Interactions between posture and locomotion: Motor patterns in humans walking with bent posture versus erect posture

Human erect locomotion is unique among living primates. Evolution selected specific biomechanical features that make human locomotion mechanically eff icient. These features are matched by the motor patterns generated in the C NS. What happens when humans walk with bent postures? Are normal motor patt erns of erect locomotion maintained or completely reorganized? Five healthy volunteers walked straight and forward at different speeds in three differ ent postures (regular, knee-flexed, and knee- and trunk-flexed) while their motion, ground reaction forces, and electromyographic (EMG) activity were recorded. The three postures imply large differences in the position of the center of body mass relative to the body segments. The elevation angles of the trunk, pelvis, and lower limb segments relative to the vertical in the sagittal plane, the ground reaction forces and the rectified EMGs were ana lyzed over the gait cycle. The waveforms of the elevation angles along the gait cycle remained essentially unchanged irrespective of the adopted postu res. The first two harmonics of these kinematic waveforms explain >95% of t heir variance. The phase shift but not the amplitude ratio between the firs t harmonic of the elevation angle waveforms of adjacent pairs was affected systematically by changes in posture. Thigh, shank, and foot angles covarie d close to a plane in all conditions, but the plane orientation was systema tically different in bent versus erect locomotion. This was explained by th e changes in the temporal coupling among the three segments. For walking sp eeds >1 m s(-1), the plane orientation of bent locomotion indicates a much lower mechanical efficiency relative to erect locomotion. Ground reaction f orces differed prominently in bent versus erect posture displaying characte ristics intermediate between those typical of walking and those of running. Mean EMG activity was greater in bent postures for all recorded muscles in dependent of the functional role. The waveforms of the muscle activities an d muscle synergies also were affected by the adopted posture. We conclude t hat maintaining bent postures does not interfere either with the generation of segmental kinematic waveforms or with the planar constraint of interseg mental covariation. These characteristics are maintained at the expense of adjustments in kinetic parameters, muscle synergies and the temporal coupli ng among the oscillating body segments. We argue that an integrated control of gait and posture is made possible because these two motor functions sha re some common principles of spatial organization.