The effect of muscle length on motor unit discharge characteristics in human tibialis anterior muscle

Summary

Muscle length influences the contractile properties of muscle in that when muscle is lengthened the relaxation phase of the muscle twitch is prolonged and when muscle is shortened, the relaxation phase is shorter in duration. As a result, the force exerted by active motor units varies with muscle length during voluntary contractions. To determine if motoneuron spike trains were adjusted to accommodate for changes in the contractile properties imposed by shortened and lengthened muscle, motor unit action potentials were recorded from the tibialis anterior muscle at different muscle lengths. Twenty subjects performed isometric ramp contractions at ankle angles of 20° dorsiflexion, neutral between dorsiflexion and plantar flexion, and 30° plantar flexion, which put the tibialis anterior muscle in a shortened, neutral, or lengthened condition, respectively. During isometric contractions where torque increased at 5% MVC/s, motor unit discharge rate at recruitment was greater in shortened muscle than in lengthened muscle (P<0.05). Brief initial interspike intervals (<40 ms) occurred more frequently in shortened muscle than in either neutral length or lengthened muscle. During steady contractions, motor unit discharge rate was greater per unit torque (N.m) in shortened muscle than in neutral length or lengthened muscle (P<0.05). These findings indicate that muscle length does influence the discharge pattern of motor unit spike trains during isometric ramp contractions. Spike trains with higher discharge rates at recruitment in shortened muscle may take advantage of the catch-like properties in muscle and be useful in taking up the slack in the passive elements of the muscle and tendon. During steady submaximal contractions, the higher discharge rate per unit torque (N.m) in shortened muscle is likely due to the decreased peak tension and shorter one-half relaxation time observed in shortened muscle, and may indicate that the tibialis anterior muscle is operating on the steep portion of the length-tension curve when the ankle is fully dorsiflexed.