Discussion

Liedtke, A., Moore, S., Witte., Spence, A.
Because horses exhibit qualitatively unchanged locomotion in the absence of distal limb sensation, it is widely assumed that loss of sensory feedback has little or no effect on the natural movement pattern. In contrast, cats require digital sensory feedback for correct foot placement and for the limb to respond appropriately to external stimuli. This suggests that animals with limited distal limb musculature (horses) may not rely on digital sensory feedback during steady-state locomotion. Given these results in cats, and the known function of sensory input to reject internally generated neuromuscular noise, we hypothesised that kinematic parameters associated with limb touchdown position in the horse (Equus caballus) would show greater variation in the absence of sensory feedback. We predicted that this would result from a reduced ability to reject internally generated neuromuscular noise. Steady state locomotion would therefore be less tightly controlled, resulting in greater variation. To test this, we measured the kinematics of horses with reduced levels of digital sensation. Methods: Optical motion capture was used to collect kinematic data from 6 horses walking and trotting on a treadmill before and after an abaxial sesamoid nerve block was administered to remove digital sensation in one and then both front limbs. Results: Contrary to our prediction, preliminary results from 5 horses show that a lack of sensory input results in less variability (Levene's test for homogeneity of variance; trotting; P<0.001, n = 3 horses) and an increase in duty factor and stance duration for the initially blocked forelimb (linear mixed model n = 5 horses, P<0.001) for both gaits. We further observed significant changes in the pitch and roll of the body. These results suggest that sensory feedback continuously adjusts foot placement to maintain postural stability. Conclusions and practical significance: These findings have significant impact for equine clinical practice, in particular in lameness diagnosis and neurectomy for lameness treatment.