Passive thigh heating improves peak force production in younger adults and early isokinetic force production in younger and older adults

Older adults often suffer from reduced physical capability relative to young adults, in part due to impaired muscle function. This study investigated the ergogenic effects of passive thigh heating on knee extensor torque production in healthy older versus younger adults. Twenty-two younger (YOUNGER; 23 ± 3 years) and 16 older (OLDER; 68 ± 8 years) adults completed an experimental visit whereby one thigh was heated via a garment circulating 50°C water for 90 min (HEAT) with the contralateral limb unheated (CONT). Four maximal contractions were performed at three isokinetic speeds (slow, 60°/s; moderate, 180°/s; and fast, 300°/s) and an isotonic set (25% maximal voluntary isometric contraction force); contractions were performed on both limbs at baseline and every 30 min thereafter for 120 min, with the final time point used to quantify the retention/decay in response. Vastus lateralis temperature was measured every 30 min, and surface electromyography was implemented throughout. HEAT increased muscle temperature from baseline (31.7 ± 1.7°C) at 30 min (36.5 ± 1.5°C), peaking at 90 min (37.5 ± 0.7°C), all P < 0.05. HEAT 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). After 30 min, rate of force development (RFD) in HEAT increased during slow contractions from baseline in both age groups (+229 ± 210 N m s^-1, P < 0.05) and early force production (EFP) increased in both age groups during slow contractions from 60 min in HEAT (+15 ± 15 N m, P < 0.05). Peak EMG amplitude was unchanged throughout. Despite a similar increase in the RFD and EFP in both young and older adults, passive thigh heating improves peak knee extensor torque in moderate and fast isokinetic contractions in young adults only.