title: Temperature affects maximum H-reflex amplitude but not homosynaptic postactivation depression.
abstract: This study aimed to determinate the effect of hyperthermia on transmission efficacy of the Ia-afferent spinal pathway. Recruitment curves of the Hoffman reflex (H-reflex) and compound motor potential (M-wave) along with homosynaptic postactivation depression (HPAD) recovery curves were obtained in 14 volunteers in two controlled ambient temperatures that resulted in significantly different core temperatures (CON, core temperature ∼37.3°C; and HOT, core temperature ∼39.0°C). Electromyographic responses were obtained from the soleus (SOL) and medial gastrocnemius (MG) muscles following electrical stimulation of the tibial nerve at varying intensities and paired pulse frequencies (0.07-10 Hz). Results showed that maximal amplitude of the H-reflex was reached for a similar intensity of stimulation in CON and HOT (both muscles P > 0.47), with a similar associated M-wave (both muscles P > 0.69) but was significantly decreased in HOT as compared to CON (all P < 0.05), whether expressed in absolute terms (-50% in SOL, -32% in MG) or when normalized to the maximum M-wave (-23% in SOL, -32% in MG). The HPAD recovery curve was not affected by the elevated core temperature (both muscles P > 0.23). Taken together, these results suggest that hyperthermia can alter neuromuscular transmission across the neuromuscular junction and/or muscle membrane as well as transmission efficacy of the Ia-afferent pathway, albeit the latter not via an increase in HPAD.
authors.affiliation: Aspetar, Qatar Orthopaedic and Sports Medicine Hospital Doha, Qatar.
authors.forename: Sébastien
authors.initials: S
authors.lastname: Racinais
authors.forename: Andrew G
authors.initials: AG
authors.lastname: Cresswell
keywords: Exercise; hyperthermia; muscle; neuromuscular function; spinal reflex