Is gravitational loading alone sufficient to maintain bone strength?

Bone mineral content decreases as a consequence of microgravity and immobilization. Harold Frost introduced the mechanostat theory, following which the bone adapts to the strain it is subjected to. A change in forces applied to the bone would then constitute the stimulus that leads to either bone resorption or bone formation.

Therefore, models like bed rest, cast immobilization, and lower limb suspension have been applied to study bone responses to unloading in humans. However, these models cannot distinguish between the relative roles of muscle contraction and gravitational loading for maintaining healthy bones. An alternative model is proposed to study the muscle-bone relationship.

HEPHAISTOS is a newly developed orthotic device that has been developed by the German Aerospace Center in cooperation with the German Sports University, in Cologne, Germany. Its intent is to maintain the gravity-derived-bone-forces while greatly reducing lower leg musculature-derived-bone-forces in the tibia. Additionally, the model should minimally impair normal life and especially the gait pattern. Kinematics (VICON), electromyography (NORAXON) as well as plantar pressure distribution (NOVEL) were used to evaluate the acute effects of HEPHAISTOS on locomotion parameters during leveled gait trials.

The long term adaptations of the musculoskeletal system to HEPHAISTOS were assessed with eleven male subjects, who wore HEPHAISTOS unilaterally for eight weeks within the frame of an ambulant study. Functional and anatomical data of the calf musculature were obtained periodically during the intervention as well as during a one year recovery period. Muscle strength was assessed by means of dynamometry (BIODEX) and muscle volume was estimated based on magnetic resonance imaging (PHILLIPS). Bone density was assessed via peripheral Quantitative Computed Tomography (XCT 3000, Stratec and Xtreme CT, Scanco).

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last updated 31.03.2023