Hyperthermia reduces electromechanical delay via accelerated electrochemical processes
The present study aimed to determine the effect of hyperthermia on both electrochemical and mechanical components of the electromechanical delay (EMD), using very-high-frame-rate ultrasound. Electrically evoked peak twitch force, EMD, electrochemical (Dm, i.e., delay between stimulation and muscle fascicle motion) and mechanical (Tm, i.e., delay between fascicle motion and force production onset) components of EMD were assessed in sixteen participants. Assessments were conducted in a control ambient environment (CON: 26°C, 34% relative humidity) and in hot ambient environment (HOT: 46-50°C, 18% relative humidity, after ~127 min of heat exposure). Following heat exposure, gastrocnemius medialis temperature was 37.0 ± 0.6°C in HOT vs. 34.0 ± 0.8°C in CON (P < 0.001). EMD was shorter (9.4 ± 0.8 ms) in HOT than CON (10.8 ± 0.6 ms, P < 0.001). Electrochemical processes were shorter in HOT than CON (4.0 ± 0.8 ms vs. 5.5 ± 0.9 ms, respectively, P < 0.001), while mechanical processes were unchanged (P = 0.622). These results demonstrate that hyperthermia reduces electromechanical delay via accelerated electrochemical processes while force transmission along the active and passive parts of the series elastic component is not affected following heat exposure. The present study demonstrates that heat exposure accelerates muscle contraction thanks to faster electrochemical processes. Further investigations during voluntary contractions would contribute to better understand how these findings translate into motor performance.