Muscle, tendon, and skeletal adaptations to spinal cord injury in humans

Musculoskeletal deterioration after spinal cord injury (SCI) limits the functional usefulness of muscle, reduces the strength of bones, and precipitates a range of secondary medical complications. A key contributing factor is the loss of physiologic loads through the paralyzed extremities. Electrical muscle stimulation is a promising strategy for restoring these loads. The development of therapeutic loading protocols requires a thorough understanding of musculoskeletal adaptations after SCI. The purpose of this study was to quantify adaptations of muscle, bone, and tendon to SCI. Each tissue was evaluated using a novel measurement technique not previously reported in the literature. Establishing the reliability of these measurements was a key focus of this study: (1) Doublet stimulation was a sensitive and reliable method to measure quadriceps adaptations to SCI without generating high forces (which carry a risk for fracture). Doublet stimulation revealed differences in quadriceps force, contractile speed, and potentiation between subjects with and without SCI. (2) Femur metaphysis bone mineral density (BMD) declined precipitously after SCI, eventually reaching 39.5% of non-SCI BMD. Potential BMD variation due to slice misplacement was small (-2.5%), but the effect of operator-selected voxel allocation strategies was considerable (up to 17%). (3) We developed a musculoskeletal ultrasound imaging protocol with low within-operator variability and low variation due to tracing technique. In subjects who perform unilateral electrical muscle stimulation, no between-limb differences in tendon cross-sectional area emerged. Muscle adaptations were less interpretable due to fibrotic changes of paralyzed muscle. Further work is needed to determine optimal muscle scan site locations. The ultrasound protocol used in this study may be useful for studies of larger SCI cohorts to determine the nature of post-SCI tendon adaptations.;The normative values in this report may be useful for future studies of post-SCI adaptation. Future studies may also use the measurement techniques developed in this report to judge the suitability of post-SCI muscle, tendon, and bone for high-load rehabilitation interventions. Likewise, the techniques presented here will serve as useful aids in quantifying adaptations that result from the reintroduction of physiologic loads during electrical muscle stimulation protocols.