DYNAMIC EXERCISE STIMULATES GROUP-III MUSCLE AFFERENTS

1. In decerebrate cats, we investigated the responses of group III mus cle afferents to dynamic exercise. The cats performed low intensity dy namic exercise on a treadmill. Group III afferent activity from the dy namically exercising triceps surae muscles was recorded from L(7)-S-1 dorsal root filaments. 2. Single-unit recordings were obtained from 15 group III afferent fibers whose receptive fields were in the triceps surae muscles and from one group III afferent whose receptive field wa s in the flexor digitorum longus muscle. Conduction velocities for the 16 group III afferents ranged from 3.0 to 27.9 m/s(15.6 +/- 1.9 m/s, mean +/- SE). 3. Ten of 16 group III muscle afferents were stimulated by dynamic exercise. Of the 10, 7 were strongly responsive and 3 were mildly responsive to dynamic exercise. Each of the 10 afferents displa yed at least some activity that was synchronized to the contraction ph ase of the step cycle. The mean developed tensions for strongly respon sive afferents, mildly responsive afferents, and afferents that did no t respond were 0.8 +/- 0.3, 1.3 +/- 0.5, and 0.7 +/- 0.3 Kg, respectiv ely (P > 0.05). Thus differences in the responsiveness of the afferent s to exercise were not attributable to differences in developed tensio ns. 4. The group III afferents that were strongly responsive to dynami c exercise were also mechanically sensitive. Each strongly responsive afferent (n = 7) was stimulated by nonnoxious pressure applied to its receptive field. Most strongly responsive afferents (n = 5) were stimu lated by stretch of the triceps surae muscles. However, group III affe rents that did not respond to dynamic exercise could also be stimulate d either by nonnoxious probing of their receptive fields or by tendon stretch. 5. Many group III muscle afferents were extremely sensitive t o small changes in muscle tension. The onset of spike activity during dynamic contraction occurred with as little as 30 g of active tension. We conclude that mechanoreceptors, many of which are innervated by gr oup III muscle afferents, are stimulated by low intensity dynamic exer cise.