Muscle history fusimotor activity and the human stretch reflex

1. The previous history of contraction and length changes of a muscle influ ences the size of the stretch reflex and H reflex. Here we ask, is this dep endence due to changes in mechanical properties of extrafusal fibres, intra fusal fibres of spindles, or both? 2. The soleus muscle of human subjects was conditioned using either a volun tary contraction or a contraction evoked by low-strength electrical stimula tion, in the range 0-25% of maximum. Following conditioning, reflexes were increased by more than twofold above the no-contraction value by a voluntar y 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 t han the test length, rather than at the test length, a depressing effect on reflexes was attributed to both the burst of impulses generated in spindle s when the muscle was stretched back to the test length and to a reduced st retch 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 contra ctions provided they recruit the intrafusal fibres of spindles.