SUMMARY Mammalian locomotion is characterized by the use of asymmetrical gaits associated with extensive flexions and extensions of the body axis. Although the impact of sagittal spine movements on locomotion is well known, little information is available on the kinematics of spinal motion. Intervertebral joint movements were studied in two metatherian and three eutherian species during the gallop and halfbound using high-speed cineradiography. Fast-Fourier transformation was used to filter out high frequency digitizing errors and keep the lower frequency sinusoid oscillations that characterize the intervertebral angular movements. Independent of their regional classification as thoracic or lumbar vertebrae, 7±1 presacral intervertebral joints were involved in sagittal bending movements. In only one species, no more than five intervertebral joints contributed to the resulting `pelvic movement'. In general, the trunk region involved in sagittal bending during locomotion did not correspond to the traditional subdivisions of the vertebral column (e.g. as thoracic and lumbar or pre- and postdiaphragmatic region). Therefore, these classifications do not predict the regions involved in spinal oscillations during locomotion. Independent of the gait, maximum flexion of the spine was observed in the interval between the last third of the swing phase and touch-down. This results in a retraction of the pelvis and hindlimbs before touch-down and, we hypothesize, enhances the stability of the system. Maximum extension occurred during the first third of the swing phase (i.e. after lift-off) in all species. In general, the observed timing of dorsoventral oscillations of the spine are in accordance with that observed in other mammals and with activity data of respiratory and epaxial back muscles. Although no strict craniocaudal pattern was observable, the more cranial intervertebral joints tend to flex and extend earlier than the more caudal ones. This is in accordance with the organization and the activation of the paravertebral musculature in mammals. The amplitude of intervertebral joint movements increased caudally, reaching its highest values in the presacral joint. The more intense sagittal bending movements in the caudal intervertebral joints are reflected by the muscle fiber type composition of the back muscles involved. Despite the highly similar amplitude of `pelvic motion', touch-down and lift-off positions of the pelvis were clearly different between the species with a long, external tail and those with no external tail.

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SUMMARYThe extrinsic limb muscles perform locomotor work and must adapt their activity to changes in gait and locomotor speed, which can alter the work performed by, and forces transmitted across, the proximal fulcra of the limbs where these muscles operate. We recorded electromyographic activity of 23 extrinsic forelimb and hindlimb muscles and one trunk muscle in dogs while they walked, trotted and galloped on a level treadmill. Muscle activity indicates that the basic functions of the extrinsic limb musclesprotraction, retraction and trunk support -are conserved among gaits. The forelimb retains its strut-like behavior in all gaits, as indicated by both the relative inactivity of the retractor muscles (e.g. the pectoralis profundus and the latissimus dorsi) during stance and the protractor muscles (e.g. the pectoralis superficialis and the omotransversarius) in the first half of stance. The hindlimb functions as a propulsive lever in all gaits, as revealed by the similar timing of activity of retractors (e.g. the biceps femoris and the gluteus medius) during stance. Excitation increased in many hindlimb muscles in the order walk-trot-gallop, consistent with greater propulsive impulses in faster gaits. Many forelimb muscles, in contrast, showed the greatest excitation at trot, in accord with a shorter limb oscillation period, greater locomotor work performed by the forelimb and presumably greater absorption of collisional energy.

SUMMARY The body axis plays a central role in tetrapod locomotion. It contributes to the work of locomotion, provides the foundation for the production of mechanical work by the limbs, is central to the control of body posture, and integrates limb and trunk actions. The epaxial muscles of mammals have been suggested to mobilize and globally stabilize the trunk, but the timing and the degree to which they serve a particular function likely depend on the gait and the vertebral level. To increase our understanding of their function, we recorded the activity of the m. multifidus lumborum and the m. longissimus thoracis et lumborum at three cranio-caudal levels in dogs while they walked, trotted and galloped. The level of muscle recruitment was significantly higher during trotting than during walking, but was similar during trotting and galloping. During walking, epaxial muscle activity is appropriate to produce lateral bending and resist long-axis torsion of the trunk and forces produced by extrinsic limb muscles. During trotting, they also stabilize the trunk in the sagittal plane against the inertia of the center of mass. Muscle recruitment during galloping is consistent with the production of sagittal extension. The sequential activation along the trunk during walking and galloping is in accord with the previously observed traveling waves of lateral and sagittal bending, respectively, while synchronized activity during trotting is consistent with a standing wave of trunk bending. Thus, the cranio-caudal recruitment patterns observed in dogs resemble plesiomorphic motor patterns of tetrapods. In contrast to other tetrapods, mammals display bilateral activity during symmetrical gaits that provides increased sagittal stability and is related to the evolution of a parasagittal limb posture and greater sagittal mobility.