Function of the oblique hypaxial muscles in trotting dogs

In trotting dogs, the pattern of activity of the obliquely oriented hypaxial muscles is consistent with the possible functions of (i) stabilization against vertical accelerations that cause the trunk to sag in the sagittal plane and (ii) stabilization against forces that tend to shear the trunk in the sagittal plane. To test these hypotheses, we compared the amount of activity of the intercostal and abdominal oblique muscles (i) when dogs carried additional mass (8-15% of body mass) supported over the limb girdles versus supported mid-trunk (test of sagittal bounce), and (ii) when dogs trotted up versus down a 10 degrees slope (test of sagittal shear). In response to the loading manipulations, only the internal oblique muscle responded in a manner that was consistent with stabilization of the trunk against forces that cause the trunk to sag sagittally. In contrast, when the fore-aft forces were manipulated by running up- and downhill, all four of the monitored muscles changed their activity in a manner consistent with stabilization of the trunk against sagittal shearing. Specifically, muscles with a craniodorsal orientation (external oblique and external intercostal muscles) showed an increase in activity when the dogs ran downhill and a decrease when they ran uphill. Muscles with a cranioventral orientation (internal oblique and internal intercostal muscles) exhibited the opposite pattern: increased activity when the dogs ran uphill and decreased activity when they ran downhill. Changes in activity of two extrinsic appendicular muscles, the serratus ventralis and deep pectoralis, during uphill and downhill running were also consistent with the sagittal shearing hypothesis. In contrast, changes in the level of recruitment of the oblique hypaxial muscles were not consistent with stabilization of the trunk against torques that induce yaw at the girdles. Hence, we suggest that the oblique hypaxial muscles of trotting dogs act to stabilize the trunk against sagittal shearing torques induced by limb retraction (fore-aft acceleration) and protraction (fore-aft deceleration).