Comparison of Crossed-spinal modulation of the H-reflex between sedentary older adults and high-risk fallers

Abstract

The purpose of this study was to compare crossed-spinal H-reflex modulation between healthy older adult subjects and older adults who are at a high risk of falling. 18 subjects participated in this study, and were divided into two different groups: 1) 14 neurologically healthy adults over the age of 65 (Age = 75.2 yrs; SD = 6.26); 2) 4 older adults determined by physicians or physical therapists to be high-risk fallers (Age = 78.3 yrs; SD = 4.02). Fall risk was based on history of previous falls and/or Tinetti balance scores. The variable being measured in this study was a spinal stretch reflex known as Hofmann reflex, or H-reflex. Subjects were asked to lie in the prone position for testing. In order to record muscle activity, EMG electrodes were placed on the left tibialis anterior and the right soleus muscles. Stimulating electrodes were then placed over the common peroneal nerve (CPN) of the left leg and the tibial nerve of the right leg. Baseline measurements were recorded for maximal motor response (M-max), maximal H-reflex response (H-max), and tibialis anterior motor threshold. For control measurements, a single stimulation was delivered to the right tibial nerve to evoke a motor response in the soleus. The intensity used for controls was set to 50% of H-max. For the conditioning protocol, stimulation was first delivered to the CPN at 1.2x motor threshold, then a tibial nerve stimulation followed at varying intervals of 25ms, 50ms, 75ms, 150ms, or 300ms. The goal of this conditioning protocol was to determine the effect that antagonist muscle activation has on its contralateral agonist’s spinal reflexes. Results showed a significant interaction between healthy older adults and high-risk fallers (F5,45 = 4.21, p < 0.05), specifically at the 75ms, 150ms, and 300ms intervals. While the healthy adults were able to modulate spinal reflexes based on the time interval, the high-risk fallers showed no modulation across intervals. This inability to properly modulate spinal reflexes could help to explain why these individuals fall more frequently.