Effects of Repeated Local Heat Therapy on Skeletal Muscle Structure and Function in Humans

The purpose of the present study was to examine the effects of repeated exposure to local heat therapy (HT) on skeletal muscle function, myofiber morphology, capillarization and mitochondrial content in humans. Twelve young adults (23.6±4.8 years, BMI 24.9±3.0 kg/m2) had one randomly selected thigh treated with HT (garment perfused with water at ~52ºC) for 8 consecutive weeks (90 min, 5 days/week) while the opposite thigh served as a control. Biopsies were obtained from the vastus lateralis muscle before and after 4 and 8 weeks of treatment. Knee extensor strength and fatigue resistance were also assessed using isokinetic dynamometry. The changes in peak isokinetic torque were higher (p=0.007) in the thigh exposed to HT than in the control thigh at weeks 4 (Control: 4.2±13.1 Nm vs. HT: 9.1±16.1 Nm) and 8 (Control: 1.8±9.7 Nm vs. HT: 7.8±10.2 Nm). Exposure to HT averted a temporal decline in capillarization around type 2 fibers (p<0.05), but had no effect on capillarization indices in type 1 fibers. The content of eNOS was ~18% and 35% higher in the thigh exposed to HT at 4 and 8 weeks, respectively (p=0.003). Similarly, HT increased the content of small heat shock proteins HSPB5 (p=0.007) and HSPB1 (p=0.009). There were no differences between thighs for the changes in fiber CSA and mitochondrial content. These results indicate that exposure to local HT for 8 weeks promotes a pro-angiogenic environment and enhances muscle strength but does not affect mitochondrial content in humans.