Spinal cord injury (SCI) is a devastating, life-changing event, that has been characterized as a form of accelerated aging, resulting in multisystem impairment. The profound declines in function result in numerous secondary conditions, increasing morbidity and mortality in the SCI population and affecting overall well-being after injury. Advances in acute care have greatly reduced co-morbidities, but there has been little progress in addressing chronic consequences post SCI. Disuse osteoporosis is among the long-term consequences of SCI, resulting in low energy fractures, which are prevalent and extremely debilitating in this population. Even though the pathogenesis of disuse osteoporosis is considered multifactorial, the primary contributor is reduced mechanical loading. After an SCI, bone loss is rapid in onset and severe in nature, bone density decreasing to about 60% bone mass within the first one to three years post injury, this bone loss continuing up to 7 years.

The central idea behind the work presented in this thesis is the mechanoadapatation of bone to external loading:​ bone continually remodels in response to the stresses and strains applied. High loads promote bone formation, while unloading results in bone resorption. During my doctoral training, I focused on bone loss in SCI and I investigated the potential of a rehabilitation exercise, namely functional electrical stimulation rowing (FES-rowing) to address disuse osteoporosis. FESrowing is a whole body exercise, that allows for the simultaneous engagement of both the innervated arms and the non-innervated legs in those with SCI.

The biomechanical analysis of FES-rowing characterized this exercise intervention within a group of individuals with chronic SCI, allowing for broader comparisons and conclusions. While FES-rowing can mirror able-bodied rowing, there are considerable differences in rowing technique and efficiency. More importantly, the results concluded that unassisted FES-rowing provides only modest loading of the legs.

The kinetics and kinematics of FES-rowing were used to develop a musculoskeletal model to non-invasively estimate lower body loading. The computational model accounts for the external forces, the rowing kinematics, as well as the force produced by the induced muscle contractions. The model estimates knee joint forces up to four times the external leg forces in FES-rowing, and up to five times the external leg forces in able-bodied rowing.

FES-rowing results in a large range of lower body forces, according to the presence of muscle tone. Given the different loading regimes, bone microarchitecture in the appendicular skeleton (ultradistal radius and ultradistal tibia) was obtained using high resolution peripheral computed tomography. The results concluded that magnitude of loading is more important than frequency of exercise in preventing bone loss, high loads being protective of trabecular microstructure degradation.

Additionally, this thesis presents a custom made device that investigates another contributor to skeletal health and disuse osteoporosis, bone blood perfusion. The custom built near infrared spectroscopy device was effectively used to non-invasaviely monitor hemoglobin concentration changes in the tibia during exercise in both able-bodied and individuals with SCI.

The work presented in this thesis suggests that FES-rowing might be insufficient to promote bone formation, but slows down bone resorption and trabecular microstructure degradation. The results indicate that the magnitude of loading is more important than the frequency of exercise in preventing bone loss and possibly addressing disuse osteoporosis in those with SCI.