Physical activity results in various types of stress in skeletal muscle including energetic, oxidative, and heat stress. Acute exposure to stress impairs skeletal muscle mitochondrial function. In contrast, chronic intermittent exposure to mild stress through exercise training results in increased mitochondrial content and respiratory capacity. While oxidative and energetic stress have received much attention regarding their long-term effect on skeletal muscle mitochondria, heat stress is not well understood. The purpose of this work was to investigate the effects of localized heat therapy on human skeletal muscle mitochondria, and to compare these effects to those of high-intensity interval exercise training. To accomplish this purpose, 35 subjects were assigned to receive 6 weeks of sham therapy, heat therapy, or exercise training; all localized to the quadriceps muscles of the right leg. Two-hour sessions of short-wave diathermy were used for the heat therapy, and identical sessions were used for sham therapy, but the diathermy units were not activated. Forty-minute sessions of single-leg extension, high-intensity interval training were used for the exercise intervention. All interventions took place three times per week. Muscle biopsies were performed at baseline, and after three and six weeks of intervention. Muscle fiber bundles were isolated and permeabilized for measurement of oxygen consumption via high-resolution respirometry. The primary finding of this work was that heat therapy improves mitochondrial respiratory capacity by 24.8 ± 6.2% compared to a 27.9 ± 8.7% improvement following exercise training. Both heat and exercise significantly increased mitochondrial respiration compared to baseline measures (p<0.05). Fatty acid oxidation and citrate synthase activity were also increased following exercise training by 29.5 ± 6.8% and 19.0 ± 7.4%, respectively (p<0.05). However, contrary to our hypothesis, heat therapy did not increase fatty acid oxidation or citrate synthase activity. Neither heat nor exercise training increased mitochondrial respiratory protein content. Overall these results suggest that heat therapy significantly improves mitochondrial function, but not to the same degree as exercise training.



College and Department

Life Sciences; Nutrition, Dietetics, and Food Science



Date Submitted


Document Type





heat stress, heat therapy, thermic stress, short-wave diathermy, mitochondrial function, mitochondrial respiration, oxidative phosphorylation, respiratory capacity, skeletal muscle, high-intensity interval training, permeabilized fibers



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Life Sciences Commons