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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]

Objective: Existing literature indicates that females generally demonstrate higher fatigue resistance than males during isometric contractions. However, when it comes to single-limb dynamic exercises, the intricate interplay between performance fatigability (PF), cardiovascular responses, and muscle metabolism in relation to sex differences remains underexplored., Purpose: This study investigates how sex affects the relationship between muscle oxidative characteristics and the development of PF during dynamic single-leg exercise., Methods: Twenty-four young healthy participants (12 males vs. 12 females) performed a constant-load single-leg knee extension task (85% peak power output; 60 rpm) to exhaustion (TTE). Neuromuscular assessments via transcranial magnetic and peripheral stimulations were conducted pre- and post-exercise to evaluate central and peripheral factors of PF. Vastus lateralis muscle biopsies were obtained for mitochondrial respiration and immunohistochemistry analyses., Results: Participants performed similar total work (28 ± 7 vs. 27 ± 14 kJ, p = 0.81) and TTE (371 ± 139 vs. 377 ± 158 sec, p = 0.98); after the TTE, females' maximal isometric voluntary contraction (MVIC: -36 ± 13 vs. -24 ± 9 %, p = 0.006) and resting twitch (RT: (-65 ± 9 vs. -40 ± 24 %, p = 0.004) force declined less. No differences were observed in supraspinal neuromuscular factors (p > 0.05). During exercise, the cardiovascular responses differed between sexes. Although fiber type composition was similar (type I: 47 ± 13 vs. 56 ± 14 %, p = 0.11), males had lower mitochondrial net oxidative capacity (61 ± 30 vs. 89 ± 37, p = 0.049) and higher Complex II contribution to maximal respiration (CII; 59 ± 8 vs. 48 ± 6%, p < 0.001), which correlated with the decline in MVIC (r = -0.74, p < 0.001) and RT (r = -0.60, p = 0.002)., Conclusions: Females display greater resistance to PF during dynamic contractions, likely due to their superior mitochondrial efficiency and lower dependence on mitochondrial CII activity., Competing Interests: Conflict of Interest and Funding Source: This research was funded by the Italian Ministry of Research and University (MIUR; Rome, Italy) 5-year special funding (https://www.miur.gov.it/dipartimenti-di-eccellenza) and the research grant “Fondo Gianesini Emma” in collaboration with UniCredit Foundation and University of Verona (Italy). The authors declare no competing interest., (Copyright © 2024 by the American College of Sports Medicine.)

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.)