Noah Lemmens

 Background: Spinal Cord Injury (SCI) leads to bone loss that is faster and more severe than other types of immobilization such as bed rest.1,2 In just 1-2 years after injury 50% of patients with SCI meet the criteria for osteopenia or osteoporosis.3 Unsurprisingly, fractures after SCI are common, especially in the distal femur and proximal tibia.2 Susceptibility to fracture and post fracture complications further limit mobility and impair rehabiliation.2 Passive cycle training and body weight supported treadmill training (BWSTT) are therapies for incomplete SCI to promote functional recovery.7 Unfortunately, despite providing the physical loading of the legs that is necessary for bone formation, BWSTT is only known to increase bone density at the distal tibia and not at the proximal tibia where bone loss is most severe after SCI.7

Objective: The purpose of this narrative review is to investigate if passive cycle training can be used as a therapy to prevent or reverse the severe bone loss induced by SCI.

Search Methods: An online search in the PubMed database was conducted from 2019 to 2025 using the keywords: “spinal cord injury”, “bone loss”, “passive cycle training”.

Results: Rats with SCI have lower bone volume and altered bone turnover even after regaining locomotor function and weightbearing.4 SCI also leads to alterations in bone turnover at the humerus, far above the injury site.4 These findings indicate that disuse is not solely responsible for SCI induced bone loss.4 The reduced bone blood flow (BF) which occurs after SCI may be responsible for the reduction in bone formation after SCI.5 Patients with SCI have lower femoral artery diameter and lower femoral artery maximal BF.5 In rats, SCI reduced blood vessel diameter and vascular volume at the proximal tibia metaphysis by 45-60%.5 Moreover, in rats with severe SCI, femur and tibia BF deficits coincide with increased bone deterioration and decreased bone formation.5 Rats that underwent passive cycle training after SCI had no significant increase in resting state femur or tibia perfusion, but bone BF was directionally highest in the femur and tibia compared with SCI rats not exposed to passive cycle training.6 Subsequently, neither BWSTT or passive cycle training altered the rate of bone loss in the acute phase of SCI.7 However, 2-4 weeks after SCI, twice daily passive cycle training reversed bone loss at the distal femur and proximal tibia and restored it to the levels of the controls.7 Passive cycle training increased the number of osteoblasts on bone and increased the activity of osteoblasts independently of locomotor and muscle recovery.7 Currently there are no studies that have evaluated the effects of passive cycle training on bone loss in adults with SCI.7 A study with 2 children reported that passive cycle training increased bone density in only one child.7

Conclusion: It is clear that disuse is not the sole cause of bone loss after SCI and therefore treatment strategies should also focus on the systemic changes driving bone loss. Decreased bone BF after SCI is one systemic change that may contribute to bone loss. Treatment strategies that improve bone BF may decrease bone loss. More research is needed to determine if passive cycle training has the potential to decrease bone loss after SCI in human patients.

Work Cited:

1. Metzger CE, Gong S, Aceves M, Bloomfield SA, Hook Osteocytes reflect a pro-inflammatory state following spinal cord injury in a rodent model. Bone. 2019;120:465-475. doi:10.1016/j.bone.2018.12.007
2. Leone GE, Shields DC, Haque A, Banik NL. Rehabilitation: Neurogenic Bone Loss after Spinal Cord Biomedicines. 2023;11(9):2581. Published 2023 Sep 20. doi:10.3390/biomedicines11092581
3. Edmiston T, Cabahug P, Recio A, Sadowsky Bone Health following Spinal Cord Injury: A Clinical Guide to Assessment and Management. Phys Med Rehabil Clin N Am. 2025;36(1):99-110. doi:10.1016/j.pmr.2024.07.007
4. Metzger CE, Moore RC, Pirkle AS, et al. A moderate spinal contusion injury in rats alters bone turnover both below and above the level of injury with sex-based differences apparent in long-term recovery. Bone Rep. 2024;21:101761. Published 2024 Apr 10. doi:10.1016/j.bonr.2024.101761
5. Yarrow JF, Wnek RD, Conover CF, et Bone loss after severe spinal cord injury coincides with reduced bone formation and precedes bone blood flow deficits. J Appl Physiol (1985). 2021;131(4):1288-1299. doi:10.1152/japplphysiol.00444.2021
6. Yarrow JF, Wnek RD, Conover CF, et Passive Cycle Training Promotes Bone Recovery after Spinal Cord Injury without Altering Resting-State Bone Perfusion. Med Sci Sports Exerc. 2023;55(5):813-823. doi:10.1249/MSS.0000000000003101
7. Kura JR, Cheung B, Conover CF, et al. Passive bicycle training stimulates epiphyseal bone formation and restores bone integrity independent of locomotor recovery in a rat spinal cord injury model. J Appl Physiol (1985). 2024;137(3):676-688. doi:10.1152/japplphysiol.00299.2024