Prof Jörn Rittweger
Institute of Aerospace Medicine
German Aerospace Centre
Cologne, Germany

Bone in Space: what are we going to do about it?

Jörn Rittweger^1,2

¹Institute of Aerospace Medicine, German Aerospace Center, Cologne, Germany
²Department of Pediatrics and Adolescent Medicine, University Hospital Cologne, Germany

Bone is readily lost during typical 6-month ISS missions at a rate of approximately 1% per month from the legs. Most of these losses recover within 2 years, albeit with possibly some remaining deficit within the trabecular compartment. Comparing these figures to the bone losses that women experience during menopause, and considering that bone adaptation is a slow business raises concerns that astronauts might have an elevated fracture risk. Even, literature does currently not support that notion, it would be prudent to devise effective countermeasures before endeavoring year-long deep space missions to Mars.

Bone losses from the astronauts’ upper extremities are negligible to non-existent, which suggests lack of mechanical loading as the main cause. Biochemically, that bone losses are also characterized by an ‘uncoupling’ of bone formation from bone resorption (1). According to the ‘mechanostat’ concept, bones adapt to variation their mechanical environment by adding or losing bone material in such a way that keeps the peak bone strains invariant (2).  From a biomechanical perspective, the largest bone forces, and thus bone strains, are caused by muscle contractions. This has served as rationale for a series of experimental bed rest studies to test the suitability of muscle training for prevention of bone loss.

From the bed rest studies that had been performed over the past two decades, resistive vibration exercise (3) and reactive jump training have emerged as the two most effective exercise modalities for muscle and bone, which notably both involve plyometric elements. Biochemical analyses suggest, notably, that the effectiveness is not conveyed by suppressing bone formation, but rather by establishing the coupling of bone formation to bone resorption (1). Moreover, resistive exercise and running in combination with lower body negative pressure have also demonstrated sizable effectiveness for muscle, but not for bone preservation.

For missions into deep space, therefore, should make use of novel exercisers that allow plyometric training modalities in microgravity. Where this is prohibited by mass and size budgets, traditional resistive exercise can also have merit. Given that full countermeasure effectiveness is demanding and can at this stage not be guaranteed, I recommend adjuvant pharmacological shielding with a bone anti-resorptive drug (4).

References:

• Lau, L. Vico, J. Rittweger, Dissociation of Bone Resorption and Formation in Spaceflight and Simulated Microgravity: Potential Role of Myokines and Osteokines? Biomedicines 10, (2022).
• M. Frost, Bone "mass" and the "mechanostat": a proposal. Anat Rec 219, 1-9 (1987).
• Rittweger et al., Prevention of bone loss during 56 days of strict bed rest by side-alternating resistive vibration exercise. Bone 46, 137-147 (2010).
• Thomasius, D. Pesta, J. Rittweger, Adjuvant pharmacological strategies for the musculoskeletal system during long-term space missions. Br J Clin Pharmacol, (2023).