The conclusions of the systematic revision with the results from the major clinical and experimental studies.
In this issue, we would complete the conclusions of the systematic revision with the results from the major clinical and experimental studies. First of all, we outline that the use of bone mineral density is controversial and opposed of the major studies in the field that show that the bone density is only one of the factors which describe bone resistance, and not the bone quality. In fact, often, a dramatic variation of bone strength is linked to the fact that the newly bone is located in the only in the surfaces where mechanical stress is greatest and this doesn't change completely the density. To study the optimal exercises program we have to remember that the ideal timing in the remodelling unit of the bone was estimated at 4-6 months: therefore any treatment which has the aim of preventing bone loss should last at least 2-3 times this period to ensure that the registered effect on bone density is evaluated in a period of balance. According to Frost, the strength of the bone is determined by 4 factors: the mechanical property characteristic of bony tissue, the amount of micro damage from fatigue, bone mass factors (amount and type of bone in the bone) and the size and shaper of the bone (architectural factors). Moreover, it is very important the role of muscular strength on the bone: the muscles work like a lever in such a manner that to move every kilo of body weight, the muscular force is usually over 2 kg. This explains why strong muscles are usually associated with strong bones. About the specific role of the strain on the bone, from studies of the past 10 years there seems to be more precise and useful information for our queries: new formation of bone took place in rabbits only with dynamic stimuli and not static, and it is very important also the frequency of the stimulus and the speed. Moreover, some authors have shown that the stimuli of ostegoenesis depends on the fluid shear stresses though the lacunar-canalicular network system. So: exercises at high impact which can produce significant deformation of the bone matrix, better carry the fluid through the canalicular network and furthermore the strains applied at high frequency stimulate in a effective manner osteogenesis. Therefore, the mechanical strain necessary to begin osteogenesis decreases with the increase of the frequency of the strain. Rubin and Lanyon have shown that the prolongation of the stimulation with strain the osteogenic response did non increase if the regimen is prolonged more. In fact, Turner demonstrated that the bone presents a phenomenon of desensitisation following a prolonged strain stimulus. He proposed the osteogenic index of exercises like the osteogenic response to exercise which could be increased in a regimen of exercises which foresee also a period of rest between brief sessions of significant strain. So, concerning the effects of exercises, the stimulus produced by the strain must be such that it exceeds a threshold of a minimum effective stimulus, must be applied in a intermittent and dynamic manner, should produce a stimulus which is distributed differently to the norm, should be applied with high speed and few repetitions.
Bonaiuti
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Unit of Phyical Medicine and Rehabilitation, S. Gerardo General Hospital, Monza, Milan, Italy.