Cognitive decrements do not follow neuromuscular alterations during passive heat exposure

To investigate what triggers cognitive and neuromuscular alterations during passive heat exposure, eight volunteers performed simple (One Touch Stockings of Cambridge, OTS-4) and complex (OTS-6) cognitive tasks as well as neuromuscular testing (maximal isometric voluntary contractions of the thumb with electrical stimulation of the motor nerve and magnetic stimulation of the motor cortex). These tests were performed at the start (T1), after 1 h 30 min (T2), 3 h (T3) and 4 h 30 min (T4) of exposure in both hot (HOT) (Wet Bulb Globe Temperature [WBGT] = 38° ± 1.4°C) and neutral control (CON) (WBGT = 19° ± 0.3°C) environments. Environmental temperatures were adjusted during the HOT session to induce target core temperatures (T(core)) (T1 ∼ 37.3°; T2 ∼ 37.8°; T3 ∼ 38.3°; T4 ∼ 38.8°C). At T1 and T4 the OTS-6 was lower in HOT than in CON in response to the rapid increase in skin temperature and to hyperthermia, respectively. In HOT, the increase in T(core) limited force production capacity possibly via alterations occurring upstream the motor cortex (from T(core) ∼ 37.8°C) but also via a decrement in motor cortical excitability (from T(core) ∼ 38.3°C). These alterations in cortex excitability failed to explain the cognitive alterations that can originate from an additional cognitive load imposed by temperature variations.