An investigation into the choice of An investigation of the effects of jumping exercise and subsequent deconditioning on the tibia and the determination of the minimum level of jumping exercise required to maintain exercise-induced tibial gains in female rats

Abstract
Although high impact jumping is considered effective in improving bone strength, little is known about the minimum level of jumping exercise needed to maintain these jump-induced bone gains. PURPOSE: This study was carried out first to determine an appropriate jumping training programme, and then to examine the effects of this jumping training programme and subsequent deconditioning on bone, and to finally determine the minimum level of jumping that will then be required to maintain the jumpinduced bone gains in 12-week old Wistar Kyoto female rats. METHODS: The study was carried out in three phases: In phase I, an appropriate standard training programme (STP) was determined. In phase II, the effects of 12 and 24 weeks of deconditioning after the STP were investigated, and in phase Ill, the minimum level of exercise that was required to maintain STP-induced bone gains over a period of 24 weeks was determined. To determine the appropriate STP, ninety rats were divided into ten experimental groups, where the groups performed either no exercise or performed jumping exercise for 2, 4, 6, 8 or 10 weeks (n=9 per group). The exercise regimen consisted of 40 jumps per day (40J/d) for 5 days per week (5d/w) at a jump height of 40 em. From this, it was found that a minimum of 6 weeks of jumping exercise was required to significantly (p<0.05) increase bone gains in mass, mechanical property and physical dimensions. Consequently, an 8-week jumping exercise programme with a regimen of 40J/d and 5d/w (8STP} was selected as the standard training programme for the ensuing phases of the study. In the second phase, ninety rats (n=10 per group) were divided into nine groups, i.e. one baseline control group and eight experimental groups, where the latter groups were given either no exercise for 8, 20 or 32 weeks, or received 8STP, or 8STP followed by 12 or 24 weeks of no STP, or 8STP followed by another 12 or 24 weeks of STP. In the third phase of the study, one hundred and thirty rats (n=10 per group) were divided into thirteen experimental groups. Two groups of rats were given no exercise for either 8 or 32 weeks. The remaining groups received 8STP, or 8STP followed by 24 weeks of no exercise, or 8STP followed by 24 weeks of further jumping exercise at intensities of either 40J/d, or 20J/d, or 10J/d, each for either 5d/w, or 3d/w, or 1d/w. At the end of the experimental period, animals were decapitated and the right hind tibiae were harvested for measurement of bone mass (tibial wet weight and fat free dry weight), bone mechanical properties (tibial mid shaft ultimate bending load/bone strength, stiffness and moment of inertia), bone physical dimensions (tibial length, tibial mid shaft maximum and minimum diameters) and bone morphometry (the cross-sectional tibial mid shaft cortical area, medullary area, periosteal perimeter and endosteal perimeter). Calcium, osteocalcin and alkaline phosphatase (bone formation markers) and C-terminal telopeptide of type 1 collagen/1 CTP (bone resorption marker) were determined in the serum. RESULTS: Fat free dry weight, ultimate bending load, maximum and minimum diameters, cortical area, periosteal perimeter and endosteal perimeter of the tibiae were significantly (p<0.05) greater in rats given 8STP when compared to those in the sedentary controls. However, no detectable changes in serum parameters were evident after 8STP. The 8STP-induced bone gains like ultimate bending load, maximum diameter and cortical area were still maintained after 12 weeks of complete cessation of jumping. However, with the exception of cortical area, all measured 8STP-induced bone gains were lost after 24 weeks of complete cessation of jumping. When the rats were further trained with different exercise regimens for another 24 weeks after 8STP, it was found that the minin'lum levels of exercise required to maintain the 8STP-induced bone gains in wet and fat free dry weight, ultimate bending load, moment of inertia, maximum diameter, periosteal perimeter and cortical area were 38 jumps per week (J/w), 31J/w, 36J/w, 27J/w, 12J/w, 25J/w and 21J/w respectively. No significant differences were observed in blood parameters between the exercised and sedentary groups except for serum alkaline phosphatase and 1CTP, which were significantly (p<O.OS) different from controls in rats receiving a work load of 40J/w and above. CONCLUSION: The data indicated that 8STP significantly (p<O.OS) increased bone gains and the decay in exercise-induced bone gains was not uniform. However, most of the exercise-induced bone gains following 8STP could not be preserved after 24 weeks of deconditioning. Nevertheless, it is possible to maintain these bone gains, over a period of at least 24 weeks, with much lower exercise frequency and intensity, ranging from 6 to 19% of the original exercise work load. The minimum level of exercise required to maintain the bone gains however varied slightly from parameter to parameter. An activity comprising of 30J/w given at 1 OJ/d and for 3d/w appeared the most appropriate. Whether this minimum level of physical activity can maintain these bone gains for a longer period remains to be ascertained.
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Keywords
Jumping exercise and subsequent deconditioning , Jumping exercise required , Exercise-induced tibial gains in female rats
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