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The skeleton's architecture is matched to the changing loads to which it is subjected because mechanical loading directly or indirectly influences the activity of cell populations to deposit, maintain, or remove bone tissue as appropriate. This responsiveness to load bearing presupposes that certain cells are sensitive to load itself or to its consequences within the tissue. The nature of this effect and the cells concerned have not yet been determined. In this series of experiments, bones were exposed in vivo to a single short period of dynamic loading, which if repeated daily had been shown to result in increased new bone formation. There was an increase in the activity of glucose 6-phosphate dehydrogenase (G6PD) in the periosteal cells adjacent to the bone surface 6 min after this single period of loading. This increase was proportional to the strain magnitude in the bone tissue in the immediate vicinity of the cells. In osteocytes, although the G6PD activity in each individual cell was unchanged by loading, the number of cells displaying activity was increased. This increase was also directly proportional to the applied strain in that area of the cortex (52% compared with 26% active osteocytes at a strain of 0.002). Activation of G6PD was unaccompanied by any equivalent changes in the activities of either glyceraldehyde 3-phosphate dehydrogenase (GA3PD) or lactate dehydrogenase (LDH). This finding is consistent with loading increasing the activity of the oxidative part of the pentose monophosphate shunt pathway. It is also consistent with stimulation of a synthetic process, such as the production of RNA from ribose 5-phosphate.(ABSTRACT TRUNCATED AT 250 WORDS)

The velocity of ultrasound was measured transcutaneously across the patella and tibia in 98 volunteers both before and after running the 26 mile Boston Marathon. Absolute sound velocities were 2.9% higher in those runners finishing before 3 h when compared to runners finishing after 3 h. Tibial velocities in males were 8.8% higher than in female runners. The mean velocity across the patella of three wheelchair racers was 28% lower than the mean combined patella velocity measured in all runners. These data suggest that 'faster' velocities are associated with bone that is better suited for high functional demands. Surprisingly, when pre- and post-race velocities were compared in each runner, there was a 1.6% increase in ultrasonic velocity across the tibia, and a 3.5% increase across the patella. An increase in ultrasonic velocity following extreme physical activity suggests that adaptive mechanisms exist in healthy bone to withstand, or possibly avoid, the microdamage which might be caused by repetitive cyclic loading.