Function of the oblique hypaxial muscles in trotting dogs

In trotting dogs, the pattern of activity of the obliquely oriented hypaxia l muscles is consistent with the possible functions of (i) stabilization ag ainst vertical accelerations that cause the trunk to sag in the sagittal pl ane and (ii) stabilization against forces that tend to shear the trunk in t he sagittal plane. To test these hypotheses, we compared the amount of acti vity of the intercostal and abdominal oblique muscles (i) when dogs carried additional mass (8-15% of body mass) supported over the limb girdles versu s 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 forc es 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 craniodo rsal orientation (external oblique and external intercostal muscles) showed an increase in activity when the dogs ran downhill and a decrease when the y ran uphill. Muscles with a cranioventral orientation (internal oblique an d internal intercostal muscles) exhibited the opposite pattern: increased a ctivity when the dogs ran uphill and decreased activity when they ran downh ill. Changes in activity of two extrinsic appendicular muscles, the serratu s ventralis and deep pectoralis, during uphill and downhill running were al so consistent with the sagittal shearing hypothesis. In contrast, changes i n the level of recruitment of the oblique hypaxial muscles were not consist ent with stabilization of the trunk against torques that induce yaw at the girdles. Hence, we suggest that the oblique hypaxial muscles of trotting do gs act to stabilize the trunk against sagittal shearing torques induced by limb retraction (fore-aft acceleration) and protraction (fore-aft decelerat ion).