Instant centres of rotation of equine limb joints and their relationship to standard skin marker locations.

The instant centres of rotation (ICRs) of each limb joint, other than the coxofemoral joint and those of the digits, were calculated for eight clinically sound horses from tracings of radiographs according to the method of Reuleaux (1963). The majority of ICR positions coincided with the attachment sites of the collateral ligaments. The ICR of the scapulohumeral joint, which does not have collateral ligaments, was caudodistal to the caudal part of the greater tubercle of the humerus. The ICR for the femorotibial joint was located just caudal to the femoral attachment of the collateral ligament. The location of the ICRs calculated for the two joint carpal joint complex was displaced caudad during extreme flexion of the carpus and differed from those calculated separately for either the antebrachiocarpal joint or the intercarpal joint. Interpretation of these ICR positions was confounded by complex spatial motion changes which occur between the carpal bones. The sites used for skin marker positions in equine locomotion and conformation research generally agree with the ICR positions, particularly for the humeroradial, tarsocrural, and metacarpo- and metatarsophalangeal joints. Traditional marker sites used for the scapulohumeral and femorotibial joints would result in overstimation and underestimation respectively of the caudal joint angles. Inaccuracy of marker placement, together with movement of skin markers and non-planar motion during joint angulation, limit accurate biokinematic measurement of limb joint angles during locomotion and conformation analysis.