Effect of temperature on spike-triggered average torque and electrophysiological properties of low-threshold motor units

The aim of the study was to jointly analyze temperature-induced changes in low-threshold single motor unit twitch torque and action potential properties. Joint torque, multichannel surface, and intramuscular electromyographic signals were recorded from the tibialis anterior muscle of 12 subjects who were instructed to identify the activity of a target motor unit using intramuscular electromyographic signals as feedback. The target motor unit was activated at the minimum stable discharge rate in seven 3-min-long contractions. The first three contractions (C1 - C3) were performed at 33 degrees C skin temperature. After 5 min, the subject performed three contractions at 33 degrees C (T1), 39 degrees C (T2), and 45 degrees C (T3), followed by a contraction at 33 degrees C (C4) skin temperature. Twitch torque and multichannel surface action potential of the target motor unit were obtained by spike-triggered averaging. Discharge rate (mean +/- SE, 7.1 +/- 0.5 pulses/s), interpulse interval variability (35.8 +/- 9.2%), and recruitment threshold (4.5 +/- 0.4% of the maximal voluntary torque) were not different among the seven contractions. None of the investigated variables were different among C1 - C3, T1, and C4. Conduction velocity and peak twitch torque increased with temperature (P < 0.05; T1: 3.53 +/- 0.21 m/s and 0.82 +/- 0.23 mN center dot m, T2: 3.93 +/- 0.24 m/s and 1.17 +/- 0.36 mN center dot m, T3: 4.35 +/- 0.25 m/s and 1.46 +/- 0.40 mN center dot m, respectively). Twitch time to peak and surface action potential peak-to-peak amplitude were smaller in T3 (61.8 +/- 2.0 ms and 27.4 +/- 5.1 mu V, respectively) than in T1 (71.9 +/- 4.1 ms and 35.0 +/- 6.5 mu V, respectively) ( P < 0.05). The relative increase in conduction velocity between T1 and T3 was positively correlated ( P < 0.05) with the increase in twitch peak amplitude (r(2) = 0.48), with the decrease in twitch time to peak (r(2) = 0.43), and with the decrease in action potential amplitude (r(2) = 0.50). In conclusion, temperature-induced modifications in fiber membrane conduction properties may have a direct effect on contractile motor unit properties.