Proximal tibia volumetric bone mineral density is correlated to the magnitude of local acceleration in male long-distance runners

Abstract

The beneficial effect of physical exercise on bone mineral density (BMD) is at least partly explained by the forces exerted directly on the bones. Male runners present generally higher BMD than sedentary individuals. We postulated that the proximal tibia BMD is related to the running distance, as well as to the magnitude of the shocks (while running) in male runners. A prospective study (three yearly measurements) included 81 healthy male subjects: 16 sedentary lean subjects, and 3 groups of runners (5-30 km/wk, n = 19; 30-50 km/wk, n = 29; 50-100 km/wk, n = 17). Several measurements were performed at the proximal tibia level: volumetric BMD (vBMD) and cortical index (CI), i.e., an index of cortical bone thickness and peak accelerations (an index of shocks during heel strike) while running (measured by a three-dimensional accelerometer). A general linear model assessed the prediction of vBMD or CI by 1) simple effects (running distance, peak accelerations, time); and 2) interactions (for instance, if vBMD prediction by peak acceleration depends on running distance). CI and vBMD 1) increase with running distance to reach a plateau over 30 km/wk; and 2) are positively associated with peak accelerations over 30 km/wk. Running may be associated with high peak accelerations to have beneficial effects on BMD. More important strains are needed to be associated with the same increase in BMD during running sessions of short duration than those of long duration. CI and vBMD are associated with the magnitude of the shocks during heel strike in runners.

Fig. 1.

Summary of the measurements performed at the proximal tibia level. A total of 19 tomograms were taken from the tibia plateau every millimeter (the first tomogram at the reference line), and 13 tomograms every 5 mm (the last tomogram at distance of 7 cm from the reference line). The densities of the metaphysis, the diametapysis, and the diaphysis were calculated from tomograms 7–8, 9–16, and 17–19, respectively. Moreover, an accelerometer was fixed on the skin at the same site to measure peak acceleration (parallel to the tibia axis) when the heel hits the ground during a running session.

Fig. 2.

An original algorithm was developed to identify the exact time of the shock exerted on the tibia, as well as its intensity. The circles indicated the moments when the ipsilateral heel hits the ground. For each subject, the mean of all peak accelerations recorded throughout the treadmill test was calculated and used for further analyses.

Fig. 3.

Interactions between peak acceleration and running distance group. Vertical axis represents the change in volumetric bone mineral density or cortical index for one unit increase in peak acceleration for each running category. One-half of the coefficient 95% confidence intervals is represented.