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.

Clinical assessment of reactive balance control in acquired brain injury: A comparison of manual and cable release-from-lean assessment methods.

OBJECTIVE: Perturbation-evoked stepping reactions are infrequently assessed directly in clinical settings even though stepping reactions in response to a sudden loss-of-balance perturbation ultimately determine whether a fall occurs. Individuals with acquired brain injury (ABI) due to stroke who are ambulatory and awaiting hospital discharge have been found to exhibit delayed stepping reactions but are typically discharged without assessment of perturbation-evoked stepping. Tests that specifically target the capacity to perform perturbation-evoked stepping reactions are important to identify those at risk for falls and to direct intervention strategies. The aim of this study was to evaluate agreement and reliability of two assessment methods used to assess rapid stepping reactions, specifically in individuals with ABI due to stroke or other causes. A secondary aim was to compare perturbation-triggered biomechanics of the two assessment methods. METHODS: Thirty-five participants who were less than 4 months post-ABI were evaluated in an inpatient unit at the Toronto Rehabilitation Institute. Stepping reactions were assessed using manual release-from-lean and cable release-from-lean perturbation-based assessment methods. RESULTS: There was moderate agreement between the assessment scores resulting from the two assessment methods (κ = 0.55) and substantial test-retest reliability (κ's > 0.61) for both assessment methods. There was no evidence that the assessment score was affected by assessment method, test-retest, or assessment order. However, the cable release from lean resulted in a more rapid release of the lean support force (2 ms vs. 125 ms) and earlier foot-off times (340 ms vs. 401 ms) compared with the manual release from lean. CONCLUSION: Delays in foot-off time associated with the manual release-from-lean perturbation raise concerns that the manual assessment method may not provide a perturbation that is sufficiently challenging to reveal a patient's balance-recovery capacity and associated fall risk. However, the manual assessment requires no equipment and the assessment score may provide a useful indication of gross reactive balance control.