Afferent contribution to locomotor muscle activity

31 Plantar flexor series elasticity can be used to dissociate muscle-fascicle and 32 muscle-tendon behaviour and, therefore, afferent feedback during human walking. We 33 used electromyography (EMG) and high speed ultrasonography concomitantly to 34 monitor muscle activity and muscle fascicle behaviour in nineteen healthy volunteers 35 as they walked across a platform. On random trials, the platform was dropped (8cm, 36 0.9g acceleration) or held at a small inclination (≤ ±3 ̊ in the parasagittal plane) with 37 respect to level ground. Dropping the platform in the mid and late phases of stance 38 produced a depression in the soleus muscle activity with an onset latency of ~50ms. 39 The reduction in ground reaction force also unloaded the plantar flexor muscles. The 40 soleus muscle fascicles shortened with a minimum delay of 14ms. Small variations in 41 platform inclination produced significant changes in triceps surae muscle activity; 42 EMG increased when stepping on an inclined surface and decreased when stepping on 43 a declined surface. This sensory modulation of the locomotor output was concomitant 44 with changes in triceps surae muscle fascicle and gastrocnemius tendon length. 45 Assuming that afferent activity correlates to these mechanical changes, our results 46 indicate that within-step sensory feedback from the plantar flexor muscles 47 automatically adjusts muscle activity to compensate for small ground irregularities. 48 The delayed onset of muscle fascicle movement after dropping the platform indicates 49 that at least the initial part the soleus depression is more likely mediated by a decrease 50 in force feedback than length sensitive feedback, indicating that force feedback 51 contributes to the locomotor activity in human walking. 52 53