Whole Lotta Shakin' Goin' On

Clinical studies have been based on animal experiments that have shown a positive effect on bone strength and mass of various forms of loading. The basis of these experiments is the concept that trabecular bone adapts to its mechanical environment—Wolff’s law. Support for these experiments has also come from epidemiological findings that greater physical activity–mechanical stimulation is associated with greater bone mass, and in some studies, fewer fractures. The question of the ideal form of stimulation has been addressed in animal studies. High-frequency (30 Hz), lowmagnitude (200 strain) signals stimulated large increases in cortical bone in turkeys. However, higher amplitude and lower frequency was not anabolic in that model. In a longer-term study in sheep over 1 year, daily 20-minute sessions of high-frequency mechanical stimulation of sheep produced a 35% increase in BMD. This kind of vibration may also affect the sarcopenia that occurs at the same time as bone loss with aging. Other animal studies have shown similar results. Low-magnitude mechanical loading became osteogenic when rest is inserted between each load cycle. Effects of loading frequency on mechanically induced bone formation and periosteal osteogenesis suggested a complex interaction between extracellular fluid forces and cellular mechanics in mechanotransduction, best predicted by a mathematical model that assumed that (1) bone cells are activated by fluid shear stresses and (2) stiffness of the bone cells and the extracellular matrix near the cells increases at higher loading frequencies because of viscoelasticity. These animal experiments have formed the scientific basis for studies in humans. SHAKIN ALL OVER