BIOMECHANICAL TESTING OF A COMPLIANT BENCH FOR STEP AEROBICS

Abstract

INTRODUCTION The purpose of this study was to determine if adding compliance to the bench used in step aerobics routines would decrease the stress on the musculoskeletal system. The stresses are imposed by the shock of the ground reaction forces (external) and due to joint compression caused by muscles attempting to absorb and generate forces (internal). Injury has been related to the amount of force and the rate of force application. Repetitive strain was not a primary consideration in this study. METHODS The present investigation involved four step aerobics instructors and 11 recreational calibre subjects. Each subject performed two types of stepping exercises at a cadence of 120 beats per minute. A compliant bench was constructed with similar dimensions to the traditional rigid bench. The major difference was that the compliant bench had an arched top of plywood that would deform by becoming more flat as it was loaded. The stiffness of the top was about 8 KN*m-l. Two force plates were used to measure the ground and bench reaction forces. The data were sampled at 100 samples per second and ensemble averaged over 15 steps for each subject and each task to obtain a representative record and a measure of the variability. The representative record was analysed for peak forces and each individual step was analysed for the rate of rise of force. The rate of rise of force was processed by filtering out the high frequency noise of the force channel and differentiating the signal with respect to time. EMG of the knee extensors of the right leg was also examined. Four two-way analyses of variance with repeated measures were applied to the peak force and rate of rise of force data. The two factors were the type of bench and state of fatigue. The significance level was chosen to be p < 0.05. RESULTS The step-to-step variability within each subject was quite low. The peak ground reaction forces of the compliant bench were not significantly different for the instructors on each bench and the EMG of the vastus lateralis did not show a significant increase in muscle activity to absorb shock on the rigid bench. The 11 recreational calibre subjects showed a main effect for three of the four analyses. The compliant bench was found to have significantly lower rates of rise of force in both the step and propulsive moves and a lower peak force in the propulsive move. The peak forces were lower in the step move but not enough for statistical significance. There was no main effect for fatigue or interaction between fatigue and bench type. CONCLUSIONS Risk of injury by either high peak force or muscle force was neither increased nor decreased when the instructors used the compliant bench. It appears that professionals are able to provide musculoskeletal shock absorption without significantly increasing the muscle activation when performing on a rigid surface. Recreational athletes, however, were not as capable and would benefit more by exercising on a compliant surface.