Whole Body Vibration: Mapping Of Transmission With High Speed Motion Analysis

Abstract

Whole body vibration (WBV) is used for its potential benefits in flexibility training and musculoskeletal strengthening. While bone-pin accelerometers have been used, vibration characteristics typically are assessed using skin mounted accelerometers which require corrections to the signal to minimize the effects of local skin-accelerometer resonance. Recent advances in camera technology allow measurement of small, low-mass skin mounted marker positions to a small fraction of a mm at sufficiently high sampling frequency to characterize local vibration. PURPOSE: Evaluate the capability of a motion analysis system to measure local vibrations over bony landmarks during WBV. METHODS: In this pilot study, two subjects experienced WBV while standing on a Freemotion iTonic machine which was set to vibrate under four conditions: low and high frequency (28 and 42 Hz) with low and high amplitude (about 1.5 and 3 mm displacement). Small (9 mm diameter) reflective markers were mounted on the vibration plate and on bony landmarks (lateral malleolus, tibial tuberosity, ASIS, sternum and forehead). Marker positions were measured using six Vicon T-20 cameras operating at 500 Hz. Each condition involved about 10 seconds of vibration exposure from which 40 consecutive cycles were extracted and analyzed. Mean (±SD) of vertical vibration amplitude was determined for each condition and subject. During measurement, knees were extended and feet were flat on the plate. RESULTS: Motion at the ankle closely matched the plate vibration however attenuation occurred at other points. Tibial vibration was about 0.8 mm and 1.0 - 1.5 mm under low and high amplitude conditions. ASIS and sternum vibrations ranged between 0.2 to 0.6 mm. At the head, high frequency vibrations had amplitude of about 0.7 mm while low frequency was transmitted more strongly (1.9 ± 0.4 mm for the high amplitude condition). CONCLUSIONS: High speed motion analysis has sufficient spatial precision to easily detect vibration of sub mm amplitude. When used with small, low-mass markers over bony landmarks, vibration transmission can be mapped across the body without the challenges associated with skin or bone mounted accelerometers.