Responses of human muscle spindle afferents during isotonic position holding and active movements

Discharge characteristics of muscle spindle primary afferents from human finger flexors were investigated during load-bearing isotonic position holding, and intentional shortening or lengthening contractions. During position holding, units mostly showed a negative relationship between muscle length and impulse rate, particularly when the load was small. When the load was increased, Ia-discharge rates increased more in the elongated than in the shortened muscle, and the inverse position response became less pronounced. During active lengthening contractions, a stretch response pattern was sometimes absent or could be abolished by increasing the load.