Aging changes in protective balance and startle responses to sudden drop perturbations

Abstract

This study investigated aging changes in protective balance and startle responses to sudden drop perturbations and their effect on landing impact forces (vertical ground reaction forces, vGRF) and balance stability. Twelve healthy older (6 men; mean age = 72.5 ± 2.32 yr, mean ± SE) and 12 younger adults (7 men; mean age = 28.09 ± 1.03 yr) stood atop a moveable platform and received externally triggered drop perturbations of the support surface. Electromyographic activity was recorded bilaterally over the sternocleidomastoid (SCM), middle deltoid, biceps brachii, vastus lateralis (VL), biceps femoris (BF), medial gastrocnemius (MG), and tibialis anterior (TA). Whole body kinematics were recorded with motion analysis. Stability in the anteroposterior direction was quantified using the margin of stability (MoS). Incidence of early onset of bilateral SCM activation within 120 ms after drop onset was present during the first-trial response (FTR) for all participants. Co-contraction indexes during FTRs between VL and BF as well as TA and MG were significantly greater in the older group (VL/BF by 26%, P < 0.05; TA/MG by 37%, P < 0.05). Reduced shoulder abduction between FTR and last-trial responses, indicative of habituation, was present across both groups. Significant age-related differences in landing strategy were present between groups, because older adults had greater trunk flexion (P < 0.05) and less knee flexion (P < 0.05) that resulted in greater peak vGRFs and decreased MoS compared with younger adults. These findings suggest age-associated abnormalities of delayed, exaggerated, and poorly habituated startle/postural FTRs are linked with greater landing impact force and diminished balance stabilization. NEW & NOTEWORTHY This study investigated the role of startle as a pathophysiological mechanism contributing to balance impairment in aging. We measured neuromotor responses as younger and older adults stood on a platform that dropped unexpectedly. Group differences in landing strategies indicated age-associated abnormalities of delayed, exaggerated, and poorly habituated startle/postural responses linked with a higher magnitude of impact force and decreased balance stabilization. The findings have implications for determining mechanisms contributing to falls and related injuries.

Keywords: aging; balance; postural control; startle.

Fig. 1.

Representative experimental setup of drop perturbation. a, acceleration; Mp, participant’s mass.

Fig. 2.

Representative muscle activation patterns between younger and older adults for sternocleidomastoid (SCM), middle deltoid (DLT), biceps brachii (BIC), vastus lateralis (VL), biceps femoris (BF), medial gastrocnemius (MG), and tibialis anterior (TA) muscles. Vertical lines indicate the onset of platform release.

Fig. 3.

Mean (SE) first-trial response (FTR) onset latencies for sternocleidomastoid (SCM), middle deltoid (DLT), biceps brachii (BIC), vastus lateralis (VL), biceps femoris (BF), medial gastrocnemius (MG), and tibialis anterior (TA) muscles. *P < 0.05, young vs. older adults.

Fig. 4.

Mean (±SE) amplitude ratios relative to first-trial response (FTR) for sternocleidomastoid (SCM), middle deltoid (DLT), biceps brachii (BIC), vastus lateralis (VL), biceps femoris (BF), medial gastrocnemius (MG), and tibialis anterior (TA) muscles for younger and older adults.

Fig. 5.

A: representative time histories of vertical ground reaction force (vGRF; top), electromyographic (EMG) recordings of vastus lateralis (VL) and biceps femoris (BF; middle), and rectified linear envelope of the EMG of tibialis anterior (TA) and medial gastrocnemius (MG; bottom) from a younger (left) and older adult (right), during drop perturbations, with the vertical lines indicating drop onset and ground contact, respectively, and the shaded regions indicating the period of 100 ms preceding ground contact. B: mean (±SD) co-contraction indexes (CoI) for ratios VL/BF and TA/MG for first-trial (FTR) and last-trial responses (LTR). *P < 0.05, young vs. older adults.

Fig. 6.

Representative kinematic profiles in order from top to bottom: trunk, knee, hip, and ankle joint angles (θ) for young and older adults.

Fig. 7.

A: group means (±SE) of margin of stability (MoS) for first-trial (FTR) and last-trial responses (LTR) at the lowest vertical center of mass position (COM_low) for young and older adults. B: scatterplots showing associations between MoS at ground contact (GC) and MoS at COM_low) (left), trunk flexion angle at COM_low (middle), and knee flexion angle at COM_low (right) for young and older adults. *P < 0.05, young vs. older adults.

Fig. 8.

Scatterplots of associations between normalized vertical ground reaction forces (BW, body weight) and co-contraction index (CoI) for ratios of vastus lateralis and biceps femoris (VL/BF; left) and tibialis anterior and medial gastrocnemius (TA/MG; right) for young and older adults.

Fig. 9.

Mean (±SE) self-assessment manikin (SAM) scores for valence (top), fear (middle), and arousal (bottom) for young and older adults. *P < 0.05, young vs. older adults.