The effects of muscle heating on hypertrophy and strength responses to resistance training

Background: Heat is a major form of stress within exercise, though its value in driving adaptation is not well understood. Acutely, various aspects of muscle function appear to respond differently to changes in muscle temperature. Increased muscle temperature can increase speed of contraction and relaxation, but also hasten fatigue. Chronically, increased core, muscle and skin temperatures have been shown to play a direct or indirect role in post- exercise hypotension and subsequent blood volume expansion, improved vascular function, and upregulation of stress proteins, which has multiple effects including regeneration of muscle primarily through facilitation of muscle protein synthesis. These studies indicate that heating of muscles could protect against disuse atrophy or even produce hypertrophy in humans. However, few studies have investigated the effects of active muscle heating as an adjunct to a hypertrophy-focused resistance training programme. As such, this could provide a novel means to increase muscle quality and mass, which could be beneficial for general, clinical, and elite populations for its effect on metabolic control, functional performance, and overall quality of life. Aim: The aim of the study was to assess whether supplemental heating (HOT) of active muscle during resistance training differentially affected physical and functional adaptations compared to those from traditional training without muscle heating (CON). Method: Ten (5 female; 21 ± 3 y) healthy, resistance-untrained individuals completed 12-wk progressive resistance training of the knee extensors, comprising 30 sessions of 64 repetitions (4 sets of 8 per leg) of unilateral knee extensions at 70% of leg-specific 1RM. In a contralateral limb-control design, one randomly allocated thigh was heated during and for 20 min after exercise in each training session, using a customised heat pad eliciting muscle temperatures of 38–39.5 °C. Thigh lean mass was measured at baseline and 12 weeks, and concentric knee extensor maximal isokinetic (90°.s-1) torque was measured at baseline and each 4 wk. Results: Within-condition quadriceps’ lean mass increased across 12-wk training, by 15 ± 7% (CON; 761 ± 280 g;p = 0.00) and 15 ± 6% (HOT; 752 ± 304 g;p = 0.00); the between- condition difference being trivial (0 ± 6%; p = 0.94). Within-condition peak torque increased by 30 ± 25% (p = 0.00) and 34 ± 33% (p = 0.01), respectively, with no significant between- condition difference (p = 0.93). Rate of torque development increased by 42 ± 47% in CON (p = 0.04) and 37 ± 34% (p = 0.02) in HOT (between-condition difference: 5 ± 44%; p = 0.73). Within-condition 3RM strength increased significantly by 75 ± 16% in HOT (p = 0.00) and by 71 ± 14% in CON (between-condition difference: 4 ± 5%; p = 0.80). No sex differences were evident for mass or strength changes. Conclusions: Heating of the active muscle mass during and after resistance training shows no clear positive (or negative) effect on training-induced hypertrophy or the improvements in concentric isokinetic strength, rate of torque development, or 3-repetition maximum strength compared to those from resistance training without heat