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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 degrees dorsiflexion, neutral between dorsiflexion and plantar flexion, and 30 degrees 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 less than 0.05). Brief initial interspike intervals (less than 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 less than 0.05). These findings indicate that muscle length does influence the discharge pattern of motor unit spike trains during isometric ramp contractions.(ABSTRACT TRUNCATED AT 250 WORDS)

Postcontraction depression of Hoffmann-reflex (H-reflex) amplitudes was examined to study the rationale underlying proprioceptive neuromuscular facilitation relaxation techniques. The time course of H-reflex amplitude depression was used to assess postcontraction changes in motoneuron reflex excitability. Sixteen healthy female subjects performed voluntary isometric plantar-flexion contractions (65%-75% of maximal voluntary contraction) in a prone position. H-reflex stimulation began at a postcontraction delay of 0.05, 0.1, 0.5, 1, or 5 seconds and continued every 10 seconds for 1 minute. Reflexes were depressed (mean = 67% decrease) by 0.05 second postcontraction, reached maximal depression (mean = 83.3% decrease) from 0.1 to 1 second postcontraction, recovered to 70% of control amplitudes (mean = 30% decrease) by 5 seconds postcontraction, and reached 90% control amplitudes (mean = 10% decrease) by 10.05 seconds postcontraction. The results indicate that proprioceptive neuromuscular facilitation techniques (eg, hold-relax) purported to produce a phase of relaxation following voluntary contraction do appear to produce a strong, but brief, neuromuscular inhibition that may be clinically useful for applying stretch.