Locomotor stability in able-bodied trunk-flexed gait across uneven ground

This study aimed to explore the control of dynamic stability of the imposed trunk-flexed gaits across uneven ground. For ten young healthy participants, we compared the anteroposterior margin of stability (MoS) and lower limb joint kinematics at foot-contact during accommodating a consecutive stepdown and step-up (10-cm visible drop) to that of level steps while maintaining four postures: regular erect, ∼30°, ∼50° and maximal trunk flexion from the vertical. Two-way repeated measures ANOVAs revealed no significant step × posture interactions for the MoS (p = .187) and for the parameters that contributed to the MoS calculation (p > .05), whereas significant interactions were found for the hip flexion, hip position (relative to the posterior boundary of the base of support) and the knee flexion. The main effect of step (p = .0001), but not posture (p = .061), on the MoS was significant. Post hoc tests, compared with the level step, showed that the decreased magnitude of the MoS during stepping down (p = .011)-mainly due to a further forward displacement of the center of mass position (p = .006)-significantly increased in the immediate following step-up (p = .002) as a consequence of a substantial increase in the base of support (p = .003). In the stepdown versus level step, the hip and knee flexions as well as the hip position did not significantly change in the trunk-flexed gaits (p > .05). In the step-up, the knee flexion increased (except for the gaits with the maximum trunk flexion), whereas other kinematic variables remained unchanged. Quantifying the step-to-step control of dynamic stability in a perturbed walking reflected continuous control adaptations through the interaction between gait and posture. In fact, the able-bodied participants were able to safely control the motion of the body's CoM with the combination of compensatory kinematic adjustments in lower-limb and adaptations in stepping pattern.