Effect of agonist load on fastest muscle contractions in human elbow flexors

The purpose of this study was to investigate whether the central nervous system (CNS) motor program subserving most-rapid voluntary force pulse generation is modified according to the level of agonist muscle load maintained prior to the pulse. Five normal subjects produced most-rapid force pulses to a target of 20% maximum voluntary force (MVF) above different levels of steady tonic contraction. Time to reach peak force, the first agonist burst duration and area were relatively constant for loads up to approximately 40% of MVF and then increased substantially with larger loads. These results indicate that the CNS adjusts agonist burst amplitude and duration in order to accomplish the same rapid motor task depending on the load. It is argued that these adjustments are necessary to compensate for physiological limitations of motor unit behavior and non-linearity between EMG activity and phasic force output.