Mechanical loading stimulates bone formation and regulates bone size, shape, and strength. It is recognized that strain magnitude, strain rate, and frequency are variables that explain bone stimulation. Early loading studies have shown that a low number (36) of cycles/day (cyc) induced maximal bone formation when strains were high (2,000 microepsilon) (Rubin CT and Lanyon LE. J Bone Joint Surg Am 66: 397-402, 1984). This study examines whether cycle number directly affects the bone response to loading and whether cycle number for activation of formation varies with load magnitude at low frequency. The adult rat tibiae were loaded in four-point bending at 25 (-800 microepsilon) or 30 N (-1,000 microepsilon) for 0, 40, 120, or 400 cyc at 2 Hz for 3 wk. Differences in periosteal and endocortical formation were examined by histomorphometry. Loading did not stimulate bone formation at 40 cyc. Compared with control tibiae, tibiae loaded at -800 microepsilon showed 2.8-fold greater periosteal bone formation rate at 400 cyc but no differences in endocortical formation. Tibiae loaded at -1,000 microepsilon and 120 or 400 cyc had 8- to 10-fold greater periosteal formation rate, 2- to 3-fold greater formation surface, and 1-fold greater endocortical formation surface than control. As applied load or strain magnitude decreased, the number of cyc required for activation of formation increased. We conclude that, at constant frequency, the number of cyc required to activate formation is dependent on strain and that, as number of cyc increases, the bone response increases.

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responses are made at 0 during the exposure of the hand when the left eye is used, then this direction of response would be expected to persist without visual guidance after the exposure period.For this experiment the distance RY was 12 cm and that of YP was 25.5 cm. Assuming an interpupillary distance of 6 cm the distance LO was 41.5 cm and OP was 12.5 cm.With feet in a fixed position and head immobilized in a support so that the eyes were 12 cm above the plane of an aperture 9.5 X 7 cm, 12 subjects (group E) were required to make 5 dots directly beneath the marked center without visual guidance before and after an exposure period. The right hand was used throughout. For the kinesthetically controlled centering responses before and after exposure, a cover bearing a central mark coincident with the left-right center of the aperture was placed over the opening and dots were made on the surface below so that they were judged to "be in vertical alignment with the center mark. For the exposure period the cover was removed and the subject now viewed his right hand through the opening with his left eye. The right eye was covered. Thirteen dots, one every 13 seconds, were made on separate sheets of paper so that they were judged to be in vertical alignment with the center. The center was indicated by a line coincident with that on the cover used for the responses betore exposure. Thus in the phases before and after exposure the position beneath the center of the aperture (which was covered) was judged kinesthetically and during the exposure period visual-kinesthetic judgments of the same position were made with only the left eye being used.There were two control conditions. Since maintained motion of a limb in a given direction can result in a sensory spatial aftereffect (3), another 12 subjects (group C1) performed the tasks before and after exposure but during the intervening period the aperture cover was left in place and the limb moved 1130 responses are made at 0 during the exposure of the hand when the left eye is used, then this direction of response would be expected to persist without visual guidance after the exposure period.For this experiment the distance RY was 12 cm and that of YP was 25.5 cm. Assuming an interpupillary distance of 6 cm the distance LO was 41.5 cm and OP was 12.5 cm.With feet in a fixed position and head immobilized in a support so that the eyes were 12 cm above the plane of an aperture 9.5 X 7 cm, 12 subjects (group E) were required to make 5 dots directly beneath the marked center without visual guidance before and after an exposure period. The right hand was used throughout. For the kinesthetically controlled centering responses before and after exposure, a cover bearing a central mark coincident with the left-right center of the aperture was placed over the opening and dots were made on the surface below so that they were judged to "be in vertical alignment with the center mark. For the exposure period the cover was removed and the subject now viewed his right hand through the opening ...