The effect of muscle length on motor-unit recruitment during isometric plantar flexion in humans

The triceps surae muscle group, consisting of the mono-articular soleus (SOL) and bi-articular gastrocnemius (GAS) muscles, primarily generates plantar flexor torque. Since the GAS muscle crosses the knee joint, flexion of the knee reduces the length of this muscle, thus limiting its contribution to torque output. However, it is not clearly understood how the central nervous system activates muscles that are at inefficient or non-optimal force-producing lengths. Therefore, the present study was designed to determine the effect of muscle length on motor-unit recruitment in the medial GAS muscle. Single motor-unit activity was recorded from the medial GAS muscle while electromyographic (EMG) activity was recorded from the SOL muscle in nine male subjects. With the ankle angle held constant at 90 degrees, the knee angle was changed from 180 degrees to 90 degrees, corresponding to a long and short GAS muscle length, respectively. Levels of voluntary plantar flexor torque were produced at a rate of 2 Nm.s-1 until motor-unit activity was detected. A total of 229 motor units were recorded, of which 121 and 108 were obtained at the long and short muscle lengths, respectively. At the short length, onset of motor-unit activity occurred at significantly higher levels of plantar flexor torque and SOL EMG. Onset of motor-unit activity occurred at 2.97 +/- 7.78 Nm and 32.14 +/- 10.25 Nm, corresponding to 0.045 +/- 0.075 mV and 0.231 +/- 0.129 mV of SOL EMG in the long and short positions, respectively. No individual GAS motor unit could be recorded at both muscle lengths. Motor units in the shortened GAS muscle may be influenced by peripheral afferents capable of reducing the excitability of the motoneurone pool. This may also reflect a specific inhibition of motor units having shortened, non-optimal fascicle lengths, and they are thereby incapable of contributing to plantar flexor torque.