The impact of landing gradient on cross-country jump biomechanics

Introduction: The event horse must execute the cross-country phase of the CCI4* competition, characterised by solid fences and varying terrain. Understanding how gradients alter kinematic variables can be advantageous to the equine sector as it provides insight into the mechanical requirements for the elite horse when jumping out on course; current research of modern eventing is limited. The current study investigated the differences in forelimb kinematics, trunk angle and time from the start of carpal extension on landing to trailing limb contact within a group of elite horses executing two palisade-style fences with different landing gradients.
Material and Methods: Two-dimensional kinematic data were collated from 12 competitors during the cross-country phase of the NAF Five Star Hartpury International Horse Trials (CCI4*) over two fences: one on the flat (fence 19) and one on a downhill gradient (fence 8a). Recordings were taken via video cameras set perpendicularly to the left of the approach to each of the two fences. After digitisation using Kinovea (version 0.9.5), angles of the scapulohumeral, humeroradial and radiocarpal joints, and trunk angle were determined at three phases of the jump effort: the start of carpal extension during the suspension phase, maximum forelimb extension prior to forelimb impact and impact of the trailing forelimb at landing. Additionally, time recordings were taken of the total duration of the three analysed phases of the jump. Data were assessed for normality (Shapiro-Wilks) and, depending on normality, a Paired t-test or Wilcoxon test was employed to test for differences, using SPSS software (IBM SPSS Statistics, version 29).
Results: The scapulohumeral joint angle was significantly more flexed (P ≤ 0.05 – P ≤ 0.01) at the start of carpal extension during landing (7.7°) and at maximum forelimb extension before trailing limb impact (6.85°) when jumping on the downhill gradient compared to the flat. Trunk angle was less steep (P ≤ 0.05 – P ≤ 0.001) over the fence on the downhill compared to on the flat (14.85°, 8.95°, 5.55°), reflecting the increased total time duration (P ≤ 0.001) from the start of carpal extension during landing to the impact of the trailing forelimb on landing. The angles of the distal limb did not significantly alter with changes in landing gradient.
Discussion and Conclusions: The increased flexion in the scapulohumeral joint recorded for the downhill landing is likely to be influenced by decreased stride frequency and decreased longitudinal propulsive forces (Chateau et al., 2014) undertaken during the approach to the fence. Variation in the time to landing is related to the trajectory over each fence. The flatter landing encouraged a more elevated trajectory, characterised by a more upright trunk angle, and thereby increasing the time in the suspension phase. The findings provide an original insight into the mechanical impacts on the elite event horse jumping on different gradients.