MOTOR PATTERNS FOR HUMAN GAIT - BACKWARD VERSUS FORWARD LOCOMOTION

Seven healthy subjects walked forward (FW)and backward (BW) at differe nt freely chosen speeds, while their motion, ground reaction forces, a nd electromyographic (EMG) activity from lower limb muscles were recor ded. We considered the time course of the elevation angles of the thig h, shank, and foot segments in the sagittal plane, the anatomic angles of the hip, knee, and ankle joints, the vertical and longitudinal gro und reaction forces, and the rectified EMGs. The elevation angles were the most reproducible variables across trials in each walking directi on. After normalizing the time course of each variable over the gait c ycle duration, the waveforms of all elevation angles in BW gait were e ssentially time reversed relative to the corresponding waveforms in FW gait. Moreover, the changes of the thigh, shank, and foot elevation c ovaried along a plane during the whole gait cycle in both FW and BW di rections. Cross-correlation analysis revealed that the phase coupling among these elevation angles is maintained with a simple reversal of t he delay on the reversal of walking direction. The extent of FW-BW cor respondence also was good for the hip angle, but it was smaller for th e knee and ankle angles and for the ground reaction forces. The EMG pa tterns were drastically different in the two movement directions as wa s the organization of the muscular synergies measured by crosscorrelat ion analysis. Moreover, at any given speed, the mean EMG activity over the gait cycle was generally higher in BW than in FW gait, suggesting a greater level of energy expenditure in the former task. We argue th at conservation of kinematic templates across gait reversal at the exp ense of a complete reorganization of muscle synergies does not arise f rom biomechanical constraints but may reflect a behavioral goal achiev ed by the central networks involved in the control of locomotion.