No clear benefit of muscle heating on hypertrophy and strength with resistance training

Abstract

Heat is a major stressor during exercise, though its value in driving adaptation is not well understood. Muscle heating can upregulate pathways facilitating protein synthesis and could thereby enhance effects of exercise training, however, few studies have investigated this possibility. We examined whether heating active muscle during resistance training differentially affected physical and functional adaptations. Within a randomised contralateral-limb control study, ten healthy, resistance-untrained individuals (21 ± 3 y; 5 female) completed 30 sessions of progressive resistance training (12 weeks), performing 4 × 8 unilateral knee extensions at 70% of 1RM. One randomly-allocated thigh was heated during, and for 20 min after, each session using an electric pad eliciting muscle temperatures of >38 °C (HOT); the contralateral limb remained unheated (CON). Training intensity was progressed using 4-weekly strength assessments. Quadricep lean mass (measured using DXA) increased by 15 ± 7% in HOT (p = 0.00) and 15 ± 6% in CON (p = 0.00); the difference being trivial (p = 0.94). Peak isokinetic torque at 90°.s^-1 increased by 30 ± 25% (HOT; p = 0.00) and 34 ± 33% (CON; p = 0.01), with no difference (p = 0.84) between limbs. Rate of torque development increased ∼40%, with no difference between limbs (p = 0.73). The increase in 3-RM strength was also similar in HOT (75 ± 16%) and CON (71 ± 14%; p = 0.80 for difference). No differences in mass or strength changes were evident between sexes. In conclusion, supplemental heating of active muscle during and after each bout of resistance training showed no clear positive (or negative) effect on training-induced hypertrophy or function.

Keywords: DXA; females; force; isokinetic; quadriceps; torque.

Figure 1.

DXA thigh region of interest (ROI), showing 1) left thigh, and 2) right thigh borders used to determine muscle mass from total body scan analyses.

Figure 2.

Mean (±95% CI) and individual (n = 10) percent change in thigh muscle mass of control (CON) and heated (HOT) limbs when measured after 12-week resistance training programme; participants' between-condition (HOT-CON) differences are highlighted with red and blue shaded areas indicating greater HOT and CON response, respectively; grey shaded area indicates zone of trivial effect; *significantly different (p < 0.01) to baseline.

Figure 3.

Mean (±95% CI) and individual (n = 10) percent change in A) peak isokinetic concentric torque, B) mean isokinetic concentric torque, and C) peak rate of torque development of control (CON) and heated (HOT) limbs when measured after 12-week resistance training; participants' between-condition (HOT- CON) differences are highlighted with red and blue shaded areas indicating greater HOT and CON response, respectively; grey shaded areas indicate zone of trivial effect; *significantly different (p < 0.05) to baseline.

Figure 4.

Mean (±95% CI) and individual (n = 10) percent change in knee extensor 3-repetition maximum (3RM) strength of control (CON) and heated (HOT) limbs when measured after 12-week resistance training; participants' between-condition (HOT-CON) differences are highlighted with red and blue shaded areas indicating greater HOT and CON response, respectively; grey shaded areas indicate zone of trivial effect; *significantly different (p < 0.05) from baseline.

Figure 5.

Mean (±95% CI) and individual (n = 10) percent change in knee extensor muscle quality of control (CON) and heated (HOT) limbs when measured after 12-week resistance training; participants' between-condition (HOT-CON) differences are highlighted with red and blue shaded areas indicating greater HOT and CON response, respectively; grey shaded areas indicate zone of trivial effect.