Forelimb kinematics and net joint moments during the swing phase of the trot.

The purpose of this study was to calculate net moments of force at the joints of the forelimb during the swing phase of the stride. An optoelectronic system was used to measure segmental kinematics for 3 strides in 5 sound, Warmblood horses trotting at a mean velocity +/- s.d. of 3.03 +/- 0.16 m/s. A link segment model was used to determine the net moments of force about the joints of the left forelimb. The model combined kinematic data with morphometric data describing the inertial parameters of the limb segments of warmblood horses, and incorporated correction factors for skin displacement. At each joint the net moment of force was on the cranial/dorsal side during the early swing phase and on the caudal/palmar side during the later swing phase. The transition (time of zero moment) occurred between 35-52% of the swing phase. The peak magnitude of the net joint moments decreased progressively in a proximal to distal direction. Published electromyographic (EMG) data correlated well with the timing of muscular activity required to generate the calculated net joint moments. The moments in the proximal limb are indicative of muscular activity accelerating the limb forward during the first 30-40% of the swing phase, then decelerating the forward swing of the upper limb segments. The net joint moments at all of the joints except the elbow work to slow the motion of the joints. The net joint moment about the elbow actively flex and then extend the joint. The low net joint moments at the distal joints during the first half of swing are consistent with their motion being primarily a result of inertial forces. Flexor muscle activity during the last half of swing indicate active control in preparation for ground contact.