Fig. 1. Illustration of hypothesized torques exerted on the trunk (circular arrows) and requisite activity from the oblique hypaxial muscles (large gray arrows) to stabilize the trunk during running uphill (A) and downhill (B). The arrows on the substratum represent the fore–aft component of the ground reaction force. To run on hills, the extrinsic limb muscles must exert retracting or protracting torques on the limbs that results in rearward-directed forces on the ground when running uphill and forward-directed forces when running downhill. Consequently, equal and opposite torques are expected on the trunk (circular arrows). These torques tend to shear the trunk in the sagittal plane and could be resisted by contraction of the internal oblique and intercostal muscles when running uphill (A) and by the external oblique and intercostal muscles when running downhill (B).