Time-of-day effects on force control during object manipulation.

The aim of this study was to determine time-of-day variations of grip force control during a manipulative task. In particular, we repeatedly tested continuous up and down movements of a hand held instrumented object on two consecutive days. The movements caused fluctuations in the load that had to be compensated for by adequate grip forces. The level of grip force, the acceleration of the arm movement, the ratio of grip and load force as well as the temporal relationship between the two force profiles and the subjective sleepiness were examined. The time-of-day protocol lasted 26 h with sessions at different times of day, and sleep at night. Diurnal variations were analyzed with an intersession interval of 2 h; detailed analyses were carried out for the evening sessions until bed time and the sessions in the morning after awake. The present study proves a time-of-day effect for the level of grip force as well as for the acceleration. The effect was most obvious during the morning sessions when both parameters increased. The ratio of grip force to load, i.e., independent of the actual load and acceleration, shows a similar modulation as grip force. This later finding suggests that grip force changed partly independently from changes of arm acceleration. Time of day did not influence the strength of the cross-correlation and the synchronicity between the two force profiles. Thus, while grip force changed with time of day, the predictive coupling of the grip force according to the load was constant.