How to Use Gravity to Accelerate Bone Adaptation: A Computational/Experimental Investigation of Exercises for Bone

Impact exercises are known to increase bone mineral density (BMD) and in turn, bone strength and resistance to fracture. The biochemical pathways driving changes in BMD take months to complete, complicating our ability to understand how specific exercises influence the remodeling stimulus received by the bone. The purpose of this study was to compare several measures that have been theoretically linked to bone remodeling stimulus, including accelerations measured by Inertial Measurement Units (IMUs) at the middle of the tibia, ground reaction forces measured by force plate, joint contact forces estimated by musculoskeletal modeling, and tibia strains estimated by finite element modeling informed by high-resolution CT imaging. Twenty healthy adults (10 male: 22.1 +/- 2.2 years; 10 female: 21.3 +/- 1.3 years) participated in a biomechanical investigation of how drop height and landing style (bilateral vs. unilateral) affect the various bone remodeling stimuli. The results showed that while drop height consistently had significant direct relationships with stimulus magnitude, there was little benefit to drop heights greater than 0.4 m. In contrast, switching from a bilateral to a unilateral landing had a large positive effect. The stimuli calculated based on IMU data showed opposite trends compared to force plate and musculoskeletal modeling-based calculations, highlighting the need for caution in how IMUs are placed, and the resulting data interpreted, in the context of bone loading. A post-hoc analysis showed that a linear regression with predictor variables of kinematics, jump height, landing type (unilateral vs. bilateral) and the Ground Reaction Force FFT Integral could explain 79% of the variance in the bone remodeling stimulus that was predicted using much more sophisticated (and labor intensive) modeling. We conclude that higher level biomechanical modeling may not be necessary to understand the magnitude of a bone remodeling stimulus of an exercise.