Vibratory Stimulation At Hip Vs. Ankle Muscles: Investigating Dynamic Balance Recovery Outcomes Due To Stochastic Stimulation In Healthy Young Adults And Older Adults

Abstract

Human balance is a complex mechanism that involves sensory units, muscular reflexes, and central nervous system control. Among older adults, falls represent a significant source of traumatic injuries, with tripping as the primary cause [1]. Research has underscored the association between proprioceptive deficits and compromised balance recovery [2]. Recent studies suggest that proprioceptive signals from proximal musculature associated with the hip joint initiate the recovery response, while distal proprioception associated with the ankle joint completes the recovery step [3]. These conclusions pivot on the assumption that the onset of muscle activity is intrinsically tied to proprioceptive performance. Yet, to gain a deeper understanding of the specific contributions of ankle versus hip proprioceptive areas to balance control, a methodology enabling direct manipulation of proprioceptive performance is necessary. Stochastic vibratory stimulation (SVS) applied to lower extremities has shown promise in impacting upright balance sway. This impact arises from the mechanical vibration of muscles, which increases the excitement of type Ia afferents in spindles. This enhanced excitement is postulated to influence both short-latency reflex mechanisms and long-latency feedback to the central nervous system [4].The primary goal of these studies was to identify how modifications in proprioceptive information originating from ankle and hip muscles impact balance recovery. By manipulating proprioceptive performance in joint muscles using SVS, we investigated the distinct contributions of ankle and hip proprioception in balance recovery within two phases. The first phase was healthy young participants aged between 18 and 30. We recruited 20 participants and categorized them into two groups: ankle stimulation (n=10) and hip stimulation (n=10). In the second phase, we recruited 24 older adults aged 65 and above. They were asked to visit twice for a complete participation, on one day, they would undergo ankle stimulation, and on another day, they would undergo hip stimulation. All participants underwent treadmill perturbations with varying levels of vibration and speed randomly, and were assessed for balance recovery outcomes, including reaction time, recovery step length, and full recovery time. The results and subsequent discussion of this study unveiled notable findings. For younger adults, ankle SVS emerged as a significant factor influencing reaction time and recovery step length (p<0.002). In contrast, hip SVS primarily affected the time required for full recovery, displaying a 61.4% increase at 40 Hz and a remarkable 99.7% increase at 80 Hz during slow speeds. Similarly, for fast speeds, full recovery time experienced a 30.8% increase at 40 Hz and a 29.2% increase at 80 Hz SVS (p=0.019). As hypothesized, the main finding of this study was that local SVS on ankle and hip muscles significantly influence recovery performance among healthy young adults. Within our sample, SVS caused a negative impact on the balance recovery performance, which was observed as a delayed reaction time when the stimulation was applied to the ankle muscles. However, findings from phase II balance recovery studies among older adults indicate that vibratory stimulation to the hip joint affects reaction time, unlike the ankle joint, which remains unaffected by the stimulation. These observations provide further support for the notion that in older adults, the deterioration of proprioceptive signals stems from nerve cell death and the demyelinating process associated with aging [5, 6] . Current findings demonstrated the role of ankle and hip muscle proprioceptive information in balance recovery. Our next step is to test SVS on older adults who are at high risk of fall. Our hypothesis is that SVS improves balance recovery by enhancing the proprioceptive afferent signal. This is based on our previous work showing that SVS can improve upright standing balance and timed up-and-go test among high fall risk older adults [7, 8]. This will ultimately guide us to engineer an easy-to-use sleeve/device that would help high fall risk older adults in performing strenuous tasks.