Bilateral deficit is larger for step than for ramp isometric contractions

The force produced by a muscle group during a maximum effort bilateral task has been reported to be less than that produced during a maximum effort unilateral task. It was hypothesized that if decreased activation of primarily fast motor units is responsible for this bilateral deficit, 1) the bilateral deficit would be larger in step than in ramp isometric contractions and 2) the rate of torque generation in step contractions would be slower in bilateral than in unilateral tasks. Twelve healthy male subjects performed unilateral and bilateral isometric knee joint extension tasks in which maximum torque was generated under ramp and step conditions. The maximum torques produced in the bilateral tasks were 17.0 and 24.6% less than those produced in the unilateral tasks during the ramp and step conditions, respectively (both P < 0.001). The bilateral torque deficit for the step condition was significantly greater than that for the ramp condition (P = 0.012). The rate of torque generation in the step condition was 19.7% slower in the bilateral than in the unilateral task (P = 0.010). Thus the results from these torque measurements support the notion that decreased activation of primarily fast motor units may be responsible for the bilateral deficit.