The effect of time constraints on reactive arm positioning prior to falling on outstretched hands

Protective arm reactions were evoked in 12 younger adults to determine the effect of delaying deployment of a protective response. Participants were supported in a forward-leaning position that was released at an unpredictable time. Upon release, participants rapidly oriented the arms to prevent collision with the support surface. Ground reaction force (GRF), elbow angle (arm extension) and hand velocity at impact, and co-contraction of the biceps and triceps were compared at four initial lean angles. Larger lean angles resulted in increased ground reaction force and increased elbow extension while hand velocity at impact decreased. Delayed impact protection responses when falling may result in insufficient elbow extension possibly increasing the risk of torso and/or head injury.