TL;DR
Ex vivo muscle physiology techniques provide a reliable and reproducible approach for quantitatively evaluating skeletal muscle function by enabling direct assessment of intrinsic contractile properties under controlled conditions.
EMS applied during LPS-induced systemic inflammation exacerbated skeletal muscle atrophy and was associated with activation of IL-6/STAT3-C/EBPδ signaling and proteolytic pathways.
Neuromuscular electrical stimulation (NMES) has been previously shown to attenuate muscle atrophy in the 5-days after colorectal surgery. However, if the protocol used was optimal in terms of intensity or time-under-tension remained unknown. Therefore, the aim of this study was to assess the effect of modified NMES protocols on acute muscle protein synthesis (MPS) in healthy volunteers age-matched to those commonly presenting for colorectal surgery. Eight older male volunteers attended 3 separate study visits each comprising a different 30-minute NMES protocol in a within-subject, randomised cross-over design. The protocols were: A) 30Hz, 1s on:1s off (original protocol); B) 100Hz, 1s on:1s off (increased intensity); C) 30Hz, 6s ON:2s OFF (increased time-under-tension). Myofibrillar MPS was assessed using a stable-isotope tracer technique. Effect size of interventions was compared using Cohen’s D. Mixed model analysis demonstrated no fixed effect of time or protocol, nor a time x protocol interaction. However, effect size analysis of before vs. after NMES for each protocol showed a moderate effect size for protocol A (+0.67), a small negative effect size for protocol B (-0.03), and a large effect size for protocol C (+0.87). Tolerability ratings were no different across the three protocols. Although there was no statistically significant difference between the three NMES protocols using mixed methods analysis, the observed effect size data indicates that NMES with prolonged time-under-tension may be the best strategy to optimally stimulate MPS, the physiological process required for muscle mass maintenance/hypertrophy. Further work is required to confirm this.
The priming effect, whereby prior exercise alters responses to subsequent exercise, has been characterized mainly using systemic measures (e.g., pulmonary oxygen uptake kinetics), whereas muscle microvascular mechanisms and sex-specific responses remain unclear. We used diffuse correlation spectroscopy combined with near-infrared spectroscopy (DCS-NIRS) to quantify relative microvascular blood flow index (rBFI) and tissue oxygenation (StO₂) and to derive relative oxygen extraction fraction (rOEF) and relative muscle oxygen consumption (rMRO₂) during rhythmic handgrip exercise in the flexor digitorum superficialis. Twenty-seven healthy young adults (13 males, 14 females) performed two 5-min bouts (Ex1, Ex2) at 10% and 30% maximal voluntary contraction (MVC); females also completed 50% MVC on a separate day. In the overall cohort, between-bout differences were modest at 10% MVC but clearer at 30% MVC, with Ex2 showing higher rBFI and rMRO₂ and lower StO₂ with higher rOEF during the pre-exercise and early-to-mid exercise phases (p < 0.05). In males, priming was evident at 30% MVC: Ex2 showed higher rBFI and rMRO₂ and lower StO₂ with higher rOEF during exercise (p < 0.05). In females, differences at 30% MVC were limited: StO₂ and rOEF did not differ during exercise and rMRO₂ differed only at an isolated early time point. At 50% MVC in females, Ex2 showed higher rBFI across exercise and higher rMRO₂ during the pre-exercise and early exercise phases, with discrete mid-exercise differences (p < 0.05), whereas StO₂ and rOEF did not differ during exercise. These findings indicate that the expression of priming-related microvascular responses is intensity-dependent and sex-specific.
BACKGROUND Intramuscular fat grafting is used for soft tissue augmentation, yet its effects on muscle structure and function remain unclear. OBJECTIVES To evaluate structural and functional outcomes of intramuscular fat grafting using human lipoaspirate in a nude mouse model and assess its regenerative potential. METHODS Fifty-nine female athymic mice received 100 µL injections of low-density fat (n = 20), high-density fat (n = 20), or saline (n = 19) into hind limb muscles. In vivo grip strength was measured weekly for 6 months. Ex vivo mechanical testing was performed at study endpoint. Histology (H&E, Masson's trichrome) assessed muscle architecture and fibrosis, while immunofluorescence with human-specific markers evaluated graft persistence and cellular contribution. RESULTS No statistically significant differences in muscle strength were observed between groups. Peak in vivo hind limb grip strength ranged from 82-96 grams-force (gf), with saline-treated limbs generally demonstrating higher final values (78 gf) than high-density (61 gf) and low-density fat groups (50 gf). Ex vivo peak force measured 9.0 ± 1.8 N (saline), 8.0 ± 0.4 N (high-density), and 7.8 ± 0.5 N (low-density). At 6 months, most adipocytes were resorbed and replaced by collagen-rich extracellular matrix. Fibrosis was greater in high-density fat grafts (25.49 ± 3.28%) than saline (19.42 ± 4.05%) or low-density fat (19.61 ± 3.53%). Limited new muscle formation was observed, predominantly of mouse origin. CONCLUSIONS Intramuscular fat grafting resulted in transient graft survival followed by fibrosis and structural remodeling without functional improvement at 6 months, suggesting a primarily structural rather than regenerative role.
Abstract Serial sarcomere number (SSN) is reduced following immobilization of a muscle in a shortened position in male rodents thereby impairing muscle contractile function. However, in female rodents the effects of oestrogen on SSN and mechanical changes following immobilization and re‐ambulation are unknown. Here we investigated the effects of oestrogen deficiency (i.e., ovariectomized; OVX model) on SSN loss, recovery and contractile function following hindlimb immobilization and 4 weeks of voluntary ambulation. In female Sprague–Dawley rats [(Intact; high oestrogen; n = 24; mass = 290.6 g ± 25.1 g), (OVX; low oestrogen; n = 24; mass = 317.8 g ± 27.3 g)], each left hind‐limb was immobilized in maximal plantar flexion (shortened muscle position) for 14 days. In vivo plantar flexor torque and power were assessed pre‐cast and at 0, 1, 2 and 4 weeks post‐cast. Six rats per group were euthanized at each post‐cast time point for measurements of soleus and medial gastrocnemius SSN. Immediately post‐cast, soleus and medial gastrocnemius SSN and plantar flexor peak torque were reduced similarly in both groups (∼16% and ∼45%, respectively; P < 0.05). After 4 weeks of re‐ambulation, intact rats demonstrated full recovery of SSN and torque, whereas OVX rats exhibited persistent deficits (−10% and −30%, respectively; P < 0.05). These findings indicate that oestrogen facilitates serial sarcomerogenesis and functional recovery following immobilization in female skeletal muscle.
ABSTRACT Background Skeletal muscle injuries significantly impair mobility and function, yet effective therapeutic interventions remain limited. Both eccentric exercise (EE) and concentric exercise (CE) promote muscle repair, but the underlying mechanisms are not fully understood. Muscle‐derived extracellular vesicles (mEVs) have emerged as critical mediators of intercellular communication; however, their role in exercise‐induced regeneration remains unclear. Methods A murine model of barium chloride‐induced muscle injury was used to compare the effects of EE and CE on muscle regeneration. mEVs were isolated from sedentary (SED), CE‐ and EE‐conditioned muscle and characterised by nanoparticle tracking analysis, transmission electron microscopy and western blotting. Metabolic profiling of mEVs was performed using LC–MS. The functional roles were assessed through intramuscular injection of mEVs and GW4869‐mediated inhibition of mEV secretion. The effects of mEVs on myogenesis were further examined in C2C12 myoblasts. Results EE significantly enhanced muscle regeneration compared with CE, as evidenced by improved histology, reduced fibrosis (F (2,15) = 59.37, p < 0.0001) and increased expression of myogenic markers such as Myod (p < 0.001), Myog (p < 0.001) and eMyhc (p < 0.001). EE also induced greater release of mEVs than CE, as indicated by higher expression of mEV markers and Rab27a/b (Rab27a, p < 0.001; Rab27b, p = 0.1222). Inhibition of mEV secretion with GW4869 abolished the regenerative benefits of exercise. Exogenous administration of EE‐mEVs enhanced muscle repair and C2C12 differentiation (Myod, F (2, 6) = 33.09, p < 0.001; Myog, F (2, 6) = 66.41, p < 0.001) more effectively than CE‐mEVs or SED‐mEVs. Metabolomic analysis revealed significant enrichment of lipid metabolites in EE‐mEVs (N = 5, p < 0.05), which was consistent with the upregulation of lipid metabolism–related genes. RNA‐seq analyses further indicated that lipid metabolites enriched in mEVs contributed to muscle repair potentially through activation of energy‐sensing pathways such as AMPK. Conclusions EE facilitates muscle repair more effectively than CE by promoting the release of mEVs enriched in pro‐regenerative lipid metabolites. These findings suggest EE‐mEVs as a promising biological therapeutic strategy for muscle injury, particularly in cases where exercise is not feasible.
