Activity of motor neurones during isometric contraction

During slowly increasing isometric contraction, motor neurones showed a stable recruitment level. As the rate of contraction increased recruitment order changed, with motor units having larger amplitude muscle action potentials firing relatively earlier. Most motor neurones showed a 1 cycle/sec rise in firing frequency for a 10% rise in muscle tension. In about 10% of measurements there was no change in firing frequency, or an actual decrease at higher tensions. When a steady tension was maintained, most motor neurones showed a monotonic decrease in firing frequency. If a constant firing frequency was maintained, muscle tension gradually increased. Two examples of abnormal motor control are illustrated.