Age-related changes in the capacity to select early-onset upper-limb reactions to either recover balance or protect against impact

Falling is a leading cause of serious injury, loss of independence and nursing home admission in seniors. Arm reactions induced by a sudden loss of balance can play an important role in preventing falls and protecting against injury. The aim of this study was to investigate the effect of unpredictable perturbation characteristics on perturbation-triggered early-onset balance-recovery and impact-protection arm reactions. Twelve healthy young adults (20-28 yrs.; 5 women) and twelve healthy older adults (65-74 yrs.; 8 women) were tested and compared. Participants were exposed to forward/backward platform translations with/without a handrail available, while perturbation magnitudes were varied over a wide range that either allowed balance recovery or resulted in a "fall" (into a safety harness). Barriers were used to deter stepping reactions, so as to simplify interpretation of the arm reactions. Early-onset arm reactions (deltoid/biceps latency <200 ms) occurred in 91% of trials. When a handrail was present, the majority of responses (71%) involved a reach-to-grasp reaction. In the absence of a handrail, the induced arm movement was consistent with efforts to either counterbalance the falling motion (27% of trials) or to protect against impact (13% of trials). In contrast to suggestions that early-onset arm reactions may be generic startle-like responses, the present results supported our hypothesis that early-onset arm reactions would be dependent on task conditions. The results also supported our hypothesis that early-onset impact-protection reactions would occur in some trials; however, these reactions were relatively infrequent and the frequency did not increase even in trials where large perturbation magnitude precluded any possibility of recovering balance. Age-related differences were limited to an increase in fall frequency among older adults (59%) compared to younger adults (44%) and a small (12 ms) delay in EMG onset latency of the right medial deltoid. Further work is needed to fully understand the complex interaction (and possible sequencing) of upper- and lower-limb balance-recovery and impact-protection reactions, and the effects of co-morbidities and other factors.