Effects of long-term episodic heat exposure on skeletal muscle morphology and capillarization in aged, obese mice

Aging is accompanied by progressive muscle fiber atrophy, changes in fiber-type composition, and reductions in capillarization, ultimately impairing skeletal muscle function. While heat therapy (HT) has demonstrated potential to stimulate angiogenic and anabolic pathways, its capacity to mitigate structural and microvascular impairments in skeletal muscle under conditions of aging exacerbated by obesity remains poorly defined. OBJECTIVE: To determine the effects of long-term, repeated episodic HT on skeletal muscle morphology and capillarization in a mouse model of aging with diet-induced obesity. HYPOTHESIS: HT would attenuate age-related reductions in muscle fiber size and distribution and promote improvements in capillarization.METHODS: Male C57BL/6J mice (52 weeks old) were fed a high-fat diet (60% kcal fat) and randomized to SHAM (n=9) or HT (n=6). HT consisted of exposure to a 39°C incubator, while SHAM animals remained at 22°C. The intervention progressed over 11 months: 30 min/day twice weekly (months 1–3), 20 min/day three times weekly (months 4–6), and 15 min/day four times weekly (months 7–11). Forty-eight hours after the final exposure, the extensor digitorum longus (EDL) and soleus (SOL) were harvested for analysis of fiber cross-sectional area, fiber-type composition, and capillarization. Data were analyzed by one-way ANOVA (Holm-Šidák post-hoc; α=0.05; mean ± S.E.M.). RESULTS: HT did not affect muscle fiber size or fiber-type composition in either muscle compared with SHAM. In contrast, capillarization indices were significantly increased in the SOL of HT-treated mice. Capillary contacts (CC) were greater in HT versus SHAM for type IIa fibers (7.3±0.2 vs 5.5±0.2, p< 0.01), type IIx fibers (6.8±0.2 vs 5.5±0.2, p< 0.01), and type I fibers (6.5±0.2 vs 5.4±0.2, p< 0.01). Capillary-to-fiber ratio (C/Fi) was also higher in HT compared with SHAM for type IIa (3.0±0.1 vs 2.4±0.1, p< 0.01) and type I fibers (2.7±0.2 vs 2.2±0.2, p< 0.05), with a trend toward elevation in type IIx fibers (2.8±0.1 vs 2.4±0.1, p=0.08). No differences were observed in the EDL. CONCLUSIONS: These findings demonstrate that long-term episodic heat therapy selectively enhances capillarization in the oxidative soleus muscle, suggesting that HT may represent a promising strategy to counteract age-related capillary rarefaction in skeletal muscle. Funding sources: R01AG073634 This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.