Effects of cold stimulation of human skin on motor unit activity

This study examined the recruitment threshold of motor units (MUs) and the effects of a maintained cold sensation that mediates the cold reflex activation of the cutaneous receptors, produced by cooling skin with cold air. The changes of the threshold force of the motor units in the human biceps muscle were measured during a slow ramp contraction (2.52 +/- 0.13 kg/s) for 3 s, while the skin temperature on the subject's muscle was continuously reduced using a cooling box fixed at -10 degrees C. The temperature range of the skin on the activated muscle was about 33-25 degrees C. In the same trials, MU activities were sampled during two controlled 30% voluntary isometric ramp contractions (VIRCs) with changes in the skin temperature to 31, 29, 27, and 25 degrees C. Data on recruitment behavior were obtained for 51 MUs in 5 subjects. A total of 13/51 MUs were newly recruited by the decrease of threshold force with the skin temperature at 25 degrees C. Also, 38/51 MUs appeared among a ramp contraction at each temperature from control (Tc) to 25 degrees C. The threshold forces of 7/38 MUs were not affected by cold stimulation to the skin, but other MUs were more affected by the stimulation. The threshold forces of 12/38 MUs were decreased and those of 19/38 MUs were increased with the reduction in skin temperature. These data suggest that either an increase or no change of the threshold force affects the low-amplitude and low-threshold-force MUs (LT-MUs), and a decrease in the threshold force affects the high-amplitude and high-threshold-force MUs (HT-MUs). The threshold force of the MUs at Tc vs four cooling temperatures was correlated in all subjects. The slope (coefficient) of the different regression line, however, clearly decreased with the reduction in skin temperature except for 27-25 degrees C (p > 0.05). Therefore, these results suggest that some recruitment thresholds might change among the HT-MUs that have synaptic connections with the peripheral afferent, because the cutaneous afferent input does not contribute to the same extent at all MUs.