Biomechanical and neuromuscular strategies for a recovery from a trip during human walking

Human locomotion is a highly complex task which requires precise reactive control to recover from an obstruction of the swing foot (e.g., a small rock). The purpose of this study was to explore the control strategies necessary for recovering from a tripping perturbation and to determine the physical requirements necessary for a successful recovery. A momentary mechanical obstruction was applied to the swing foot at two different points of the swing phase (early and late) and the muscular responses were integrated with the kinematic and kinetic outcome. The recovery strategies demonstrated specific sequencing and organization of the reflex responses which were critical in generating functionally appropriate strategies for overcoming the obstacle and maintaining the ongoing locomotion. The early swing perturbation elicited an elevating strategy: excitatory responses of the swing limb flexors and stance limb extensors removed the limb from the obstacle prior to accelerating the limb over the obstacle. This response was primarily controlled by a triphasic swing knee moment pattern which facilitated hip flexion and ankle dorsiflexion via the joint accelerational components. The late perturbation elicited two different muscle activation patterns (an inhibition of the vastus lateralis and tibialis anterior or an excitation of the biceps femoris) which both resulted in a lowering strategy. Despite the two activation patterns, the kinetic outcome was similar; increased hip extensor and knee flexor moments decelerated the swing limb and a decreased ankle dorsiflexor moment lowered the foot to the ground. A fixed ratio was found between the hip and knee moments which may be indicative of a neuromuscular constraint to simplify the task and a function of the biarticulate hamstring muscles.

The physical demands for a recovery from a trip require rapid production of forces and precise agonist/antagonist coordination to bring the perturbed swing limb to the ground. Secondly, eccentric control of the trunk flexion is necessary to prevent the centre of mass from continuing its forward and downward acceleration upon the landing. This research demonstrates the adaptive control of locomotion and provides preliminary material which can be used to develop strategies for those prone to falls.