MOVEMENT-INDUCED GAIN MODULATION OF SOMATOSENSORY POTENTIALS AND SOLEUS H-REFLEXES EVOKED FROM THE LEG .2. CORRELATION WITH RATE OF STRETCHOF EXTENSOR MUSCLES OF THE LEG

Attenuation of initial somatosensory evoked potential (SEP) gain becom es more pronounced with increased rates of movement. Manipulation of t he range of movement also might alter the SEP gain. It could alter joi nt receptor discharge; it should alter the discharge of muscle stretch receptors. We hypothesized that: (1) SEP gain reduction correlates wi th both the range and the rate of movement, and (2) manipulation of ra nge and rate of movement to achieve similar estimated rates of stretch of a leg extensor muscle group (the vasti) results in similar decreas es in SEP gain. SEPs from Cz' referenced to Fpz' (2 cm caudal to Cz an d Fpz, respectively, according to the International 10-20 System), alo ng with soleus H-reflexes were elicited by electrical stimulation of t he tibial nerve at the popliteal fossa. Stable magnitudes of small M-w aves indicated stability of stimulation. A modified cycle ergometer wi th an adjustable pedal crank and electric motor was used to passively rotate the right leg over three ranges (producing estimated vasti stre tch of 12, 24 and 48 mm) and four rates (0, 20, 40 and 80 rpm) of move ment Two experiments were conducted. Ranges and rates of pedalling mov ement were combined to produce two or three equivalent estimated rates of tissue stretch of the vasti muscles at each of 4, 16, 32 and 64 mm /s. Tibial nerve stimuli were delivered when the knee was moved throug h its most flexed position and the hip was nearing its most flexed pos ition. Means of SEP, H-reflex and M-wave magnitudes were tested for ra te and range effects (ANOVA). A priori contrasts compared means produc ed by equivalent estimated rates of vasti stretch. Increasing the rate of movement significantly increased the attenuation of SEP and H-refl ex gain (P<0.05). Increasing the range of movement also significantly increased these gain attenuations (P<0.05). Combining these to achieve equivalent rates of stretch, through different combinations of rate a nd range, resulted in equivalent depressions of SEP gain. H-reflex gai ns were similarly conditioned. These results suggest that muscle stret ch receptors play a more important role than joint or cutaneous recept ors in regulating SEP gain consequent to movement. We note that the pr esent calculation only considers the knee extensors; however, the biom echanical model of stretch applies also to receptors in the hip extens ors. This paper and the companion one show that primary factors in the kinaesthetic components of the movement regulate activity-induced gai n attenuation of SEPs.