Article

Friction and gravity represent two basic physical constraints of terrestrial
locomotion that affect both motor patterns and the biomechanics of bipedal gait.
To provide insights into the spatiotemporal organization of the motor output in
connection with ground contact forces, we studied adaptation of human gait to
steady low-friction conditions. Subjects walked along a slippery walkway (7 m
long; friction coefficient approximately 0.06) or a normal, nonslippery floor at
a natural speed. We recorded gait kinematics, ground reaction forces, and
bilateral electromyographic (EMG) activity of 16 leg and trunk muscles and we
mapped the recorded EMG patterns onto the spinal cord in approximate rostrocaudal
locations of the motoneuron (MN) pools to characterize the spatiotemporal
organization of the motor output. The results revealed several idiosyncratic
features of walking on the slippery surface. The step length, cycle duration, and
horizontal shear forces were significantly smaller, the head orientation tended
to be stabilized in space, whereas arm movements, trunk rotations, and lateral
trunk inclinations considerably increased and foot motion and gait kinematics
resembled those of a nonplantigrade gait. Furthermore, walking on the slippery
surface required stabilization of the hip and of the center-of-body mass in the
frontal plane, which significantly improved with practice. Motor patterns were
characterized by an enhanced (roughly twofold) level of MN activity, substantial
decoupling of anatomical synergists, and the absence of systematic displacements
of the center of MN activity in the lumbosacral enlargement. Overall, the results
show that when subjects are confronted with unsteady surface conditions, like the
slippery floor, they adopt a gait mode that tends to keep the COM centered over
the supporting limbs and to increase limb stiffness. We suggest that this
behavior may represent a distinct gait mode that is particularly suited to
uncertain surface conditions in general.

Langue : ANGLAIS

Tiré à part : OUI