Circadian variations in the kinematics of handwriting and grip strength.

The present study determined whether the motor process of handwriting is influenced by a circadian rhythm during writing tasks of high everyday relevance and analyzed the relationship to the circadian rhythm of grip strength. Ten healthy young male subjects underwent a 40 h sleep-deprivation protocol under constant routine conditions. Starting at 09:00 h, subjects performed three handwriting tasks of increasing perceptual-motor complexity (writing a sentence, writing one's signature, and copying a text for 3 min) and assessed grip strength of both hands every 3 h. Handwriting performance was analyzed by writing speed, writing fluency, script size, break times, and pen pressure. The handwriting tasks revealed a coincident circadian rhythm for the frequency of handwriting as a measure of movement speed, with slowest writing speed at 03:16 h. A weak effect of task complexity was evident for the non-writing episodes: while copying a text, break times were influenced by a circadian rhythm, whereas during sentence writing, the non-writing episodes remained constant. The circadian rhythm of grip strength paralleled the time course of motivation ratings, with least motivation and weakest grip strength around 06:00 h concurrently for both hands. The rate of force production also displayed circadian rhythmicity and sharply decreased with the onset of melatonin secretion. Neither grip strength nor the kinematics of handwriting was influenced by sleep deprivation; only the level of the force rate was decreased the second day. The results show a clear circadian rhythm in the speed of handwriting and grip strength.