1. Introduction
Landing after a jump is the most strenuous phase for the forelimbs of jumping horses. However quantifying forces between the hooves and the ground, or measuring limb joint angles, at landing, has been performed in very few studies. Ground reaction forces (GRF) during landing were studied by Schambardt et al. (Citation1993). Differences between the trailing (TF, first to land) and leading (LF, second to land) forelimbs were described, but not statistically tested. Meershoek et al. (Citation2010), using experienced horses jumping a 1 m high vertical fence, combined kinematics and GRF measurements to calculate joint moments on both forelimbs at landing. These authors concluded that the peak flexor joint moments of the distal limb joints are larger in the TF than in the LF, suggesting larger flexor tendon forces in the TF. However time (stance duration) difference between both limbs was not considered in the reflection. Crevier-Denoix et al. (Citation2013), using a dynamometric horseshoe (DHS) - and not a force-plate as in both previous studies - and an actual equestrian arena surface, statistically compared LF and TF’ forces and impulses in jumping horses, at landing after jumping a 1.0 m vertical fence. It was demonstrated that total vertical impulse was larger in LF, with conclusions opposite to those of Meershoek et al. (Citation2010) regarding consequences for the main palmar tendons. As stresses on these tendons are conditioned by metacarpo-phalangeal (fetlock) joint movements, measuring fetlock angle during stance at landing should provide new insight into this issue.
Therefore the aims of the present study were: 1. to test if fetlock joint kinematics would confirm that LF is more stressed than TF at landing; 2. to determine the effects of fence’s height on both kinetic and kinematic variables.
2. Methods
Three jumping horses (geldings, mean (SD): 556 (55) kg) clinically sound were used. After trimming, the right forehoof of each horse was equipped with a DHS (Crevier-Denoix et al. Citation2013). A horseshoe with matching height and weight was attached to the left fore, as well as to both hind hooves. Data were acquired at 7.8 kHz. The acquisition system was placed in saddle-bags. The GRF was expressed in a local reference frame with the normal Z-axis perpendicular to the solar plane (positive downward). Reflective markers were stuck to the skin facing the elbow, distal radius and fetlock joint of the right forelimb. A T-square with 3 reflective markers was screwed on the right forehoof. A high-speed camera (1000 Hz, Phantom v5.1, Vision Research), filming the right side of the horse, was placed on the side of the landing area, at a distance of about 7 m from the middle of the recording corridor. The films were synchronized with the DHS recordings using the lighting of a LED (in saddle-bag). In an outdoor arena covered with microsand mixed with fibres (ProSol*, Toubin & Clément) a 10 m corridor was delimited with two parallel series of reflective markers (3.6 m apart) placed by pair face to face every meter. A vertical fence was placed midlength of the corridor. After warming-up, each horse had to perform a total of 28 jumps of the fence, the height of which was successively increased then decreased (1.1, 1.2, 1.3, 1.4, 1.3, 1.2, 1.1 m), twice at each lead (right/left) canter, alternatively.
Knowing the angle made by the right forehoof with respect to the ground surface during stance (using markers of the T-square and those along the track), GRF of the right forelimb was projected in the track’s reference frame. Customised softwares developed in Matlab (MathWorks) were used to calculate stance duration as well as peak vertical force and vertical impulse (integral of force over time). Kinematic markers were semi-automatically tracked on each film using a custom program. The markers’ 2-D coordinates were low-pass Butterworth filtered with a 50 Hz cut-off frequency. Fetlock joint angle, measured on the dorsal side, as well as the angle of the elbow–fetlock (limb) axis with respect to the track, were computed during stance. Angles at the beginning of stance, and minimal fetlock angle, were determined, and the area of the fetlock angle-time curve (reference value at landing) was calculated. Linear mixed-effects regression models (SAS v 9.2) were used to test the association between kinetic or kinematic variables and fence’s height, taken into account repetitions within a given horse, for each lead on one hand, for both leads together, adjusting for the lead, on the other hand (p < 0.05).
3. Results and discussion
Stance duration of the forelimb at landing was associated with fence’s height only in TF (), and it was 1.2 to 1.3 larger in LF compared with TF. At beginning of stance, both forelimbs were more vertical to the ground (angle closer to 90°) as fence’s height increases, and TF was more vertical than LF.
Table 1. Selected kinetic and kinematic variables of the trailing and leading forelimb of 3 horses at landing after jumping a fence.
Fetlock joint at beginning of stance, and at maximal extension (minimal angle), was more extended (smaller dorsal angle) as fence’s height increases, without significant limb difference (). The area of the fetlock angle-time curve revealed 1.2 to 1.3 larger in LF, but it showed no significant association with fence’s height.
Figure 1. Average charts of the dorsal metacarpo-phalangeal (fetlock) joint angle of the leading (LF) and trailing (TF) forelimb of 3 horses during the landing stance after jumping 1.1 to 1.4 m high fence.
Maximal vertical force (1.6 times body weight in average) increased with fence’s height, in both forelimbs. Lead effect was not found significant.
Vertical impulse, larger (1.3 in average) in LF, showed no significant association with fence’s height, although it tended to increase in LF (p = 0.07). As already described by Schambardt et al. (Citation1993), interindividual variation were observed, with the least experienced horse having maximal vertical force about 20% larger than the most experienced horse. The third horse revealed afterwards to suffer from cervical arthropathy (compatible with national-level competitions). Interestingly this was the only horse in which maximal vertical force showed no relation with fence’s height (although association was significant when the 3 horses were tested together).
4. Conclusions
Although based on a small sample, this study is the first to include fetlock angle measurement in horses landing after a jump under sport conditions (jumping arena surface, fence up to 1.4 m). It is also the first to demonstrate an association between vertical force, as well as fetlock angle, and fence’s height. Kinematics confirms what kinetics suggested (Crevier-Denoix et al. Citation2013): anatomical structures involved in the support phase of stance are more stressed over time in the leading forelimb. This should be considered when interpreting reported preferred lead at landing in horses examined for lameness or poor performance.
Acknowledgements
Région Normandie, FEDER, Fonds Eperon and IFCE for financial support; ENE (IFCE), CSEM, and J.-P. Villauld, for loaning horses; firms Toubin & Clément and Normandie drainage for providing and maintaining the surface.
References
- Crevier-Denoix N, Camus M, Chateau H, Pourcelot P. 2013. External loads on the leading and trailing forelimbs of a jumping horse at landing measured with a DHS. CMBBE. 16(Suppl 1):145–146.
- Meershoek LS, Roepstorff L, Schamhardt HC, Johnston C, Bobbert MF. 2010. Joint moments in the distal forelimbs of jumping horses during landing. Equine Vet J. 33(4):410–415.
- Schambardt HC, Merkens HW, Vogel V, Willekens C. 1993. External loads of the limbs of jumping horses at the take-off and landing. Am J Vet Res. 54(5):675–680.