Fig. 1. Location of markers and explanation of the GRF distribution method. Retroreflective markers (small circles) were placed bilaterally on the skin overlying bony landmarks of the forelimb (tuber of spina scapulae, greater tubercle of humerus, a point midway between the two, lateral epicondyle of humerus, carpal joint, fetlock joint, lateral side of the hoof at the level of the coffin joint), hindlimb (tuber coxae, greater trochanter of femur, lateral epicondyle of femur, tarsal joint, fetlock joint, lateral side of the hoof at the level of the coffin joint), and on the head and neck (crista facialis, zygomatic arc, wing of atlas, transverse process of vertebra C7). Furthermore, spherical markers were placed on the trunk (spinous processes of vertebrae T6, T10, T13, T17, L1, L3, L5, S3, S5 and C2). Left limbs are indicated with dotted lines, markers on hoofs in contact with the ground have been filled. The GRF distribution method involved the following steps. The acceleration of the centre of mass (CM) of the horse was calculated from the marker position–time histories using a segmental model. The acceleration of CM, combined with the acceleration due to gravity, provided the magnitude and direction of the calculated total ground reaction force vector F_R,total. The product of the ground reaction force and its moment arm relative to CM (a_R) equals the rate of change of angular momentum. The latter was also calculated from marker position–time histories, so a_R could be calculated. This then fully defined the line of action of the calculated F_R,total, and therewith the point P where it passed between the hoofs of the limbs in contact with the ground. When only one forelimb and one hindlimb were in contact with the ground, the relative contribution of the forelimb to the calculated F_R,total was taken to be equal to a_h/(a_f+a_h) and that of the hindlimb was taken to be equal to a_f/(a_f+a_h). The situation shown in the diagram occurred at the instant that the vertical component of F_R,total (F_Ry,total) reached its peak at trot (for illustrative purposes a trial was selected in which the angular momentum and horizontal component of F_R,total were exceptionally large at the instant that F_Ry,total attained its peak).