Force and EMG correlates of constant effort contractions.

The relation between the force exerted by the left arm and the electromyographic (EMG) activity of the brachial biceps and triceps muscles was examined during constant effort contractions maintained for 120 s. The initial force levels were set at 35%, 50% and 65% of each subject's maximal strength, but thereafter no feedback was provided. In contrast to previous results it was found that the direction of the change in force and EMG during constant effort contractions was dependent on the level of force initially exerted. During the lowest initial force contraction the force remained constant, while for the other two force levels there was an exponential decline in the force exerted. These changes in force during the three contractions were well described by an exponential equation with two free parameters. The EMG also varied as a function of initial force. For the higher two forces the amplitude of the EMG fluctuated during the first 40 s but thereafter remained constant, while it increased steadily during the lowest initial force contraction. These results suggest that depending upon the initial level of exertion either peripheral sensory cues relating to the actual force exerted, or centrally-generated signals reflecting the magnitude of the descending motor command may be used by subjects to maintain a constant level of muscular effort.