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Function of the oblique hypaxial muscles in trotting dogs

Mathew M. Fife, Carmen L. Bailey, David V. Lee and David R. Carrier^*

Department of Biology, University of Utah, Salt Lake City, UT 84112, USA

View larger version (22K): [in a new window]   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).

View larger version (23K): [in a new window]   Fig. 2. Average electromyographic (EMG) activity in the left abdominal external oblique muscle from four dogs trotting at intermediate speeds. Each trace represents the average rectified signal from 15 strides. The EMG signals are plotted relative to an average limb support cycle (N=5) for each dog. The phases of limb support are labeled in A: LH, left hind, top trace; LF, left fore, second trace; RF, right fore, third trace; RH, right hind, bottom trace. (A) 24.5kg dog trotting at 2.2ms-1. (B) 21.6kg dog trotting at 2.4ms-1. (C) 34.1kg dog trotting at 2.0ms-1. (D) 22.7kg dog trotting at 2.5ms-1.

View larger version (26K): [in a new window]   Fig. 3. Average electromyographic (EMG) activity in the left abdominal internal oblique muscle from four dogs trotting at intermediate speeds. Each trace represents the average rectified signal from 15 strides. The EMG signals are plotted relative to an average limb support cycle (N=5) for each dog. The phases of limb support, dog masses and running speeds are as described in Fig.2. LH, left hind, top trace; LF, left fore, second trace; RF, right fore, third trace; RH, right hind, bottom trace.

View larger version (25K): [in a new window]   Fig. 4. Average electromyographic (EMG) activity in the left fourth external intercostal muscle from four dogs trotting at intermediate speeds. Each trace represents the average rectified signal from 15 strides. The EMG signals are plotted relative to an average limb support cycle (N=5) for each dog. The phases of limb support, dog masses and running speeds are as described in Fig.2. LH, left hind, top trace; LF, left fore, second trace; RF, right fore, third trace; RH, right hind, bottom trace.

View larger version (27K): [in a new window]   Fig. 5. Average electromyographic (EMG) activity in the left fourth internal intercostal muscle from four dogs trotting at intermediate speeds. Each trace represents the average rectified signal from 15 strides. The EMG signals are plotted relative to an average limb support cycle (N=5) for each dog. The phases of limb support, dog masses and running speeds are as described in Fig.2. LH, left hind, top trace; LF, left fore, second trace; RF, right fore, third trace; RH, right hind, bottom trace.

View larger version (21K): [in a new window]   Fig. 6. Bar charts of the average activity of the oblique (A) and intercostal (B) muscles from four dogs during trotting while carrying additional mass distributed over the pectoral and pelvic girdles (filled columns) and centered over the middle of the trunk (open columns). Values are means + S.E.M. The rectified and integrated area of 20 stride cycles was averaged for each dog, and an average value was then calculated from the mean values of the four dogs (N=4). Results are presented as a percentage of values obtained when the dogs trotted without additional mass. Paired t-tests were used to evaluate whether there was a significant difference between the girdle and mid-trunk trials. Muscles with P-values of greater than 0.05 were taken to be not significantly different and are marked NS.

View larger version (19K): [in a new window]   Fig. 7. Bar charts of the average activity of the serratus ventralis and deep pectoralis muscles from three dogs during trotting while (A) carrying additional mass divided between the girdles or over the mid-trunk and (B) while running uphill and downhill. Values are means + S.E.M. The rectified and integrated area of 20 stride cycles was averaged for each dog, and an average value was then calculated from the mean values of the three dogs (N=3). Results are presented as a percentage of values obtained when the dogs trotted without additional mass (A) or trotted on the level (B). Paired t-tests were used to evaluate whether there was a significant difference between the girdle and mid-trunk trials (A) and between the slope and level trials (B). Muscles with P-values of greater than 0.05 were taken to be not significantly different and are marked NS.

View larger version (25K): [in a new window]   Fig. 8. Sample electromyographic signals of the external (A) and internal (B) oblique muscles recorded during running on the level, uphill and downhill. These recordings are from a 34kg dog running at 2ms-1. They illustrate the different patterns displayed by the external versus the internal hypaxial muscles. In each graph, the horizontal bars represent the period of support of the left hindlimb. Compared with running on the level, activity in the external oblique muscles decreased when the dogs ran uphill and increased when the dogs ran downhill. In contrast, the internal oblique muscles displayed the opposite pattern.

View larger version (16K): [in a new window]   Fig. 9. Bar charts of the average activity of the oblique (A) and intercostal (B) muscles from four dogs during running uphill (filled columns) and downhill (open columns). Values are means + S.E.M. The rectified and integrated area of 20 stride cycles was averaged for each dog, and an average value was then calculated from the mean value of the four dogs (N=4). Results are presented as a percentage of values obtained during running on the level. Paired t-tests were used to evaluate whether there was a significant difference between the slope and level trials. Muscles with P-values of greater than 0.05 were taken to be not significantly different and are marked NS.