Effects of Dorisflexion Perturbation Velocity and Amplitude on Sex Comparisons of Soleus Spinal Stretch Reflexes

Abstract

1133 Variability in velocity and amplitude of muscle stretch, and background EMG may contaminate spinal stretch reflex (SSR) data. Servo-controlled perturbation devices and EMG biofeedback have been used to control these factors. Spring-loaded perturbation devices and static muscle loading methods reduce experimental complexity, but may introduce measurement error. PURPOSE: To determine the efficacy of a springloaded dorsiflexion perturbation device (PD) designed to stretch the soleus and a static muscle loading method in evaluating sex differences in SSR latency and amplitude. METHODS: Subjects (20 males, 20 females) were seated with the hip and knee of the right leg at 90°, and the right foot on the PD. A static load was placed on the distal thigh via a custom device, resisting plantarflexion. The forefoot was placed on a block fixed to the PD such that plantarflexion brought the calcaneus out of contact with the PD. The static load was standardized at ∼15% soleus MVC. Ankle position was maintained at 90°, making the contraction isometric. SSR amplitude was standardized to the maximal soleus motor response (S/M ratio). RESULTS: While S/M ratio did not differ across sex (P>.05), SSR latency was significantly shorter in females (t38 = 2.16, p = .01). However, SSR latency and leg length were related (r = .57, p<.01), and the sex difference in SSR latency was not significant when variance in leg length was accounted for (ANCOVA;F1,37 = .03, p = .45). Due to inter-subject differences in angular inertia and the fact that the calcaneus was not in contact with the PD, perturbation amplitude varied from 2 to 7°, and perturbation velocity varied from 55 to 177°/s. Perturbation amplitude did not differ across sex, and was not related to S/M ratio (r = .06) or SSR latency (r = .20). Perturbation velocity was significantly greater in females, but was not significantly related to S/M ratio (r = .05) or SSR latency (r = .21). The sex difference in background soleus EMG was not significant, and did not demonstrate significant relationships with S/M ratio (r = .31) or SSR latency (r = .19). Activity of the tibialis anterior and knee fiexors and extensors was negligible. CONCLUSIONS: Though spring-loaded platforms and static loading mechanisms do introduce variation to perturbation characteristics between individuals, the effects on SSR characteristics are limited and do not have significant implications for group comparisons. Additionally, it is necessary to account for differences in the distance traveled by the neural impulse (leg length) when comparing SSR latencies between individuals. These results suggest that this methodology is effective for comparing SSR characteristics between groups.