The overall objective of this project is to compare two self-managed treatment regimens for PFPS: (1) NMES-Exercise supplemented with high BFR LOP of 80% and (2) NMES Exercise augmented with a low percentage of BFR LOP at 20 mmHG (BFR-sham). Each of the two treatment arms will perform a fixed exercise protocol singularly and in combination with BFR-NMES, and NMES alone. The specific aims of the study are 1) To determine whether self-managed NMES-Exercise program supplemented with a high BFR of 80% LOP (limb occlusion pressure) is significantly more efficacious than NMES-Exercise with low BFR LOP set at 20 mmHG (sham) in improving muscle strength of the lower extremity, daily physical activity and mobility; and 2) To determine whether NMES-Exercise with high LOP BFR therapy improves QOL and PFPS symptoms significantly more than NMES-Exercise with low LOP BFR.

The long-term goal is to identify effective self-managed therapies for military members that will return them to duty as quickly as possible. The overall objective of this project is to compare two self-managed treatment regimens for PFPS: (1) NMES-Exercise supplemented with high BFR LOP of 80% and (2) NMESExercise augmented with a low percentage of BFR LOP at 20 mmHG (BFR-sham). Each of the two treatment arms will perform a fixed exercise protocol singularly and in combination with BFR-NMES, and NMES alone.

The rationale for this study is that by limiting blood flow to an exercising limb using both active and passive exercise is expected to significantly improve lower extremity muscle strength, mobility, quality of life (QOL) and pain. The central hypothesis is that the combination of NMES/exercise of the injured leg supplemented with BFR set at 80% LOP will show significantly greater improvements in muscle strength, mobility, pain, daily activity and QOL than the NMES-Exercise with BFR set at a very low percentage of LOP at 20 mmHG. To test this hypothesis, the investigators will address the following specific aims:

a1. To determine whether self-managed NMES-Exercise program supplemented with a high BFR of 80% LOP (limb occlusion pressure) is significantly more efficacious than NMES-Exercise with low BFR LOP set at 20 mmHG (sham) in improving muscle strength of the lower extremity, daily physical activity and mobility. Muscle strength will be measured by isometric peak torque of the left and right knee and hip posterolateral stabilizers; physical activity will be measured by steps walked and energy (kcal) expenditure/day; and mobility by time to complete a stair climb, step down test, distance walked in 6 minutes, and number of times to complete the chair rise.

a2. To determine whether NMES-Exercise with high LOP BFR therapy improves QOL and PFPS symptoms significantly more than NMES-Exercise with low LOP BFR. QOL will be measured by the SF-12 Health Survey and the Center for Epidemiological Studies-Depression (CES-D) scale. Symptoms of PFPS will be quantified by the Anterior Knee Pain Scale and IKDC subjective knee form.

The investigators expect to demonstrate that these combined multimodality therapies of a self-managed NMES-Exercise program supplemented with high (80%) LOP BFR-NMES-Exercise is more efficacious than a low percentage of LOP BFR-NMES-Exercise (sham). Positive results will translate into therapy options for PFPS that can be used by health care providers to accelerate rehabilitation, speed return to world-wide duty status, decrease symptoms, and lower medical costs. Moreover, these approaches are safe, easy to use and non-pharmacological.

Inclusion Criteria:

- Active duty military personnel who are:

1. diagnosed with knee pain, categorized as anterior or retropatellar in one or both knees;
2. self-reported difficulty performing at least two or more of the following activities associated with knee pain: prolonged sitting, stair climbing, running, jumping, and repetitive movements such as kneeling or squatting or stooping;
3. military service member at the time of diagnosis and length of study;
4. age ≥18 and <45 years; and
5. able to provide freely given informed consent.

Exclusion Criteria:

1. fracture or injury to external knee structures such that knee extension or flexion is impaired;
2. a significant co-morbid medical condition (such as severe hypertension, neurological disorder, sickle cell anemia, or pacemaker/defibrillator) in which BFR, NMES strength training, or unsupervised exercise is contraindicated and would pose a safety threat or impair ability to participate;
3. previous knee surgeries (i.e., tibiofemoral, patellofemoral) excluding knee arthroscopy > 3 years;
4. knee instability or recurrent patella dislocation or subluxation;
5. inability or unwillingness to participate in an exercise or strengthening program;
6. reduced sensory perception in the lower extremities;
7. pregnancy;
8. Any vascular disorder (varicose veins, Hx of DVT); or
9. unwillingness to accept random assignment.