The hindlimb in walking horses: 1. Kinematics and ground reaction forces.

This research focuses on assessing the correlation between kinematics and ground reaction forces in the hindlimb of horses when they walk, providing valuable data that can be used for further studies into equine health issues such as lameness.

Study Design and Procedure

• The researchers gathered video and force data from five sound horses at walk. This data was collected across eight strides for each horse.
• The video was processed at 60 Hz (frames per second) while the force data was captured at a frequency of 2000 Hz.
• The team studied the kinematics—the physics of motion—in the sagittal plane, which is the vertical plane that divides the horse’s body into right and left parts.
• Simultaneously, vertical and longitudinal ground reaction forces – the forces exerted by the ground in response to the forces exerted on it by the horse’s limb – were also measured.

Findings

• The hindlimb experienced rapid loading and braking within the initial 10% of the stride. This means in the first part of the stride, weight was rapidly applied and slowed down.
• During this initial loading stage, the stifle (the horse’s knee), the tarsal (analogous to the human ankle) and the coffin joints (located in the horse’s hoof) flexed, while the fetlock joint (similar to the human wrist) extended.
• The vertical ground reaction force displayed two peaks, separated by a dip, mirroring the fetlock joint angle-time graph. These peaks did not seem to relate directly to specific movement events.
• The total braking impulse was equal to the total propulsive impulse throughout the stride, indicating the energetic forces applied in slowing the limb and pushing off are essentially equal.
• The hip’s flexion (bending) and extension (straightening) was responsible for the protraction (moving forward) and retraction (pulling back) of the entire limb.
• The maximum protraction occurred shortly before the swing phase ended, and the maximum retraction occurred during breakover, the phase when a horse lifts its hoof off the ground.
• In the middle of the stance phase, when the hoof is grounded fully, the tarsal joint extended slowly while the stifle started to flex as the limb was retracted beyond mid-stance at the 28% stride point.
• The flexion cycles of the stifle and tarsal joints were well-coordinated during the swing phase, to lift the distal (furthest from the body) part of the limb as it moved forward.

Conclusion and Implication for Future Studies

• This study demonstrated a relationship between the limb kinematics and vertical limb loading in the hindlimbs of sound, or healthy, horses.
• The findings can be used as a reference point for further studies into equine health problems, specifically lameness – a condition that affects the gait of a horse. By observing the changes in kinematics and ground reaction forces, researchers can detect abnormalities in the horse’s movement and thus diagnose and treat lameness.