Effects of repeated local heat therapy on skeletal muscle structure and function in humans

Abstract

Introduction Short‐term exposure to heat stress has been shown to enhance muscle strength and elicit mitochondrial adaptations in human skeletal muscle. We recently documented that exposure to five sessions of heat therapy (HT) accelerates the recovery of fatigue resistance following exercise‐induced muscle damage in humans. However, the long‐term impact of repeated exposure to local heat stress on contractile function and skeletal muscle structure remains unknown. The purpose of the present study was to examine the effects of 8 weeks of daily treatment with local HT on muscle strength, fatigability, fiber size and mitochondrial content in humans. Methods Twelve healthy young adults (23.6±1.4 years, BMI 24.9±0.9 kg/m2) had one randomly selected thigh treated with daily HT for 8 consecutive weeks while the opposite thigh served as a control. A water‐circulating garment perfused with water at ~52ºC was used to apply HT for 90 min daily. Knee extensor strength (single peak torque during 3 repetition maximums at 180°/s and 60°/s) and fatigue resistance (total work during 40 repetition maximums at 180°/s) were assessed using isokinetic dynamometry before and after 4 and 8 weeks of treatment. Biopsies were obtained from the vastus lateralis muscle for the determination of myofiber cross‐sectional area and maximal citrate synthase activity. Results Treatment with HT elicited an increase in thigh skin temperature (39.8±0.1ºC), while skin temperature remained at baseline levels in the control thigh (32.4± 0.1ºC). A significant treatment effect was observed for the changes from baseline in peak isokinetic torque at 180º/s (p=0.04). On average, the changes in peak isokinetic torque from baseline for HT were ~2 fold higher than control at week 4 (Control: 4.1±3.7 Nm vs. HT: 9±4.6 Nm) and ~4 fold higher at week 8 (Control: 1.7±2.7 Nm vs. HT: 7.7±2.9 Nm). There were no differences between treatments for peak isokinetic torque at 60º/s, myofiber cross‐sectional area and citrate synthase activity. Conclusion These results indicate that daily exposure to HT for 8 weeks enhances muscle strength in humans. Local HT may be a practical therapeutic approach to improve muscle function in individuals with limited mobility and skeletal muscle weakness. Support or Funding Information Support: American College of Sports Medicine Research Endowment This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Skeletal Muscle Signaling Following Whole-Body and Localized Heat Exposure in Humans

“…Consistently, we found no effect of SL on upstream kinases (i.e., AMPK, p38 MAPK) regulating the mitochondrial biogenesis program. Instead, our findings align with Kim et al (2020) , who demonstrated no effect on mitochondrial adaptations following 8 weeks of localized (quadriceps) heat therapy. As discussed, such divergent findings may be attributed to differences in the duration of heating and changes in muscle temperature.…”

“…In addition, heating modality may also account for the observed differences. Specifically, pulsed shortwave diathermy utilized by Hafen et al (2018 , 2019) enables rapid and substantial deep tissue heating, and resultantly may be more effective at activating mitochondrial adaptations compared with superficial heat modalities as employed in the current and previous report ( Kim et al, 2020 ). Taken together, the current and previous findings ( Hafen et al, 2018 , 2019 ; Kim et al, 2020 ) tend to indicate muscle temperature, core temperature and duration of stimulus as important factors that determine the extent to which whole body or localized heat stress may be beneficial.…”

“…For instance, despite substantial increases in core (∼1.5°C) and muscle (∼3.6°C) temperatures, 60 min of one-legged hot water immersion has been shown to demonstrate no effect on the mRNA levels of HSP 27, HSP 60, or HSP 70 ( Morton et al, 2007 ). Moreover, protein expressions of HSP 70 and HSP 90 were not altered following 8 weeks of localized heat therapy to the quadriceps ( Kim et al, 2020 ). In contrast, a recent study reported increased HSP mRNA levels following 90 min of localized quadriceps heat therapy ( Kuhlenhoelter et al, 2016 ).…”

“…For instance, short-term (2 h/day for 6–10 days) localized heat therapy targeting the quadriceps has been shown to attenuated decline in muscle mass in participants immobilized over a 2-week period ( Hafen et al, 2019 ), with longer term (8–10 weeks) treatment resulting in increased muscle mass, muscle capillarity, and improved knee extensor muscle strength in healthy weight-bearing participants ( Goto et al, 2011 ; Kim et al, 2020 ). While these studies support the therapeutic potential of localized heat treatment, the mechanisms involved show discord, with some showing evidence of a HSP-mitochondrial-centered mechanism ( Hafen et al, 2018 , 2019 ), whilst others demonstrating limited involvement of such pathways in human models of localized heat therapy ( Kim et al, 2020 ). Similar discrepancies are observed in passive whole-body (WB) modalities, where acute heat stress has been shown to repress PGC-1α mRNA, a key regulator of mitochondrial adaptations ( Petrie et al, 2016 ).…”

A Role for Heat Therapy in Low Back Pain in Modern Clinical Practice

“…Connective tissues may also change viscosity and density in response to heat, thereby improving the range of movement and enhancing tissue extensibility. Recent evidence also suggests that localized, repeated heat therapy may promote an angiogenic environment and enhance muscle strength [17,18,29,41,42]. One advantage of superficial heat therapy is its safety profile.…”