The present paper deals with new health aspects of clothing. These research studies were undertaken to determine the influence of natural and synthetic fibres which temporarily cover the forearm muscles on the activity of the motor units in these muscles. The electrophysiological parameters of motor units were measured with electromyographical methods. The results of the tests showed that temporarily covering the examined muscles in the forearm with synthetic clothing changed the pattern of the motor units' activity. Covering the forearm with natural clothing evoked no such effect. one of the determinants causing this phenomenon was changes in the transmission of the motor fibres which innervate the muscles investigated. The circadian cycle of the muscles' effi- ciency originating from the fatigue- relaxation rhythm was also considered in these studies (7). It is commonly known that in a typically healthy human the motor units after relaxation during sleep (between 1.00 a.m. and 7 a.m.) show fluctuating activity up to the period of their maximal efficiency in the pre- and post-midday hours (more or less between 11.00 a.m. and 1.00 p.m.) as well as later. Furthermore, about 6.0 p.m. - 10.00 p.m., the effi- ciency of the motor units undergoes exhaustion because of their natural fatigue, reaching a peak in the late evening hours (about 11.00 p.m.) (6). Therefore, the period of the day when tests were performed (between 2.00 p.m. and 9.00 p.m.) was chosen to adjust changeable activity to the opti- mal period of muscle motor unit effi- ciency in a healthy human. The circa- dian cycle of the temperature rhythm is, in general, the same as above. The variability in motor unit efficiency