Load rather than length sensitive feedback contribute to soleus 1 muscle activity during human treadmill walking

39 Walking requires a constant adaptation of locomotor output from sensory afferent 40 feedback mechanisms to ensure efficient and stable gait. We investigated the nature of 41 the sensory afferent feedback contribution to the soleus motoneuronal drive and to the 42 corrective stretch reflex by manipulating body load and ankle joint angle. The 43 volunteers walked on a treadmill (~3.6km/h) connected to a body weight support 44 (BWS) system. To manipulate the load sensitive afferents the level of BWS was 45 switched between 5% and 30% of body weight. The effect of transient changes in 46 BWS on the soleus stretch reflex was measured by presenting dorsiflexion 47 perturbations (~5 ̊, 360-400 ̊/s) in mid and late stance. Short (SLR) and medium 48 latency reflexes (MLR) were quantified in a 15ms analysis window. The MLR 49 decreased with decreased loading (p=0.045), but no significant difference was 50 observed for the SLR (p=0.13). Similarly, the effect of the BWS was measured on the 51 unload response, i.e. the depression in soleus activity following a plantar flexion 52 perturbation (~5.6 ̊, 203-247 ̊/s), quantified over a 50ms analysis window. The unload 53 response decreased with decreased load (p>0.001), but was not significantly affected 54 (p=0.45) by tizanidine induced depression of the MLR (p=0.039, n=6). Since 55 tizanidine is believed to depress the group II afferent pathway, these results are 56 consistent with the idea that force-related afferent feedback contributes both to the 57 background locomotor activity and to the medium latency stretch reflex. In contrast, 58 length-related afferent feedback may only contribute to the medium latency stretch 59 reflex. 60 61