Aging effects on Kinematics, Kinetics, and Neuromuscular Control of Landing Movements in Response to Sudden Loss of Ground Support Surface while Standing

Abstract

Background. With advancing age, the capacity to maintain balance after perturbations deteriorates due to a number of age-related sensorimotor deficits, and likely increases the risk for falls. The unexpected nature of falls triggers startle-like whole body postural responses. Startle responses are characterized by exaggerated whole body postural responses with increased muscle co-activity causing co-contraction during the first trial response (FTR) and normally diminish with repeated exposure due to behavioral habituation. Previous work suggested that age-related abnormalities of exaggerated startle responses and habituation might influence protective balance and startle FTRs to sudden loss of the ground support surface. Furthermore, while startle-like effects in younger adults are modifiable during self-activated (SLF) drop perturbations due to motor prediction, the extent to which this capacity is retained with aging is unknown. Aim. This dissertation 1) compared changes in protective balance and startle responses to unexpected and expected drop perturbations in relation to age during a) FTRs and b) subsequent trials; and 2) determined the modulatory effects of repeated self-triggered drop perturbations on reducing FTR magnitude evoked by externally triggered drop perturbations. Methods. Participants stood atop a moveable platform and received blocks of twelve consecutive trials of externally triggered (EXT) and self-triggered (SLF) drop perturbations. Following the last SLF trial, participants received an additional EXT trial spaced 20 minutes apart to assess retention (EXT RTN) of any modulation effects. Electromyographic (EMG) activity was recorded bilaterally over the sternocleidomastoid (SCM), middle deltoid (DLT), biceps brachii (BIC), vastus lateralis (VL), biceps femoris (BF), medial gastrocnemius (MG), and tibialis anterior (TA). Whole-body kinematics were recorded with motion analysis. Stability in the antero-posterior direction was quantified using the margin of stability (MoS). To quantify landing strategies, the mechanical work performed during drop landings was measured. Results. Incidence of early onset of bilateral SCM activation within 120ms after drop onset was present during the first trial response (FTR) for all participants. Co-contraction indices (CoI) during FTRs between VL and BF as well as TA and MG were significantly greater in older adults compared to young (VL/BF by 26%, p<.05 and TA/MG by 37%, p<0.05). Reduced shoulder abduction between FTR and last trial responses (LTR) was present across both groups and indicative of habituation. Significant age-related differences in landing strategy were present between groups as older adults had greater trunk flexion (p<0.05) and less knee flexion (p<0.05) which resulted in greater peak vGRFs and decreased MoS compared to young adults. Motor prediction via self-triggered drop perturbations (SLF) reduced peak SCM response and VL/BF and TA/MG CoIs (p <0.05) across both groups. Older adults significantly reduced peak vGRFs during SLF FTR compared to EXT FTR (2.40 ± 0.07 vs. 2.74 ± 0.07, p<0.05). Similarly, young adults significantly reduced peak vGRF during SLF FTR compared to EXT FTR (1.43 ± 0.08 vs. 1.83 ± 0.07, p<0.001). Lastly, in both groups, more eccentric work was performed during SLF trials compared to EXT (p <.05). Conclusion. Drop perturbations of standing balance evoke startle-like reactions in young and older adults. Age-associated abnormalities of delayed, exaggerated, and poorly habituated startle/postural FTRs linked with less balance stability and increased joint stiffening with increased impact forces were present among older adults. However, protective balance and startle responses were modulated with motor prediction resulting in reduced knee and ankle co-contraction, reduced ground impact forces and improved balance stability. The observed difference coincided with a reduced SCM response amplitude indicative of a reduced startle influence.