Hot ambient conditions shift the Force / EMG relationship

PURPOSE: This manuscript reports the data from two interventions on human subjects aiming to determine the effect of elevated core (HOT-core, study 1, 14 participants) and skin (HOT-skin, study 2, 11 participants) temperature on the force / EMG relationship. METHODS: In both studies, participant underwent an experimental trial and a control (CON) trial, in which maximal voluntary contractions (MVC) of the plantar flexors, surface EMG recordings of both the soleus and gastrocnemius medialis (GM), and electrical stimulation of the tibial nerve were performed to determine the percentage of voluntary activation (VA). During the HOT-core trial, rectal temperature was passively increased and then clamped at 39°C by adjusting the room temperature in the range of 46-50°C. During the HOT-skin trial, tests were performed in a temperate environment (~20°C) and skin temperature was locally affected by applying a cool or a hot pack during 1 min. RESULTS: HOT-core resulted in a decrease in MVC torque (−19%) and VA (−5%) (p < 0.05). HOT-skin did not induce any changes in MVC torque (−1%) or VA (+0%). However, the EMG activity (RMS) was decreased both in HOT-core (soleus −40%, GM −33%) and HOT-skin (soleus −10%, GM −13%), compared with CON (p < 0.05). CONCLUSION: The effect of skin temperature on EMG recordings may be attributed to both methodological and physiological factors. Hot ambient conditions shift the Torque / EMG relationship to the left, with the decrement in torque induced with passive hyperthermia lower than the decrement in EMG.