This investigation is based on a within subject design. The investigators aim to recruit 16 individuals with diagnosed incomplete spinal cord injuries (level C3-L1, AISA C and D) to partake in two 8-week lower body exercise programs consisting of either traditional knee extension and flexion exercise or knee extension and flexion with blood flow restriction (Figure 4). Prior to the start of each program a series of muscle and vascular function measurements will be obtained (for details on these measurements see specific aim 1 and 2 below). These measurements will be repeated at the end of each 8-week training period. In addition, to monitor risk for thrombosis, blood samples will be collected and analyzed at 4 time points throughout each 8 week period: baseline, after a single bout of exercise, after 4 weeks of training and again at the end of the 8-week training period (for details on these assays see specific aim 3 below).

Subjects Subjects will be recruited through the SCI/D department at the Cleveland VA Medical Center. In brief, the subjects must be between 18 and 55 years of age, at least 1-year post injury and free of cardiovascular or hematological diseases. Subjects must also have enough strength in their lower limbs to walk unassisted for 10 meters (see the complete list of subject inclusion/exclusion criteria in human subjects document). The primary variables for this investigation are changes in muscle strength, cross sectional area and vascular function. Due to the lack of data on BFR exercise in the SCI population, estimates of required sample size for this study were based on previously published BFR data in other clinical populations with musculoskeletal weakness and atrophy. In general, effect size of BFR exercise training on muscle strength and cross sectional from several investigations were 1.13, 0.995 and 1.9. Thus, a priori power analysis indicated that a sample size of 16 participants are needed to provide sufficient statistical to detect a change between pre- and post-training using an alpha of 0.05 and power of 0.80.

Training program: The training program will consist of 8 weeks of either traditional or BFR knee extension/flexion exercise on a Biodex System 3 dynamometer. Subjects will be required to come to the laboratory 2-3 days per week for the duration of the training program with the ultimate' goal of 20 training sessions distributed across each 8-week period. Two recent published review papers indicate that BFR training 2-3 times per week is sufficient for muscular benefits in the clinical populations. BFR training throughout the 8 weeks will consist of the standard BFR protocol (30reps-15reps-15reps-15reps) of knee extension/flexion with 60 seconds of recovery between each set. The dynamometer resistance will be set to 30% of maximal voluntary contraction (determined on initial visit) and a metronome will be used to pace the knee extension/flexion at a rate associated with 180 degrees per second (middle of the knee joint velocity range associated with comfortable walking). Performing resistance knee extension/flexion on the isokinetic dynamometer will result in concentric contractions only (there will be no eccentric phase). Concentric contractions combined with BFR have been reported to increase muscle cross sectional area and strength to a greater extent than eccentric exercise with BFR. Blood flow restriction will be applied through the use of a 6 cm cuff inflated to 70% of the pressure required to occlude limb blood flow at rest (see determining arterial occlusion pressure below). The cuff pressure will be inflated prior to the initial set and remain inflated throughout the entire protocol (exercise and recovery periods). This prescribed training plan for the BFR exercise (ie. frequency of training sessions, number of repetitions within each set, duration of rest between sets, intensity and cuff pressure) is based on recommendations from two recently published systematic reviews of BFR training. The traditional resistance training program will be a standardized exercise protocol used in rehabilitation: 3 sets of knee extension/flexion exercise with the resistance set at 60% MVC. To approximate the overall volume (number of total repetitions x weight) of the BFR protocol, the three sets will consist of 13, 12 and 12 repetitions, respectively. During both 8-week periods, peak torque will be re-evaluated every two weeks and subsequent training loads will be modified based on improvements in peak torque.

Determining arterial occlusion pressure: the pressure to occlude femoral arterial blood flow will be determined on an initial visit. Subjects will sit quietly at rest. A cuff will be placed around their upper thigh and a Doppler/ultrasound machine will be used to obtain an image and blood velocity spectrum of the superficial femoral artery (distal to the cuff). The cuff will then be slowly inflated, approximately 2mmHg every second until the velocity spectrum disappear, indicating the absence of blood flow. The cuff pressure at which blood flow stops will be considered the arterial occlusion pressure.

Potential problems and alternative strategies with training protocol: Due to the wide range of functional capacity in the iSCI population, some subjects may not be able to initially complete the exercise protocol as outlined above. To increase the likelihood that subjects can obtain the prescribed number of total repetitions, especially in the first couple weeks of training, the repetitions within each set and number of sets may be modified (ex 3 sets of 13, 12, 12 repetitions may be replaced by 4 sets of 10, 9, 9, 9 repetitions as this would maintain the overall number of repetitions but add an additional recovery period). Furthermore, training sessions in the initial 8-week period will be well documented such that overall volume can be replicated in the second 8-week training session. Although there is limited experimental evidence suggesting that BFR training carries a higher risk compared to typical high intensity exercise, Spranger and colleagues have previously outlined potential risks associated with BFR exercise in patients with hypertension, heart failure and peripheral artery disease. Specifically, BFR exercise might lead to dangerously high blood pressure responses in those with exaggerated exercise pressor reflexes. However, the SCI population, if anything, would have a reduced exercise pressor reflex due to interrupted afferent feedback from the lower limb. Finally, two 8-week training sessions is a major time commitment for subjects. Thus, the investigators will provide financial reimbursement for participation and attempt to recruit patients who are already visiting the hospital on a regular basis for rehabilitation. If it is still difficult to find subjects who are willing to participate in such a lengthy study the investigators will modify the protocol such that there will be two independent groups of 16 subjects assigned to one of the two interventions (rather than repeated measures) which would require recruitment of more subjects but each subject to participate in only one 8-week training intervention.