Dynamic exercise stimulates group III muscle afferents

1. In decerebrate cats, we investigated the responses of group III muscle afferents to dynamic exercise. The cats performed low intensity dynamic exercise on a treadmill. Group III afferent activity from the dynamically exercising triceps surae muscles was recorded from L₇-S₁ 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 was 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 displayed at least some activity that was synchronized to the contraction phase of the step cycle. The mean developed tensions for strongly responsive afferents, mildly responsive afferents, and afferents that did not respond were 0.8 ± 0.3, 1.3 ± 0.5, and 0.7 ± (1.3 Kg, respectively (P > 0.05). Thus differences in the responsiveness of the afferents to exercise were not attributable to differences in developed tensions. 4. The group III afferents that were strongly responsive to dynamic 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 stimulated by stretch of the triceps surae muscles. However, group III afferents that did not respond to dynamic exercise could also be stimulated either by nonnoxious probing of their receptive fields or by tendon stretch. 5. Many group III muscle afferents were extremely sensitive to 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 group III muscle afferents, are stimulated by low intensity dynamic exercise.