous system during voluntary muscle contractions. We showed that fiber type I proportional area influences the TMS-induced muscle relaxation, suggesting that TMS could be used for the noninvasive estimation of muscle relaxation in unfatigued and fatigued human muscles when the feasibility of more direct method to study relaxation properties (i.e., muscle biopsy) is restricted. [ABSTRACT FROM AUTHOR]

Aim: Force expression is characterized by an interplay of biological and molecular determinants that are expected to differentiate males and females in terms of maximal performance. These include muscle characteristics (muscle size, fiber type, contractility), neuromuscular regulation (central and peripheral factors of force expression), and individual genetic factors (miRNAs and gene/protein expression). This research aims to comprehensively assess these physiological variables and their role as determinants of maximal force difference between sexes. Methods: Experimental evaluations include neuromuscular components of isometric contraction, intrinsic muscle characteristics (proteins and fiber type), and some biomarkers associated with muscle function (circulating miRNAs and gut microbiome) in 12 young and healthy males and 12 females. Results: Male strength superiority appears to stem primarily from muscle size while muscle fiber‐type distribution plays a crucial role in contractile properties. Moderate‐to‐strong pooled correlations between these muscle parameters were established with specific circulating miRNAs, as well as muscle and plasma proteins. Conclusion: Muscle size is crucial in explaining the differences in maximal voluntary isometric force generation between males and females with similar fiber type distribution. Potential physiological mechanisms are seen from associations between maximal force, skeletal muscle contractile properties, and biological markers. [ABSTRACT FROM AUTHOR]

Introduction: This study investigated the magnitude and etiology of neuromuscular fatigue and muscle damage induced by eccentric cycling compared to conventional concentric cycling in patients with breast cancer., Methods: After a gradual familiarization protocol for eccentric cycling, nine patients with early-stage breast cancer performed three cycling sessions in eccentric or concentric mode. The eccentric cycling session (ECC) was compared to concentric cycling sessions matched for power output (CONpower, 80% of concentric peak power output, 95 ± 23 W) or oxygen uptake (10 ± 2 mL.min.kg-1). Pre- to postexercise changes (30s through 10 min recovery) in knee extensor maximal voluntary contraction force (MVC), voluntary activation, and quadriceps potentiated twitch force (Qtw) were quantified to determine global, central, and peripheral fatigue, respectively. Creatine kinase (CK) and lactate dehydrogenase (LDH) activities were measured in the plasma before and 24 h postexercise as markers of muscle damage., Results: Compared to CONpower (-11 ± 9%) and (-5 ± 5%), the ECC session resulted in a greater decrease in MVC (-25 ± 12%) postexercise (P < 0.001). Voluntary activation decreased only in ECC (-9 ± 6% postexercise, P < 0.001). The decrease in Qtw was similar postexercise between ECC and CONpower (-39 ± 21% and -40 ± 16%, P > 0.99) but lower in (P < 0.001). The CONpower session resulted in twofold greater compared to the ECC and sessions (P < 0.001). No change in CK or LDH activity was reported from preexercise to 24 h postexercise., Conclusions: The ECC session induced greater neuromuscular fatigue compared to the concentric cycling sessions without generating severe muscle damage. ECC is a promising exercise modality for counteracting neuromuscular maladaptation in patients with breast cancer., Competing Interests: Conflict of Interest and Funding Source: This work was supported by the Institute of Cancerology Strasbourg Europe (ICANS) and by the initiative of excellence IdEx Unistra (ANR-10-IDEX-0002-02) from the ‘Investment for the Future' French national program. The authors report no conflicts of interest., (Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American College of Sports Medicine.)

Muscle fatigue induced by voluntary exercise, which requires central motor drive, causes central fatigue that impairs endurance performance of a different, nonfatigued muscle. This study investigated the impact of quadriceps fatigue induced by electrically induced (no central motor drive) contractions on single-leg knee-extension (KE) performance of the subsequently exercising ipsilateral quadriceps. On two separate occasions, eight males completed constant-load (85% of maximal power-output) KE exercise to exhaustion. In a counterbalanced manner, subjects performed the KE exercise with no pre-existing quadriceps fatigue in the contralateral leg on one day (No-PreF), whereas on the other day, the same KE exercise was repeated following electrically induced quadriceps fatigue in the contralateral leg (PreF). Quadriceps fatigue was assessed by evaluating pre- to postexercise changes in potentiated twitch force (ΔQ tw,pot ; peripheral fatigue), and voluntary muscle activation (ΔVA; central fatigue). As reflected by the 57 ± 11% reduction in electrically evoked pulse force, the electrically induced fatigue protocol caused significant knee-extensors fatigue. KE endurance time to exhaustion was shorter during PreF compared with No-PreF (4.6 ± 1.2 vs 7.7 ± 2.4 min; P < 0.01). Although ΔQ tw,pot was significantly larger in No-PreF compared with PreF (-60% vs -52%, P < 0.05), ΔVA was greater in PreF (-14% vs -10%, P < 0.05). Taken together, electrically induced quadriceps fatigue in the contralateral leg limits KE endurance performance and the development of peripheral fatigue in the ipsilateral leg. These findings support the hypothesis that the crossover effect of central fatigue is mainly mediated by group III/IV muscle afferent feedback and suggest that impairments associated with central motor drive may only play a minor role in this phenomenon.