Ground reaction forces associated with an effective elementary school based jumping intervention.

BACKGROUND Mechanical loading during childhood plays a critical role in normal growth and development of the skeleton. Ground reaction forces (GRFs) may provide a surrogate measure for the strain experienced by bone on landing and at take off. However, there appear to be no paediatric studies that assess GRFs across a variety of loading activities. OBJECTIVES To measure biomechanical variables in commonly performed childhood activities used in an elementary physical education intervention study which augmented bone health in boys and girls. METHODS Maximal GFR, maximal rates of force, and time to maximum force were measured for 12 different jumping activities on a force platform. The jumps measured were drop jumps from 10, 30, and 50 cm, all followed by a plyometric jump, submaximal and maximal jumping jacks, alternating feet jump, counter movement jumps, and side to side jumps over 10 and 20 cm foam barriers. The subjects were 70 children (36 boys and 34 girls), 8.3-11.7 years old. RESULTS Subjects ranged in height from 128.4 to 172.6 cm and had a mass of 25.0-57.0 kg. Mean (SD) for vertical jump was 24.2 (5.5) cm and 135.2 (16.6) cm for standing long jump. The children engaged in loaded physical activity 5.7 (5.3) hours a week, on average. The highest mean maximal GRFs, normalised for body weight (BW), were generated from the plyometric portion of the drop jumps and the counter movement jump (about 5 times BW) compared with 3.5 times BW for jumping jacks. Similarly, the highest rates of change in force were 514 times BW/s for the drop jump from 10 cm and 493 times BW/s for the counter movement jump. CONCLUSIONS Simple jumps requiring minimal equipment produce GRFs of 3.5-5 times BW and rates of force of around 500 times BW/s. As children appear to attenuate higher impact forces when jumping from increased heights, it cannot be assumed that merely increasing the height of the jump will necessarily "progress" the exercise intervention.