Active load forces (the pull on bone from muscle contractions) and impact load forces (incurred during weight-bearing physical activities) cause mechanical strain or deformation of bone. When these mechanical strains are greater than needed for steady-state remodeling, a modeling response occurs that increases bone mass and improves the internal structural supports at the high strain locations. Bone hypertrophies to reduce later strains. The response is regional, allowing specific bone to meet increasing load requirements. The threshold for modeling response--the point at which mechanical stress triggers bone hypertrophy--may be elevated or depressed by pharmacological and physiological stimuli such as exogenous and endogenous hormones.

Laboratory research demonstrates that this osteogenic response to active load and impact load forces saturates with repetitions. It also demonstrates that load magnitude and deviations from normal load patterns are more important to bone modeling than the duration of the stimulus (6). This is because strains that stay at or below the threshold of normal load magnitude or patterns turn modeling off. This observation is consistent with epidemiologic studies demonstrating that the strongest relationship between physical activity and bone measures involves physical activities producing high active or impact load strains in unusual patterns such as jumping and strength training (6). Consistent with this work are observational studies reporting that children and adolescents engaged in weight-bearing activities with high impact load forces such as basketball, gymnastics, and tennis have greater bone mass when compared to individuals involved in low impact weight-bearing and non-weight-bearing activities such as walking and swimming (7).

6. Umemura Y, Ishiko T, Yamauchi T, Kurono M, Mashiko S. Five jumps per day increase bone mass and breaking force in rats. J Bone Miner Res 1997;12:1480-1485.

7. Bailey DA, Faulkner RA, McKay HA. Growth, physical activity, and bone mineral acquisition. Exercise and Sport Sciences Reviews,1996;24:233-266.