Muscle history, fusimotor activity and the human stretch reflex

1 The previous history of contraction and length changes of a muscle influences the size of the stretch reflex and H reflex. Here we ask, is this dependence due to changes in mechanical properties of extrafusal fibres, intrafusal fibres of spindles, or both? 2 The soleus muscle of human subjects was conditioned using either a voluntary contraction or a contraction evoked by low‐strength electrical stimulation, in the range 0–25% of maximum. Following conditioning, reflexes were increased by more than twofold above the no‐contraction value by a voluntary contraction of 5% of maximum, or more, but not by electrical stimulation which presumably did not contract the intrafusal fibres of spindles. 3 When the muscle was conditioned with a contraction at a length shorter than the test length, rather than at the test length, a depressing effect on reflexes was attributed to both the burst of impulses generated in spindles when the muscle was stretched back to the test length and to a reduced stretch sensitivity of muscle spindles. 4 The experiments demonstrate the importance of keeping the muscle and its spindles in a defined mechanical state when measuring reflexes. They also point to the powerful facilitating influences of conditioning muscle contractions provided they recruit the intrafusal fibres of spindles.