Core and skin temperature influences on the surface electromyographic responses to an isometric force and position task

Abstract

The large body of work demonstrating hyperthermic impairment of neuromuscular function has utilized maximal isometric contractions, but extrapolating these findings to whole-body exercise and submaximal, dynamic contractions may be problematic. We isolated and compared core and skin temperature influences on an isometric force task versus a position task requiring dynamic maintenance of joint angle. Surface electromyography (sEMG) was measured on the flexor carpi radialis at 60% of baseline maximal voluntary contraction while either pushing against a rigid restraint (force task) or while maintaining a constant wrist angle and supporting an equivalent inertial load (position task). Twenty participants performed each task at 0.5°C rectal temperature (Tre) intervals while being passively heated from 37.1±0.3°C to ≥1.5°C Tre and then cooled to 37.8±0.3°C, permitting separate analyses of core versus skin temperature influences. During a 3-s contraction, trend analysis revealed a quadratic trend that peaked during hyperthermia for root-mean-square (RMS) amplitude during the force task. In contrast, RMS amplitude during the position task remained stable with passive heating, then rapidly increased with the initial decrease in skin temperature at the onset of passive cooling (p = 0.010). Combined hot core and hot skin elicited shifts toward higher frequencies in the sEMG signal during the force task (p = 0.003), whereas inconsistent changes in the frequency spectra occurred for the position task. Based on the patterns of RMS amplitude in response to thermal stress, we conclude that core temperature was the primary thermal afferent influencing neuromuscular response during a submaximal force task, with minimal input from skin temperature. However, skin temperature was the primary thermal afferent during a position task with minimal core temperature influence. Therefore, temperature has a task-dependent impact on neuromuscular responses.

Fig 1. Arm brace setup.

Brace isolating the flexor carpi radialis muscle during wrist flexion for both the isometric force and position tasks, illustrating the skin temperature thermistor for local forearm temperature (T_loc) and sEMG electrodes. Resistance for the position task provided by weights suspended on a pulley at the end of the wooden arm.

Fig 2. Schematic of the experimental timeline, outlining the neuromuscular test battery along with the heating and cooling protocol.

BASE: testing at baseline prior to passive heating; H-H: Hot core, hot skin, taken at the highest point of T_re rise tolerated (>1.5°C T_re for all participants); H-C: Hot core, cool skin, taken <5 min after initiation of cooling; POST: Neutral core, cool skin, taken at the end of cooling after T_re returned to <+1.5°C from BASE.

Fig 3

Rectal temperature (A) and skin temperature responses (B) of mean skin (open squares) and local forearm temperature (closed squares) responses to the passive heating and cooling protocol. *Significantly different versus initial T_re and $\overline{\mathrm{T}}\mathrm{sk}$ (BASE; p < 0.001). †Significantly different versus hot T_re, hot ${\overline{\mathrm{T}}}_{\mathrm{sk}}$ (H-H; p < 0.001).

Fig 4

Electromyographic responses of root-mean-square amplitude (RMS; A), mean power frequency (MPF; B) and median power frequency (MDF; C) to passive heating and cooling for the 3-second isometric force (closed bars) and position (open bars) task in 20 participants. Temperature states: initial T_re and ${\overline{\mathrm{T}}}_{\mathrm{sk}}$ (BASE); hot T_re, hot ${\overline{\mathrm{T}}}_{\mathrm{sk}}$ (H-H); hot T_re, cool ${\overline{\mathrm{T}}}_{\mathrm{sk}}$ (H-C); and end of the protocol where T_re returned to normal and ${\overline{\mathrm{T}}}_{\mathrm{sk}}$ was cool (POST). *Force task significantly different from position task (p < 0.05). ^aSignificantly different from baseline (BASE). ^bSignificantly different from hot core-hot skin (H-H). ^cSignificantly different from hot core-cool skin(H-C). ^dSignificantly different from end of protocol (POST).

Fig 5

Electromyographic responses of root-mean-square amplitude (RMS; A), mean power frequency (MPF; B) and median power frequency (MDF; C) to passive heating and cooling for the 1-minute isometric force (black bars) and position (open bars) task in 18 participants. Temperature states: initial T_re and ${\overline{\mathrm{T}}}_{\mathrm{sk}}$ (BASE); hot T_re, hot ${\overline{\mathrm{T}}}_{\mathrm{sk}}$ (H-H); hot T_re, cool ${\overline{\mathrm{T}}}_{\mathrm{sk}}$ (H-C); and end of the protocol where T_re returned to normal and ${\overline{\mathrm{T}}}_{\mathrm{sk}}$ was cool (POST). ^aSignificantly different from baseline (BASE). ^bSignificantly different from hot core-hot skin (H-H). ^cSignificantly different from hot core-cool skin(H-C). ^dSignificantly different from end of protocol (POST).