Localized Heat Therapy Improves Mitochondrial Respiratory Capacity but Not Fatty Acid Oxidation

Abstract

AIM: Mild heat stress can improve mitochondrial respiratory capacity in skeletal muscle. However, long-term heat interventions are scarce, and the effects of heat therapy need to be understood in the context of the adaptations which follow the more complex combination of stimuli from exercise training. The purpose of this work was to compare the effects of 6 weeks of localized heat therapy on human skeletal muscle mitochondria to single-leg interval training. METHODS: Thirty-five subjects were assigned to receive sham therapy, short-wave diathermy heat therapy, or single-leg interval exercise training, localized to the quadriceps muscles of the right leg. All interventions took place 3 times per week. Muscle biopsies were performed at baseline, and after 3 and 6 weeks of intervention. Mitochondrial respiratory capacity was assessed on permeabilized muscle fibers via high-resolution respirometry. RESULTS: The primary finding of this work was that heat therapy and exercise training significantly improved mitochondrial respiratory capacity by 24.8 ± 6.2% and 27.9 ± 8.7%, respectively (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. CONCLUSION: Six weeks of muscle-localized heat therapy significantly improves mitochondrial respiratory capacity, comparable to exercise training. However, unlike exercise, heat does not improve fatty acid oxidation capacity.

Keywords: exercise; heat stress; high-intensity interval training; mitochondria; skeletal muscle.

Figure 1

Study timeline (A), and internal muscle temperature during sham, HT and EX interventions (B). Statistical difference in the HT group compared to time 0 is indicated by § (p < 0.001). Statistical difference in the EX group compared to time 0 is indicated by # (p < 0.001). Results are represented as mean ± SEM (n = 7–8).

Figure 2

Work rate max attained during single-leg extension graded exercise test to exhaustion. Statistical difference in the EX group compared to time 0 is indicated by # (p <0.0001). Results are represented as mean ± SEM (n = 11–12).

Figure 3

Mitochondrial protein content assessed by Western blot. (A–F) All individual values are reported as a fold change from their own baseline measure. Results are represented as mean ± SEM (n = 8–10). (G) Representative blots.

Figure 4

Citrate synthase and HAD activity. (A) Citrate synthase activity. (B) HAD activity. Statistical difference in the EX group compared to baseline is indicated by # (p < 0.05). Statistical differences between groups are indicated by % (p < 0.05). Results are represented as mean ± SEM (n = 8–10).

Figure 5

Mitochondrial respiration expressed as absolute respiration over the course of the intervention (A–C) and respiratory control ratio (D). Respiration expressed as a percent change from baseline in each group (E–G). Statistical difference in the HT group compared to baseline is indicated by § (p < 0.05). Statistical difference in the EX group compared to baseline is indicated by # (p < 0.05). In panels E-G, shared letters are no different from each other, and an asterisk (*) indicates a significant difference from baseline (p < 0.05). Results are represented as mean ± SEM (n = 8–11).

Figure 6

(A–C) Mitochondrial respiration normalized to citrate synthase activity. Units for this measure are nmol O₂*s⁻¹/U CS. Statistical difference in the HT group compared to baseline is indicated by § (p < 0.01). Results are represented as mean ± SEM (n = 8–10).

Figure 7

Mitochondrial fatty acid oxidation expressed as absolute respiration over the course of the intervention (A–D) and respiratory control ratio (E). Respiration expressed as a percent change from baseline in each group (F–I). Statistical difference in the EX group compared to baseline is indicated by # (p < 0.05). In panels (E–G), shared letters are no different from each other, and an asterisk (*) indicates a significant difference from baseline (p < 0.05). Results are represented as mean ± SEM (n = 9–10).