The influence of handrail predictability on compensatory arm reactions in response to a loss of balance.

The current study examined whether compensatory arm reactions are influenced by the participant's knowledge of the handrail location prior to losing their balance. Thirteen young adults stood on a motor driven platform that could translate in the forward or backward directions. A handrail was positioned in a location that was either predictable (i.e., always on the participant's right) or unpredictable (i.e., on either the participant's right or left) to the participant. Unpredictability of the handrail location was ensured by using liquid crystal goggles to occlude the participant's vision until the onset of each translation. In response to each surface translation, participants were instructed to reach for and grasp the handrail as fast as possible. EMG activity from the posterior and anterior deltoids of the left and right arms as well as kinematic data of the wrist were recorded to quantify the resulting arm responses. It was found that in response to a loss of balance, participants activated the reaching arm 7 ms earlier (p = 0.020) and with a 21-30% greater amplitude (p = 0.010-0.029) during the predictable compared to unpredictable handrail condition. The earlier and larger EMG activity resulted in a 19% earlier initiation of arm movement (p = 0.016) and a 24% earlier handrail contact (p = 0.002) when the handrail was in a predictable compared to unpredictable location. These findings indicate that when a handrail is predictably located, individuals will pre-select their upcoming compensatory arm reactions prior to losing their balance and may be more effective in re-gaining stability.