Sex-based comparisons of muscle cellular adaptations after 10 weeks of progressive resistance training in middle-aged adults

Sexual dimorphisms in skeletal muscle: current concepts and research horizons

The complex compositional and functional nature of skeletal muscle makes this organ an essential topic of study for biomedical researchers and clinicians. An additional layer of complexity is added with the consideration of sex as a biological variable. Recent research advances have revealed sexual dimorphisms in developmental biology, muscle homeostasis, adaptive responses, and disorders relating to skeletal muscle. Many of the observed sex differences have hormonal and molecular mechanistic underpinnings, whereas others have yet to be elucidated. Future research is needed to investigate the mechanisms dictating sex-based differences in the various aspects of skeletal muscle. As such, it is necessary that skeletal muscle biologists ensure that both female and male subjects are represented in biomedical and clinical studies to facilitate the successful testing and development of therapeutics for all patients.

Sex-Based Differences in Muscle Stem Cell Regulation Following Exercise

Sexual dimorphism, driven by the sex hormones testosterone and estrogen, influences body composition, muscle fiber type, and inflammation. Research related to muscle stem cell (MuSC) responses to exercise has mainly focused on males. We propose a novel hypothesis that there are sex-based differences in MuSC regulation following exercise, such that males have more MuSCs, whereas females demonstrate a greater capacity for regeneration.

Skeletal muscle adaptations to high-intensity, low-volume concurrent resistance and interval training in recreationally active men and women

This study compared the structural and cellular skeletal muscle factors underpinning adaptations in maximal strength, power, aerobic capacity, and lean body mass to a 12-week concurrent resistance and interval training program in men and women. Recreationally active women and men completed three training sessions per week consisting of high-intensity, low-volume resistance training followed by interval training performed using a variety upper and lower body exercises representative of military occupational tasks. Pre- and post-training vastus lateralis muscle biopsies were analyzed for changes in muscle fiber type, cross-sectional area, capillarization, and mitochondrial biogenesis marker content. Changes in maximal strength, aerobic capacity, and lean body mass (LBM) were also assessed. Training elicited hypertrophy of type I (12.9%; p = 0.016) and type IIa (12.7%; p = 0.007) muscle fibers in men only. In both sexes, training decreased type IIx fiber expression (1.9%; p = 0.046) and increased total PGC-1α (29.7%, p < 0.001) and citrate synthase (11.0%; p < 0.014) content, but had no effect on COX IV content or muscle capillarization. In both sexes, training increased maximal strength and LBM but not aerobic capacity. The concurrent training program was effective at increasing strength and LBM but not at improving aerobic capacity or skeletal muscle adaptations underpinning aerobic performance.

Fiber-type traps: revisiting common misconceptions about skeletal muscle fiber types with application to motor control, biomechanics, physiology, and biology

Skeletal muscle is a highly complex tissue that is studied by scientists from a wide spectrum of disciplines, including motor control, biomechanics, exercise science, physiology, cell biology, genetics, regenerative medicine, orthopedics, and engineering. Although this diversity in perspectives has led to many important discoveries, historically, there has been limited overlap in discussions across fields. This has led to misconceptions and oversimplifications about muscle biology that can create confusion and potentially slow scientific progress across fields. The purpose of this synthesis paper is to bring together research perspectives across multiple muscle fields to identify common assumptions related to muscle fiber type that are points of concern to clarify. These assumptions include 1) classification by myosin isoform and fiber oxidative capacity is equivalent, 2) fiber cross-sectional area (CSA) is a surrogate marker for myosin isoform or oxidative capacity, and 3) muscle force-generating capacity can be inferred from myosin isoform. We address these three fiber-type traps and provide some context for how these misunderstandings can and do impact experimental design, computational modeling, and interpretations of findings, from the perspective of a range of fields. We stress the dangers of generalizing findings about "muscle fiber types" among muscles or across species or sex, and we note the importance for precise use of common terminology across the muscle fields.

Effect of heat stress on heat shock protein expression and hypertrophy-related signaling in the skeletal muscle of trained individuals

Muscle mass is balanced between hypertrophy and atrophy by cellular processes, including activation of the protein kinase B-mechanistic target of rapamycin (Akt-mTOR) signaling cascade. Stressors apart from exercise and nutrition, such as heat stress, can stimulate the heat shock protein A (HSPA) and C (HSPC) families alongside hypertrophic signaling factors and muscle growth. The effects of heat stress on HSP expression and Akt-mTOR activation in human skeletal muscle and their magnitude of activation compared with known hypertrophic stimuli are unclear. Here, we show a single session of whole body heat stress following resistance exercise increases the expression of HSPA and activation of the Akt-mTOR cascade in skeletal muscle compared with resistance exercise in a healthy, resistance-trained population. Heat stress alone may also exert similar effects, though the responses are notably variable and require further investigation. In addition, acute heat stress in C2C12 muscle cells enhanced myotube growth and myogenic fusion, albeit to a lesser degree than growth factor-mediated hypertrophy. Though the mechanisms by which heat stress stimulates hypertrophy-related signaling and the potential mechanistic role of HSPs remain unclear, these findings provide additional evidence implicating heat stress as a novel growth stimulus when combined with resistance exercise in human skeletal muscle and alone in isolated murine muscle cells. We believe these findings will help drive further applied and mechanistic investigation into how heat stress influences muscular hypertrophy and atrophy.NEW & NOTEWORTHY We show that acute resistance exercise followed by whole body heat stress increases the expression of HSPA and increases activation of the Akt-mTOR cascade in a physically active and resistance-trained population.

Influence of sexual dimorphism on satellite cell regulation and inflammatory response during skeletal muscle regeneration

After injury, skeletal muscle regenerates thanks to the key role of satellite cells (SC). The regeneration process is supported and coordinated by other cell types among which immune cells. Among the mechanisms involved in skeletal muscle regeneration, a sexual dimorphism, involving sex hormones and more particularly estrogens, has been suggested. However, the role of sexual dimorphism on skeletal muscle regeneration is not fully understood, likely to the use of various experimental settings in both animals and human. This review aims at addressing how sex and estrogens regulate both the SC and the inflammatory response during skeletal muscle regeneration by considering the different experimental designs used in both animal models (i.e., ovarian hormone deficiency, estrogen replacement or supplementation, treatments with estrogen receptors agonists/antagonists and models knockout for estrogen receptors) and human (hormone therapy replacement, pre vs. postmenopausal, menstrual cycle variation…).

Exercise metabolism and adaptation in skeletal muscle

Viewing metabolism through the lens of exercise biology has proven an accessible and practical strategy to gain new insights into local and systemic metabolic regulation. Recent methodological developments have advanced understanding of the central role of skeletal muscle in many exercise-associated health benefits and have uncovered the molecular underpinnings driving adaptive responses to training regimens. In this Review, we provide a contemporary view of the metabolic flexibility and functional plasticity of skeletal muscle in response to exercise. First, we provide background on the macrostructure and ultrastructure of skeletal muscle fibres, highlighting the current understanding of sarcomeric networks and mitochondrial subpopulations. Next, we discuss acute exercise skeletal muscle metabolism and the signalling, transcriptional and epigenetic regulation of adaptations to exercise training. We address knowledge gaps throughout and propose future directions for the field. This Review contextualizes recent research of skeletal muscle exercise metabolism, framing further advances and translation into practice.