The purpose of this study was to clarify the relationship between rectal temperature and loop time estimated by the Hoffmann reflex (H-reflex) on daily rhythms in human, and to investigate whether the circadian rhythm is present in the recruitment threshold of motor unit. Six healthy males were voluntarily participated in this study. H-reflex was evoked in the right soleus muscle, and rectal temperature was recorded at 7:00, 11:00, 16:00, 20:00, 22:00 and 7:00 h through the two days. We compared the time interval between the times on positive peak of M and H wave (loop time), which indicates the recruitment threshold of motor units. The rectal temperature significantly changed during 24 hours (p< 0.05) ; natural circadian rhythms were seen in all subjects. The rectal temperature was significantly lower in the morning and midnight compared to the daytime (p< 0.05) through the two days. In contrast, the loop time was significantly shorter in the daytime than in the morning and midnight (p< 0.05) through the two days. The patterns of temperature and loop time showed in a mirror image. The significant negative correlations were found between the loop time and the rectal temperature (r= 0.77, p< 0.05). These results suggest the circadian rhythm is present in the recruitment threshold of motor unit.

DESCENTE SPORTS SCIENCE Vol.31/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE