Fig. 1.

Path diagrams specifying the hypothesized influence of limb length, joint posture, limb posture and substrate reaction force (SRF) orientation on joint load arm length. (A) Shoulder, (B) elbow, (C) hip and (D) knee joints. (E) A schematic representation of average forelimb and hind limb postures across the current dataset to help visualize the predicted effects of variation in posture and SRF orientation on load arm lengths. Red and blue lines indicate the average orientation of forelimb and hind limb SRF, respectively. Within the path diagrams, single-headed arrows indicate a directed causal link between predictor variables and moment arm lengths. Double-headed arrows indicate an undirected correlation between predictor variables. Red + and – symbols indicate the predicted direction of the relationship. For all joints, limb length was predicted to have a positive effect on load arm length. Because joint extension is a well-established means of shortening joint load arms (Gray, 1968; Biewener, 1989), larger, more extended, joint angles were predicted to decrease joint load arm lengths. The influence of limb angles was predicted to vary according to limb and joint (cf.E). Increasing forelimb protraction and hind limb retraction while maintaining SRF orientation should bring the limb axis more in line with the SRF vector, thus shortening shoulder/hip load arms but increasing the distance between the middle joint and the SRF vector and lengthening elbow/knee load arms. Similarly, cranial deviation of forelimb SRF angles should decrease shoulder load arm lengths and increase elbow load arm lengths, whereas caudal deviation of hind limb SRF angles should increase hip load arm lengths but increase knee load arm lengths.