MOTOR PATTERNS AND KINEMATICS DURING BACKWARD WALKING IN THE PACIFIC GIANT SALAMANDER - EVIDENCE FOR NOVEL MOTOR OUTPUT

Kinematic and motor patterns during forward and backward walking in th e salamander Dicamptodon tenebrosus were compared to determine whether the differences seen in mammals also apply to a lower vertebrate with sprawling posture and to measure the flexibility of motor output by t etrapod central pattern generators. During treadmill locomotion, elect romyograms (EMGs) were recorded from hindlimb muscles of Dicamptodon w hile simultaneous high-speed video records documented movement of the body, thigh, and crus and allowed EMGs to be synchronized to limb move ments. In forward locomotion, the trunk was lifted above the treadmill surface. The pelvic girdle and trunk underwent smooth side-to-side os cillations throughout the stride. At the beginning of the stance phase , the femur was protracted and the knee joint extended. The knee joint initially flexed in early stance and then extended as the foot pushed off in late stance, reaching maximum extension just before foot lift- off The femur retracted steadily throughout the stance. In the swing p hase, the femur rapidly protracted, and the leg was brought forward in an ''overhand crawl'' motion. In backward walking, the body frequentl y remained in contact with the treadmill surface. The pelvic girdle, t runk, and femur remained relatively still during stance phase, and mos t motion occurred at the knee joint. The knee joint extended throughou t most of stance, as the body moved back, away from the stationary foo t. The knee flexed during swing. Four of five angles showed significan tly smaller ranges in backward than in forward walking. EMGs of forwar d walking showed that ventral muscles were coactive, beginning activit y just before foot touchdown and ceasing during the middle of stance p hase. Dorsal muscles were active primarily during swing. Backward loco motion showed a different pattern; all muscles except one showed prima ry activity during the swing phase. This pattern of muscle synergy in backward walking never was seen in forward locomotion. Also, several m uscles demonstrated lower burst rectified integrated areas (RIA) or du rations during backward locomotion. Multivariate statistical analysis of EMG onset and RIA completely separated forward and backward walking along the first principal component, based on higher RIAs, longer dur ations of muscle activity, and greater synergy between ventral muscles during early stance in forward walking. Backward walking in Dicamptod on uses a novel motor pattern not seen during forward walking in salam anders or during any other locomotor activity in previously studied te trapods. The central neuronal mechanisms mediating locomotion in this primitive tetrapod are thus capable of considerable plasticity.