Influence of local temperature on motor unit behavior during rapid contractions in humans.

Purpose: The rate of torque development (RTD) is temperature dependent, but the temperature effects on motor unit behavior during rapid contractions remain largely unknown. This study aimed to clarify the influence of local limb temperature on motor unit behavior and RTD during rapid contractions in humans.

Methods: Ten healthy male participants rested in a sitting position while immersing their right lower leg in water at different temperatures (Hot: ~43 °C, Neutral: ~33 °C, and Cold: ~10 °C) for 20 min each. The participants then completed a series of voluntary isometric contractions of dorsiflexors while maintaining water immersion in each temperature condition. Specifically, they were instructed to perform two maximal voluntary contractions (MVC) followed by six rapid-hold contractions. High-density surface electromyography was recorded from the tibialis anterior muscle and decomposed into individual motor unit spike trains.

Results: We found that the late RTD (from 0 to 150 ms after the torque onset) was significantly lower in Cold than in the other conditions even when normalized by MVC torque. The motor unit discharge rate at recruitment was significantly higher in Cold (51.4 ± 4.1 pps) than in Hot (42.0 ± 3.8 pps), while the recruitment threshold decreased with the temperature (Cold: 23.9 ± 2.7%, Neutral: 29.2 ± 2.5%, Hot: 36.2 ± 2.4% of MVC). The temperature-induced changes in the late RTD were significantly related to the changes in recruitment time and recruitment threshold.

Conclusion: These findings suggest that local cooling induces earlier motor unit recruitment and higher discharge rate, mitigating the decrease in RTD.