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There is no known cure for spinal cord injury (SCI) nor for the muscle, bone, or metabolic deficits that impair physical function, increase fracture risk, and worsen health after SCI. These deficits are preceded by the neural insult and impaired motor function and may be impacted by low testosterone, present in many men with SCI. Testosterone therapy (TRT) is known to improve musculoskeletal health, body composition, and physical funciton in older men with low testosterone who do not have SCI and some of the effects of TRT are known to be mediated by the 5-alpha reductase type II (5AR2) enzyme, which converts testosterone to dihydrotestosterone (DHT; a more potent endogenous metabolite). However, it remains unknown whether TRT improves muscle mass, muscle function, bone mineral density, and body composition / metabolic health in men with low testosterone and ambulatory dysfunction after chronic incomplete SCI, and whether actions of 5AR2 mediates any of the effects of testosterone in the impaired lower limbs and in other tissues with reduced neural input after SCI. For this study, men with low testosterone and ambulatory dysfunction after chronic motor incomplete SCI will be randomized to receive TRT with or without the 5AR2-inhibitor finasteride or a placebo treatment for 9 months. TRT or placebo injection will be administered every other week; finasteride or placebo will be administered daily. Participants will be assessed at study entry and regularly thereafter. Assessments will include measurements of body composition and bone mineral density by dual energy x-ray absorptiometry (DXA) scans, bone microstructure and strength by high-resolution peripheral quantitative computerized tomography (HR-pQCT) scans, and tests of muscle strength and physical function. Participants will also undergo safety tests, including electrocardiogram (ECG) for cardiac electrophysiology, questionnaires of health status, and blood tests to assess prostate specific antigen (PSA), hematocrit, liver enzymes (AST and ALT), blood lipids, sex hormones, and other markers of health.