The aim of this study was to investigate the effect that changing step height had on ground reaction force. Using a randomised crossover design, 12 volunteers with no previous experience of step aerobics were recruited to perform at three different step heights: 6, 8 and 10 inches. Subjects performed a basic step at a cadence of 120 beats/min and performed three one minute trials during which ground reaction force was measured. Measurement of peak impact force, time to achieve peak impact, and total time of foot contact was made, and impulse of the force was calculated. Statistically significant differences were found to exist for peak impact force between the 6 and 8 inch and 6 and 10 inch, but not between the 8 and 10 inch conditions. No significant differences were found in any other parameters. The study supports the present advice that participants should use low step heights, and possible mechanisms of injury are discussed.