Motor strategies in landing from a jump: the role of skill in task execution

Summary

A motor performance which involves multijoint coordination and belongs to the natural repertoire of motor behavior has been studied. Displacements have been related to EMG in the lower limb when taking off and landing from a jump down (45 cm) onto two surfaces of differing compliance in two populations of teenage girls: skilled and unskilled. To evaluate the performance, an index was defined taking into account: 1) the time required for reaching stability (1 body weight) after landing, and 2) the amount of sway during the stabilization time. Despite the apparent intra and inter subject similarities in performing the jump-down, slight differences were observed in both the kinematics and electromyogram patterns. During takeoff, two strategies were identified that were not related to either skill or landing surface compliance. The most common strategy, “Push Off”, is characterized by almost full joint extension when departing from the jump platform and includes a swing period during flight. The other strategy, “Roll Off”, is characterized by joint flexion at departure and continual extension during midflight. While the ankle dorsiflexor, tibialis anterior, is active in preparation for the takeoff phase in both strategies, it is followed by activation of the ankle plantarflexors, lateral gastrocnemious and soleus and the hip/knee musculature, rectus femoris, biceps femoris, and vastus lateralis, only in the push off strategy. The roll off strategy is characterized by a lack of other muscle activation prior to takeoff. At landing, regardless of the strategy used in takeoff, onset of muscles followed the same sequence for both landing surfaces; ankle musculature activity began first followed by activity in the knee and hip musculature. The onset of the musculature occurred closer to landing when landing on the more compliant surface. Skilled subjects were characterized by adjustments in amount of ankle extension present at landing and concomitant flexion post-landing with respect to landing surface. When landing on the rigid surface, the ankle was more plantarflexed and onset of the dorsiflexor occurred after that of the plantarflexors; on foam, dorsiflexor activity was coincident with the plantarflexors. Ankle joint range of motion post-landing was subsequently larger when landing on the rigid surface. In contrast, unskilled individuals used a default strategy for landing on both surfaces where the ankle position and movement was between that seen for the two conditions in the skilled individual. It is suggested the landing and takeoff phases are programmed independently in both skilled and unskilled subjects. Further, it is hypothesized that the skilled individuals may be more adept at making subtle adjustments to landing surfaces by continual update during execution of the movement, while in unskilled subjects this capability is less evident. The effect of long term learning as well as the adaptive capabilities of the nervous system during the execution of the movement in skilled and unskilled subjects is discussed.