Voluntary discharge frequencies of human motoneurons at different muscle lengths.

The relationship between the discharge frequencies of motoneurons and the voluntary force at high contraction strengths (50% to 100% maximum) was determined for tibialis anterior with the ankle: (i) at 90 degrees (control length); and (ii) this angle reduced by about 15 degrees (short length), an amount sufficient to reduce the maximal voluntary force by about 35%. In the shortened position, twitch contraction and half-relaxation times were reduced by 11% and 18%, respectively. At both muscle lengths, the ability to activate the muscle fully by voluntary effort was confirmed by twitch interpolation. Motor unit firing rates were recorded during isometric voluntary contractions lasting 10 seconds, performed at 50%, 75%, and 100% maximal force. At each length, discharge rates varied in proportion to the relative force exerted. Despite the difference in twitch contractile speed at the two lengths, no significant differences were found between motoneuron discharge rates recorded at each muscle length during maximal or submaximal contractions.