EFFECTS OF THE TURNING POINTS OF DECREASING SKIN TEMPERATURE ON THE THRESHOLD FORCE OF MOTOR UNITS DURING SLOW RAMP CONTRACTION / 体力科学

ABSTRACT

The present study examined the recruitment threshold of motor units (MUs) and the cold reflex activation of the cutaneous receptors at the first turning point (TP1) and the second turning point (TP2) of decreasing skin temperature. The skin temperatures of the biceps brachii were continuously reduced using a cooling chamber fixed at -10°C. TP1 and TP2 appeared at 25.5±0.5°C and 18.5±2.21°C, respectively. The data were collected at±1°C of TP1 and TP2 (TP1-B, TP1-A, TP2-B and TP2-A) . The MUs was collected during a slow ramp contraction for 3 sec to 20% maximal voluntary contraction (20%MVC) at the each measure points (TE) . The rates of decrease in skin temperature were 1.242±0.349°C min at slope-1 (TP1-B), 0.627±0.284°C rain at slope-2 (TP1-A and TP2-B), and 0.201±0.045°C/min at slope-3 (TP2-A) . The difference of the threshold force value (&Delta;TF= TE-control value) of LT-MUs were positive value, on the other hand, &Delta;F of HT-MUs were negative value at TP1-B, TP1-A, TP2-B and TP2-A. The changes of &Delta;TF of LT-MUs were a little at TP1-B, TP1-A, TP2-B, and increased markedly more at TP2-A than at TP1-B (p<0.05) . On the other hand, the &Delta;TF of HT-MUs decreased significantly more at TP1-B than at TP1-A and TP2-B (p<0.05), however, it did not significantly differ at TP2-A. These results suggested that the threshold force of HT-MUs depend on skin temperature and LT-MUs depend on decrease speed of skin temperature.