EFFECTS OF PROLONGED BENCH STEPPING ON IMPACT FORCES
AbstractINTRODUCTION - This study examined the effects of prolonged bench stepping on the vertical ground reaction forces (GRF). Ten college-aged females experienced in bench step aerobics performed a 40 minute bench step aerobic routine on a 20 cm bench using an arm and leg crossover, alternate lead leg routine at a 100 bpm cadence. Twenty seconds of force platform data was collected at 0 min, 10 min, 20 min, 30 min and 40 min of the aerobics activity. The vertical GRFs were collected by an Ariel APAS system using a Kistler force plate at 1000 Hz sampling rate. Three right and 3 left stride impacts were selected from the middle of the 20 second interval for analysis. Each stride was delineated into 3 phases 1) landing contact, 2) full weight double support, and 3) toe push-off. - RESULTS - The time duration for the 3 phases for each stride was measured and an 5x3~3A NOVA (Time x Steps x Phase) with repeated measures on all factors was used for analysis. The analysis found significant differences in the time of contact over the exercise duration. No differences in the contact time were found between the right and left strides. The mean contact time was 1.262 seconds at the beginning of the workout and decreased to 1.19 seconds over 30 minutes of step bench aerobics. This change of time was the result of the subjects standing on the bench longer and using a more ballistic stepping technique on :he platform as the exercise was prolonged. Analysis of the GRF data found a significant phase factor, where the contact phase's forces were 946.7 Nt(l62%BWT), the support phase was 1044.1 Nt (183%BVVT), and the push-off phase was 816 Nt (134%BVVT). These vertical GRFs were of similar magnitudes reported by Humphries & Newton (1991 ), Johnson et al., (1991) and Ricard & Veatch (1990). Differences in the GRFs in the 40 minute workout were marginally significant (p=.10). These GRF forces recorded would place the impact loading in the range of easy jogging. CONCLUSION - In summary, the timing of the step bench technique was altered as the exercise was prolonged and the GRFs during the 3 phases of stepping were significantly different. An 8% increase in the double support phase GRFs after 20 to 30 min of step aerobics was the result of greater loading due to fatigue. The GRFs observed during the 40 minute stepping exercise would be classified as a mild impact activity. The combination of increased GRFs during fatigue and thousands of foot impacts have the potential for the development of musculoskeletal injuries if sufficient recovery time is not provided between workouts. REFERENCES - Humphries, B. & Newton, R. (1991). Peak ground reaction forces during step aerobics, walking and jogging. Biomechanics in Sports 1X:67-71. Johnson, B., Berry, S., Rupp, D. & Rupp, J. (1991). Peak vertical ground reaction forces and time to peak force in bench step aerobics and other activities. Research Quarterly for Exercise & Sport, 22:370-376. Ricard, M. & Veatch, S. (1990). Comparison of impact forces in high and low impact aerobic dance movements. International Journal of Sports Biomechanics, 6:67-77.
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