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Ground reaction force analysis of a variety of jumping activities in growing children Tsang, Garry


In recent years, the role of physical activity for the development and maintenance of a healthy skeleton and for the prevention of osteoporosis has garnered significant research interest. These studies have revealed that high impact loads with an unusual strain distribution are generally thought to be more effective in eliciting an osteogenic response than low impact repetitive activities. Also, immature bones appear to have a greater capacity to adapt to mechanical loads than mature bone. Childhood intervention programs that utilized different weight bearing activities and games including jumping have demonstrated a positive bone response to mechanical loading (McKay et al., 2000, Bradney et al., 1998, Morris et al., 1997, Heinonen et al., in press). However, the biomechanical characteristics of effective interventions have never been described. We addressed the question "what ground reaction forces (GRFs) are associated with pediatric mechanical loading intervention programs?" To accomplish this we measured the maximum GRF, rates of force, impulses and time to maximum force for twelve different jumping activities on a Kistler 9251A force platform (Winterthur, Switzerland). Jumps measured included drop jumps from 10, 30 and 50 cm, followed by a plyometric jump, submaximal and maximal jumping jacks, alternating feet, counter movement jumps and side to side jumps over 10 and 20 cm foam barriers. We also examined the relationship between bone mineral density (BMD) at the proximal femur, physical activity (PA) and dynamic power. The subjects were 70 children (36 boys and 34 girls), 8.3-11.7 years old. Height (cm) and mass (kg) were measured using standard techniques. BMD (g/cm²) at the hip and lean and fat mass (g) from the total body scan were assessed by dual energy X-ray absorptiometry (DXA, Hologic Inc). PA was assessed by questionnaire and a composite loading activity score was derived for each subject. Dynamic power was assessed with a vertical and standing long jump using standard procedures. Subjects ranged in height from 128.4 - 172.6 cm and with mass of 25.0 - 57.0 kg, on average. 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 PA an average of 5.7 (5.2) hours per week. BMD (g/cm²) for total proximal femur, femoral neck and trochanter was 0.70 (0.09), 0.67 (0.08) and 0.58 (0.08), respectively The highest mean maximum GRFs, normalized for body weight (BW), were generated from the plyometric portion of the drop jumps and the counter movement jump (on average 5 BW) compared to 3.5 BW for jumping jacks. Similarly, highest fates of force were 514 BW/sec for the plyometric jump from 10 cm and 493 BW/sec for the counter movement jump. In hierarchical regression, lean mass (β = 0.56) and long jump distance (β = 0.33) were significant predictors of femoral neck BMD accounting for 42% of the total variance. Our findings demonstrated that relatively high and diverse GRFs and rates of force are generated by jumps included in a pediatric exercise intervention trial. As forces at the hip are known to be approximately 3 times the measured GRF (Bassey et al., 1997), the GRFs measured in the present study would be associated with forces 15 BW at the proximal femur. These findings could be used to modify ongoing interventions or to develop new targeted interventions for bone health in children.

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