Effect of repeated hot water immersion on muscle strength, power, function and physical activity in healthy older adults: A randomised crossover trial.

Ageing leads to an increased prevalence of sarcopenia and frailty, characterised by progressive declines in muscle strength, power and function and reduced physical activity. Hot water immersion (HWI) could potentially improve muscle function, but this is yet to be explored in older adults. Twelve middle-aged to older adults completed a randomised, controlled, crossover trial (ClinicalTrials.gov ID NCT05618197), undergoing assessments before and after a 6-week HWI intervention (two to three 60-min HWIs per week) or control condition with a 6-week washout between study arms. During HWIs, body position was adjusted to maintain rectal temperature at 38.5-39.0°C. Pre- and post-intervention and control measurements of peripheral muscle strength (isokinetic and handgrip dynamometry), lower body power and functional performance (Short Physical Performance Battery consisting of balance, walking and sit to stand tests with motion and external force capture) and physical activity (accelerometry) were taken. Repeated HWI had no effect on the primary outcome peak quadriceps torque (P = 0.127, η2 p = 0.125; n = 7), whilst grip strength increased in the control arm (P = 0.004) and decreased post-intervention compared to control (P = 0.039). SPPB total and component scores, lower body power, gait measures and physical activity levels were unchanged (all P > 0.05). Repeated HWI under the conditions employed did not improve strength, power, lower extremity function or physical activity levels in this cohort, and does not appear to be an effective method to improve indices of muscle function in healthy older adults.