The isometric exercise induced tibial bone strength adaptations due to various prescribed net resultant forces generated by a leg arranged with specified hip, knee, and ankle joint angles were examined using a computationally based bone shape adaptation model. With the leg straight, or the knee flexed at 90°, and the hip either neutral or flexed by 45°, a force was repeatedly generated at the toe either anteriorly, posteriorly, superiorly, or inferiorly, in paired sequences of these directions, or in a clockwise pattern of all four directions. Model-predicted individual muscle forces, amounts of cortical bone surface accretion or resorption, and the resulting changes in bone stress distributions were compared. Similarities between adaptations to specific combinations of joint angles and net resultant force directions were found to follow similarities in the bone stress distributions rather than in muscle activity. The ratio of standard deviation to the surface-averaged measure of local stress was a good predictor of potential improvement in overall stress state uniformity. With flexed knee and neutral hip, greater adaptive improvements in tibial surface stress uniformity were found than with both joints flexed. Flexed configurations caused greater changes and overall strength improvements than exercises performed with a straight leg. Regions of local tibial cortical thickening were identified for specific joint angles and single-load resultant force directions. Sequential series of loads were also identified to enhance these effects and mitigate concurrent thinning in other regions. The presented information can aid in the design and analysis of targeted muscle and bone strengthening exercises.
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