Determining the role of exercise-induced heat in resistance exercise conditioning

Introduction: Exercise is known to increase body temperature, but the temperature of exercising muscle is under-examined, particularly in resistance exercise. Muscle temperature is of interest because muscle heating per se can promote hypertrophy and protect against atrophy. The aims of this project were to: (i) characterise muscle temperature responses to typical resistance exercise training regimes, (ii) investigate the feasibility of preventing the muscle temperature rise, and (iii) determine the extent to which exercise-induced heat underpins adaptations from resistance training. The hypotheses were that: i) high repetition, short-rest exercise would be the most thermogenic exercise regimen, and (ii) resistance training with prevention of exercise-induced rises in muscle temperature from the active muscle would attenuate hypertrophic and strength adaptation, when compared to matched training with exercise-induced heat accumulation. Methods: Two studies were completed. In Study 1, five physically-active participants (two females) undertook three work-matched resistance exercise sessions in randomised order, on separate days. Unilateral bicep curls were used in sessions representing hypertrophy training (3x10 repetitions at 67% 1RM), strength-endurance training (3x20 repetitions at 34% 1RM), and strength training (6x4 repetitions at 84% 1RM). Thereafter, the feasibility of preventing muscle temperature rise during a strength session was assessed using arm immersion in 14°C water for 10 minutes preceding the first exercise set and between each remaining set. Study 2 was a preliminary study on the effects of muscle temperature on adaptations to resistance exercise. Five healthy non-resistance trained participants (three females) completed a 6-week bicep curl resistance training programme using a contralateral limb-control design. Eighteen strength training sessions (6x4 repetitions at ~80% 1RM) were completed with one arm randomised to train in a cool state (“cool”, as described above) and the other arm training with natural heat accumulation (“warm”). Results: Study 1: The three regimes increased biceps brachii temperature to a similar extent; 2.0±0.8°C for hypertrophy, 2.5±1.0°C for strength-endurance, and 2.2±0.5°C for strength training (baseline: 35.3±0.8°C; time: p<0.001; condition: p=0.489; interaction: p=0.609). The first third of the exercise session accounted for 46±18%, 62±13% and 60±9% of the total muscle temperature rise for hypertrophy, strength-endurance and strength regimes, respectively (condition: p=0.147). Almost half (44±23%) of the muscle temperature increase was still evident after 15-min recovery, with no effect of condition (condition: p=0.649). Resistance exercise with cooling prevented muscle temperature exceeding its baseline (35.7±0.9°C; post- exercise: 34.6±1.2°C; p=0.164). Study 2: Peak isometric torque increased in both arms, with no effect of condition (warm: 11±11%; cool 4±7%; time: p=0.033; condition: p=0.310). Bicep curl 1RM increased similarly for both conditions (warm: 25±11%; cool 26±11%; time: p