THE EFFECTS OF RESISTANCE LEVEL ON MUSCLE COORDINATION PATTERNS AND MOVEMENT PROFILE DURING TRUNK EXTENSION

The objective of this research was to investigate the effects of varyi ng resistance level on patterns of muscle activity and trunk motion du ring the performance of dynamic trunk extension. Twenty-five female su bjects were tested at four resistance levels: nominal resistance, and 30%, 50%, and 70% of maximum isometric extension strength. Peak trunca l velocity, acceleration, and deceleration decreased with increased re sistance, although no difference existed between the nominal and 30% m aximum voluntary contraction (MVC) conditions. Coactivation of abdomin al muscles was present at all resistance levels, although it was great er for the nominal resistance condition than the other conditions. The timing of EMG activity of the extensor and abdominal muscles was sign ificantly affected by resistance. The amplitude and time course of pea k muscle activity are discussed in terms of the implications for spina l loading.