The effect of tendon stiffness on the development and degeneration of locomotion in the sport horse

Abstract

Introduction Injury to the flexor tendon is unfortunately a common cause of retirement of the sport horse, leading to concerns for equine welfare and economic loss. Maximising the quality of the tendon is only possible before tendon matrix synthesis ceases at around three years old. This cease is reflected in a change in tendon stiffness from an increase to a plateau. Throughout the rest of the horse’s life, repeated loading cycles gradually weaken the tendon and lead to decrease in stiffness. Since the two flexor tendons support the joint of the distal limb, this change in stiffness is likely to be reflected in the joint angles, which could be used to determine both when tendon matrix synthesis has ceased and if degenerative changes are normal. A second area of investigation is into the development and deterioration of stride parameters such as length and frequency. Since these depend on both leg length and on the input of the central nervous system (CNS), normalising to leg length allows investigation of the influence of the CNS alone. Method The distal limbs of 57 horses from three months to 17.5 years old were videoed over one complete stride. Horses were divided into three age groups; young horses up to 35 months whose tendon stiffness and leg length was assumed to increase; adult horses 36 to 99 months old whose tendon stiffness and leg length was assumed to remain constant; and finally horses over 100 months old whose deteriorating tendons were assumed to show a decrease in stiffness but whose leg length would not change. To determine joint angles, the videos were processed with a new marker-free tracking system that was found to show comparable results to previously-published data. Since joint angles of the passive equine distal limb depend not only on tendon stiffness but also mass of the horse, angles were normalised to calculated mass to allow investigation of tendon stiffness alone. Results Maximum normalised angles decreased in the young horse, remained constant in the adult horse and increased towards old age; consistent with changes in tendon stiffness. Most stride parameters changed during growth, and all except velocity showed a change when normalised to leg length. Towards old age, two parameters showed a slight decrease but there was no change in the normalised parameters. Conclusion In terms of joint angle, a trend was seen that was consistent with previously-reported trends in tendon stiffness. An owner could track their horse’s joint angles from birth to identify the characteristic change that accompanies the cease of matrix synthesis and therefore the limit of adaptive ability of the tendon. Tracking joint angles towards old age would also allow early identification of any abnormal change before the injury becomes severe enough to cause an overt change in the horse’s gait. The change seen in normalised stride parameters in the young horse indicates that these parameters are influenced by CNS maturation as well as a simple increase in leg length. Any abnormal change in stride parameters could indicate a lack of stability and the risk of a fall.