Modelling fore- and hindlimb peak vertical force differences in trotting horses using upper body kinematic asymmetry variables

Differences in peak vertical ground reaction forces (dFz_peak) between contralateral forelimbs and hindlimbs are considered the gold standard for quantifying weight-bearing lameness. However, measuring kinematics for the same purpose is more common and practical. Vertical movement asymmetries (VMA) of the horse's upper body have previously been correlated to fore- and hindlimb lameness. But the combined response of head, withers and pelvis VMA to fore- and hindlimb dFz_peak has not yet been thoroughly investigated. Deriving the kinetic responses from kinematics would help the interpretation and understanding of quantified weight-bearing lameness. In this retrospective study, 103 horses with a wide range of fore- and hindlimb dFz_peak had been trotted on a force-measuring treadmill synchronized with an optical motion capture system. VMA of the head, withers and pelvis as well as dFz_peak were extracted. Multiple linear mixed models and linear regressions of kinematic variables were used to model the dFz_peak. It was hypothesised that all included VMA would have a significant influence on the dFz_peak outcome variables. The results showed a complex relationship between VMA and dFz_peak where both amplitude and timing of the VMA were of importance. On average, the contribution percentage of VMA to fore/hind dFz_peak were 66/34% for head, 76/24% for withers and 33/67% for pelvis. The linear regressions for the fore/hindlimb models achieved mean measurement root mean squared errors of 0.83%/0.82% dFz_peak. These results might help determine the clinical relevance of upper body VMA and distinguish between primary fore, hind, ipsilateral and diagonal weight-bearing lameness.