A passive increase in muscle temperature enhances rapid force production and neuromuscular function in healthy adults

To test the effects of hot-water immersion on the rapid force production and parameters of neuromuscular function in healthy adults.

Cross-sectional study.

Fifteen healthy adults (24.9±5.6 years; 178±11.4cm; 72.8±16.2kg) performed neuromuscular assessments before, after and ∼15min after either 90min of 42°C (hot) or 36°C (sham-condition) water immersion (lower body). Knee extensors rate of torque development (RTD) was measured during explosive voluntary contraction in the interval of 0-50ms (RTD_V50) and 0-150ms (RTD_V150) and during electrically-evoked contractions by single twitches (RTD_twitch) and low- and high-frequencies doublets (RTD_20Hz and _100Hz). Rate of EMG rise (RER) was calculated for voluntary contractions and half-relaxation time (HRT) and electromechanical delay (EMD) was measured during single twitches.

After the hot-water immersion (when rectal and muscle temperature were elevated [↑1°C and ↑2.4°C, respectively]), RTD_V50, RTD_20Hz and RTD_100Hz significantly increased and HRT decreased when compared to baseline and sham-condition (p<0.05). Approximately 15min after the hot-water immersion (when muscle temperature was still higher [↑1.4°C], but rectal temperature at baseline level), RTD_V50 remained higher and RTD_twitch presented higher values than baseline and sham-condition. The RTD_20Hz and RTD_100Hz showed further increases compared to post hot-water immersion trials. HRT showed no changes compared to post water immersion, but the EMD presented lower values than baseline and sham-condition. No changes were observed for RTD_V150 and RER at any moment.

Increased muscle temperature provoked by 42°C hot-water immersion increases the early phase of the RTD (<70ms) (voluntary and evoked) and decreases HRT and EMD of the knee extensors.