Neuromechanistic causes of timber falls in older adults with mild cognitive impairment: Is response initiation or motor execution the problem?

Older adults with mild cognitive impairment (OAwMCI) show reactive balance deficits compared to cognitively intact older adults (CIOA), which could increase the likelihood of 'Timber' falls (i.e., backward falls caused by extremely delayed reactive stepping). This study investigated potential neuromechanistic causes of Timber falls in OAwMCI, including delayed muscle onset latencies and/or altered muscle synergies during reactive stepping. 36 OAwMCI, 38 CIOA, and 20 young adults were exposed to a large anterior stance perturbation, with electromyography collected from the biceps femoris (BF), vastus lateralis (VL), medial gastrocnemius (MG), and tibialis anterior (TA). Timber falls were identified by falling (> 30% weight in harness) without initiating stepping within 430 ms (perturbation duration). Timber falls only occurred in OAwMCI, who were subcategorized into MCI: Timber (36%) or MCI: Step (intact stepping, 64%). MCI: Timber had higher fall rate, lower reactive stability, shorter step length, and longer step initiation time compared to groups with intact stepping (p < 0.05), and delayed onsets of stepping limb muscles and the stance limb MG (p ≤ 0.03). MCI: Timber also showed many structural differences in muscle synergies (M1-6), such as recruiting a unique synergy (M4) which might affect coordination of the stepping limb MG, and failing to recruit M6, which might be involved in taking a long recovery step (stepping limb VL). These results suggest that reactive balance deficits in OAwMCI may be related to problems with both initiating and executing reactive stepping, possibly due to neural pathology and sensorimotor processing deficits which delay response initiation and/or affect motor command execution.