Muscular function, largely absolute strength, predicates regular human function and contributes to a higher quality of life by supporting independent bodily movements and locomotion. Older adults often suffer from a reduced capability to complete physical activity or daily tasks relative to young adults due to limitations in muscle function. Prior work in young adults has identified passive heating as a potential ergogenic aid to improve muscle function, however the efficacy of the intervention has not been comprehensively explored in older adults. The primary aim of this study was to quantify the effect of 90 min of passive thigh heating on peak torque, rate of force development and early force production during isokinetic contractions in younger vs. older adults. The secondary aim was to measure the retention/decay of the anticipated improvements in function in younger vs. older adults 30 min post cessation of heating.

Twenty-two healthy young (YOUNGER; 23 ± 3 y) and sixteen healthy older adults (OLDER; 68 ± 8 y) completed an experimental visit whereby one thigh was heated via a garment circulating 50°C water for 90 min with the contralateral limb remaining unheated. Four maximal knee extension contractions were performed at three isokinetic speeds (slow, 60°/s, moderate, 180°/s and fast, 300°/s) on both limbs at baseline and every 30 min thereafter for 120 min with the final timepoint used to quantify the retention/decay in response. Thigh (Vastus lateralis) muscle temperature was measured every 30 min with surface electromyography (EMG) implemented to monitor muscle activation.

Heating increased muscle temperature from baseline (31.7 ± 1.7°C) at 30 min (36.5 ± 1.5°C), 60 min (37.1 ± 1.4°C), 90 min (37.5 ± 0.7°C) and 120 min (35.5 ± 2.3°C), all p <0.05. Heating increased peak torque during moderate (+11 ± 12 N.m) and fast (+7 ± 11 N.m) contractions in only YOUNGER participants relative to their control leg which remained unchanged (p < 0.05). Rate of force development was increased from baseline in HEAT by 229 ± 210 N.m.s-1(p < 0.05) in YOUNGER and OLDER. Early force production increased from baseline in YOUNGER and OLDER during the slow contractions (+15 ± 15 N.m) in the heated leg (p < 0.05). Peak EMG was unchanged throughout the experiment across all contraction speeds.

Passive thigh heating increased muscle temperature by ~5°C during the protocol. Peak isokinetic force in younger adults improved during heating at moderate (+8%) and in younger and older during fast (+10%) contractile speeds. Rate of force development during slow isokinetic contractions increased from baseline by 29%. Early force production during slow isokinetic contractions increased from baseline in younger (+13%) and older adults (+28%) during passive heating. Whilst there are some observable differences in the peak torque responses of younger and older adults to passive heating, in aggregate, the intervention can be considered beneficial when implemented prior to exercise given improvements in the rate of force development and early force production irrespective of age.