Fig. 4

A schematic showing the mechanical loads borne by bone in normogravity. με: microstrain, BMD: bone mineral density, ALP: alkaline phosphatase, OCN: osteocalcin. Several studies have suggested that the rate (determined by the frequency and amplitude) rather than the magnitude alone of the applied loading stimulus correlates with bone formation.^75,247 This implies that bone formation is enhanced by dynamic loading, and therefore, both the magnitude (or amplitude) and the frequency of loading are important parameters. It has been shown that low magnitude [<10 με (<1 g; 1 g = 9.8 m·s⁻²)] and high frequency (10–100 Hz) loading stimulate bone growth, inhibiting disuse osteoporosis.^248,249 Peak dynamic strain magnitudes within the physiological range of 1 500–3 000 microstrain (μɛ) are reported to result in bone modeling and an increase in mass. Strain within the disuse range of 100–300 μɛ activates osteoclastic activity and bone resorption. Strain levels above 3 000 μɛ are considered overuse, and those above 5 000 are considered pathological overload²⁵⁰