STRATEGIES FOR RECOVERY FROM A TRIP IN EARLY AND LATE SWING DURING HUMAN WALKING

The movement strategies and the underlying organization of the muscula r responses for recovery from a tripping perturbation applied in early and late swing during walking were studied in humans. The latencies o f the reflex response (60-140 ms) suggested that polysynaptic pathways are involved. The most common movement outcome was an elevating strat egy of the swing limb in response to the early swing perturbation and a lowering strategy in response to the late swing perturbation. The el evating strategy comprised a flexor component of the swing limb and an extensor component of the stance limb. There was a temporal sequencin g of the swing limb biceps femoris prior to the swing limb rectus femo ris response to remove the limb from the obstacle prior to acceleratin g the limb over the obstacle. The extensor response of the stance limb generated an early heel-off to increase the height of the body. Thus, the lower limb joints collaborated to increase the height of the cent re of mass and provide extra time to extend the swing limb in preparat ion for the landing. Flexion of the swing limb would be dangerous in r esponse to the late swing perturbation as the swing limb is approachin g the ground and the body mass has passed forward of the stance foot. Instead, a lowering strategy was accomplished by inhibitory responses of the swing limb vastus lateralis and/or excitatory responses of the swing limb biceps femoris. Both these responses resulted in a rapid lo wering of the limb to the ground with a flat foot or forefoot landing and a shortening of the step length. Thus, in response to the late swi ng perturbation, the same recovery strategy was achieved by different patterns of muscle activation. These results demonstrate that the reco very strategies provided a functionally appropriate response for overc oming the obstacle and maintaining the ongoing locomotion.