Passive Cycle Training Promotes Bone Recovery after Spinal Cord Injury without Altering Resting-State Bone Perfusion

Abstract

Introduction Spinal cord injury (SCI) produces diminished bone perfusion and bone loss in the paralyzed limbs. Activity-based physical therapy (ABPT) modalities that mobilize and/or reload the paralyzed limbs (e.g., bodyweight-supported treadmill training (BWSTT) and passive-isokinetic bicycle training) transiently promote lower-extremity blood flow (BF). However, it remains unknown whether ABPT alter resting-state bone BF or improve skeletal integrity after SCI. Methods Four-month-old male Sprague-Dawley rats received T9 laminectomy alone (SHAM; n = 13) or T9 laminectomy with severe contusion SCI (n = 48). On postsurgery day 7, SCI rats were stratified to undergo 3 wk of no ABPT, quadrupedal (q)BWSTT, or passive-isokinetic hindlimb bicycle training. Both ABPT regimens involved two 20-min bouts per day, performed 5 d·wk-1. We assessed locomotor recovery, bone turnover with serum assays and histomorphometry, distal femur bone microstructure using in vivo microcomputed tomography, and femur and tibia resting-state bone BF after in vivo microsphere infusion. Results All SCI animals displayed immediate hindlimb paralysis. SCI without ABPT exhibited uncoupled bone turnover and progressive cancellous and cortical bone loss. qBWSTT did not prevent these deficits. In comparison, hindlimb bicycle training suppressed surface-level bone resorption indices without suppressing bone formation indices and produced robust cancellous and cortical bone recovery at the distal femur. No bone BF deficits existed 4 wk after SCI, and neither qBWSTT nor bicycle altered resting-state bone perfusion or locomotor recovery. However, proximal tibia BF correlated with several histomorphometry-derived bone formation and resorption indices at this skeletal site across SCI groups. Conclusions These data indicate that passive-isokinetic bicycle training reversed cancellous and cortical bone loss after severe SCI through antiresorptive and/or bone anabolic actions, independent of locomotor recovery or changes in resting-state bone perfusion.