Protective arm movements are modulated with fall height.

Protective arm reactions were evoked in 14 younger adults to determine the effect of fall height on protective arm reaction biomechanics. Participants were supported in a forward-leaning position on top of an inverted pendulum that isolated arm reaction by preventing any fall arresting contribution that may come from the ankle, knees, or hip. At an unpredictable time, the pendulum was released requiring participants to rapidly orient their arms to protect the head and body. Vertical ground reaction force (vGRF), arm kinematics, and electromyographic (EMG) measures of the biceps and triceps were compared at four initial lean angles. The time following perturbation onset and prior to impact consisted of two phases: rapid extension of the elbows and co-activation of the biceps and triceps in preparation for impact. The rapid orientation phase was modulated with fall height while the co-activation of the biceps and triceps in preparation for landing was minimally affected. Larger lean angles resulted in increased vGRF, increased elbow extension at impact, decreased elbow angular extension velocity at impact, and increased neck velocity at impact while hand velocity at impact was not significantly affected. The neuromuscular control strategy appears to optimize elbow extension angle/angular velocity prior to co-activation of the biceps and triceps that occurs about 100 ms prior to impact. Future work should investigate how the neuromuscular control strategy handles delayed deployment of protective arm reactions.