ASYMMETRIC BIPEDAL LOCOMOTION - AN ADAPTIVE RESPONSE TO INCOMPLETE SPINAL-INJURY IN THE CHICK

The purpose of this study was to compare the asymmetric gait induced b y unilateral spinal cord injury in chicks with asymmetric gaits of oth er bipeds and quadrupeds. After lateral hemisection of the left thorac ic spinal cord, kinetic (ground reaction forces) and kinematic (distan ce and timing) data were recorded as chicks moved overground unrestrai ned. Ground reaction forces were analyzed to obtain the mechanical ene rgy changes throughout the stride. Kinematic measurements were obtaine d over a range of speeds to determine the velocity-dependent character istics of the gait. Hemisected chicks adopted an asymmetric hopping ga it in which the animals hopped from the right leg (contralateral to th e lesion) onto the left (ipsilateral) leg but then fell forward onto t he right leg. Mechanical energy fluctuations throughout a single strid e (i.e., two steps) approximated the oscillations that occur during a single walking step of control animals. When examined over a range of velocities, asymmetries in limb timing remained constant, but distance measurements such as step length became more symmetric as speed incre ased. The results show that, after spinal hemisection, adaptations of the remaining neural circuitry permitted the production of a locomotor pattern that, in addition to providing effective support and propulsi on, incorporated some of the energy-conserving mechanisms of the norma l walk. Adjustment of this novel locomotor pattern for different veloc ities further demonstrates the flexibility of locomotor circuitry. Com parisons with other studies shows that this gait shares some temporal and energetic features with asymmetric gaits of several bipedal specie s, including humans. In particular, hemisected chicks and some hemiple gic humans adopt an asymmetric gait in which maximum energy recovery o ccurs during the stance of the affected limb; these similarities proba bly relate to common mechanical constraints imposed on bipedal forms o f terrestrial locomotion.