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Nikki Harris, Center for Healthy Living & Longevity, The University of Texas at Arlington, Arlington, TX. Advisors: Dr. Mark Ricard, Dr. Chris Ray, Dr. Judy Wilson.

THE EFFECT OF A WEIGHTED VEST ON POSTURAL REFLEXES: A PILOT STUDY

Introduction

Purpose

Methods

Methods (cont.)

Results

Results (cont.)

Conclusions

When loss of balance occurs, most people will rely on either an ankle strategy which is used for small movement adjustments, a hip method for larger movement adjustments, or take a step to prevent falling. In normal subject’s, the muscle’s corrective movements begin within 70-100 ms which leads to the re-positioning the person’s center of gravity.

The purpose of this experiment was to determine if wearing a weighted vest would cause a difference in the reflexes of the individual when a loss of balance was induced.

Twelve healthy college age volunteers (height, 163.8 cm; mass , 69.6 kg) served as subjects. The subjects visited the lab on two different occasions: initial orientation and to practice the Motor Control Test (MCT) and eliminate the possible learning effects, and a second time for the actual experiment. Each MCT was performed using the NeuroCom Equitest balance system. The MCT test consisted included 3 forward and 3 backward perturbations (large-6 cm) with and without a weighted vest with the order counterbalanced. A protective safety harness was worn for every test by every participant.

Surface EMG data were recorded from the gastrocnemius (Ga) and tibialis anterior (Ta) muscles with single differential electrodes. The electrodes were placed on the midline of the muscle belly with the detection surface of the sensor perpendicular to the direction of the muscle fibers and connected to a Bagnoli EMG system (Delsys, Boston, MA). A PCB single axis accelerometer (Model 352C33) was encased in Styrofoam and rigidly attached to NeuroCom Equitest force platform to measure floor perturbations. The EMG and accelerometer signals were sampled at 1000 Hz using a National Instruments (Austin, TX) PCI-6224, 16-bit A/D card. A Visual Basic.Net 2008 computer program was used for data collection and analysis.

The data for six subjects were excluded as these subject’s exhibited muscular preactivation prior to the floor perturbation. The mean ± sd for the postural reflex variables in the forward floor translations with and without the weighted vest are shown in Table 1. There was no difference in either the peak or the onset latency for the tibialis anterior muscle with and without the weighted vest. While not significant, the EMG amplitude and IEMG was greater in the weighted vest condition.

Table 1. Tibialis Anterior Reflex Responses for Forward Floor Translations (Mean ± SD)Variable No Weighted Vest Weighted Vest Dep. t-test sig.

Reflex Amplitude 197.07 ± 99.7 213.39 ± 99.2 0.408Integrated EMG 36.00 ± 58.4 48.76 ± 91.3 0.316

Peak Latency 157.53 ± 18.7 151.64 ± 14.6 0.254Onset Latency 107.08 ± 6.8 108.11 ± 14.4 0.830

Duration 110.97 ± 44.7 101.56 ± 32.1 0.497

Table 2. Gastrocnemius Reflex Responses for Backward Floor Translations (Mean ± SD)Variable No Weighted Vest Weighted Vest Dep. t-test sig.

Reflex Amplitude 108.73 ± 44.7 117.97 ± 64.8 0.692Integrated EMG 2.98 ± 2.1 14.20 ± 25.8 0.317

Peak Latency 136.61 ± 19.9 137.22 ± 15.6 0.856Onset Latency 105.33 ± 13.2 144.56 ± 102.7 0.367

Duration 62.78 ± 13.2 67.28 ± 17.7 0.331

The EMG signals were full wave rectified and low pass filtered with a 4th order recursive Butterworth digital filter (50 Hz cutoff). The reflex onset latency was defined as the time from the start of the floor acceleration until the EMG signal was 5 SD’s above baseline, see Figure 1 for an explanation. The peak latency was defined as the time from the start of floor acceleration to the peak EMG response. The peak amplitude was the peak EMG within the first 180 ms after the start of floor acceleration. The reflex duration was defined as the time from the first EMG point above 5 SD x baseline until the last point above 5 SD x baseline. The IEMG was the area under the EMG curve from the first EMG point above 5 SD x baseline until the end of the floor acceleration.

Statistical MethodsDependent t-tests were used to compare the effects of the weighted vest (no vest, vest) in the forward and backward floor translations for the following dependent variables: reflex amplitude, IEMG, peak latency, onset latency and duration. Alpha was set at 0.05.

Figure 1. Definition of computed variables.

The mean ± sd for the postural reflex variables in the backward floor translations with and without the weighted vest are shown in Table 2. There was no difference in either the peak or the onset latency for the gastrocnemius muscle with and without the weighted vest. While not significant, the EMG amplitude and IEMG was greater in the weighted vest condition.

Postural reflexes are automatic responses that are initiated by feedback from somatosensory, vestibular and visual sensors. These automatic responses facilitate corrective responses required to maintain postural orientation and equilibrium. It was anticipated that the weighted vest would alter both the magnitude of the EMG response and the IEMG area. Further work on this project will focus on the role of the muscles in stabilizing postural equilibrium following forward and backward floor translations in both young and elderly subjects.