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 varying resistance level on patterns of muscle activity and trunk motion during the performance of dynamic trunk extension. Twenty-five female subjects were tested at four resistance levels: nominal resistance, and 30%, 50%, and 70% of maximum isometric extension strength. Peak truncal velocity, acceleration, and deceleration decreased with increased resistance, although no difference existed between the nominal and 30% maximum voluntary contraction (MVC) conditions. Coactivation of abdominal muscles was present at all resistance levels, although it was greater for the nominal resistance condition than the other conditions. The timing of EMG activity of the extensor and abdominal muscles was significantly affected by resistance. The amplitude and time course of peak muscle activity are discussed in terms of the implications for spinal loading.