The purpose of this study is to assess the efficacy of a 12-week low-load neuromuscular electrical stimulation with volitional contraction (NMES-VC) training program to improve quadriceps strength and activation, while not adversely affecting knee-related pain, activities of daily living or quality of life in women with knee pain. The primary outcome will be change in maximal isokinetic knee extensor torque.

The investigators will test the following hypotheses. In comparison with low-load (40%) resistance training without electrical stimulation, a 12-week NMES-VC training program will:

Hypothesis 1: Increase maximal isokinetic knee extensor torque

Secondary questions and response variables

Hypothesis 2: Not adversely affect knee pain or quality of life, assessed by the Knee injury and Osteoarthritis Outcome Score (KOOS) questionnaire

Additional hypotheses in women with risk factors for incident symptomatic or progressive KOA:

1. Determine the extent to which NMES-VC-enhanced low-intensity resistance training increases quadriceps muscle rate of force development
2. Determine the extent to which NMES-VC enhanced low-intensity exercise is tolerated (using numeric rating scale survey "level of pain you experienced during the hybrid training or 40% isokinetic exercise")
3. Determine the extent to which NMES-VC-enhanced low-intensity resistance training increases physical function (20m walk, chair stand)

Arthritis is the most common cause of disability in the United States. Approximately 42.1% of women and 31.2% of men over the age of 60 have knee osteoarthritis (KOA). KOA is associated with pain, quadriceps weakness, swelling, instability, decline of range of motion, physical function, and quality of life (QOL). The presence of KOA significantly decreases QOL, while quadriceps strengthening has the ability to improve QOL. However, one challenge with strengthening is that knee pain influences muscle strength and physical function. Not only quadriceps strengthening but also reduction of knee pain may be necessary to reduce functional limitations from KOA. However, at this time, there is insufficient evidence regarding effective training to not only increase quadriceps strength but also reduce knee pain and improve neural function. Thus, there is a need for interventional studies to build on observational findings to evaluate the relationships of neural function, knee muscle strength, knee pain, and physical function.

The critical barrier to studying whether quadriceps strengthening is protective against worsening physical function is the lack of an effective strengthening program for people at elevated risk for KOA. Factors that place older adults at elevated risk for symptomatic KOA, (e.g. sedentary lifestyle, obesity, knee pain, knee injury or surgery), may also contribute to reduced tolerance of high load quadriceps strengthening programs. If a well-tolerated and effective means of strengthening could be identified, it may also have additional benefits in reducing knee pain, improving physical function and avoiding disability in individuals with or at risk of KOA.

Neuromuscular electrical stimulation (NMES) is widely used to strengthen muscles and improve function in people who cannot exercise at medium-high intensity. For KOA, NMES is effective for enhancing quadriceps strength, knee pain, physical function. Therefore, NMES could contribute to decreased neural inhibition from knee pain and promote muscle function (e.g. muscle strength, power, contraction speed, and co-contraction). Recently, it was reported that the combined application of NMES and volitional contractions (NMES-VC) is effective for making up for a limitation of NMES. NMES-VC could help to improve motor recovery. The results of recent studies suggest that a hybrid training system that utilizes both volitional contractions and NMES simultaneously, might be an effective method that can improve physical function by strengthening muscles and relieving knee pain in people at risk for incident or progressive symptomatic KOA.

This line of research could have a significant positive impact on public health, by leading to the introduction of an inexpensive means of well-tolerated and safe exercise that can be completed in community rehabilitative environments. Successful completion of this initial investigation will enable pursuit of research to determine whether NMES-VC is effective in quadriceps strengthening and pain relief while improving neural function in symptomatic and progressive KOA. Reduction of this primary cause of disability through cost-effective preventive exercise has a high potential to reduce the burden of disease and disablement, thereby improving the quality of life for older adults and significantly reducing the costs to individuals and society.

The specific aim of the proposed research is to assess the efficacy of a twelve-week, efficient and tolerable, low-intensity exercise program with NMES-VC for improving quadriceps strength, knee pain and physical function in women with risk factors for incident symptomatic or progressive KOA. This specific aim will be achieved through a randomized, controlled trial, comparing low intensity exercise with and without NMES-VC. This research is novel in that it will be the first to use a low load regimen that will minimize the potential for adverse loading on the knee joint while still having a high likelihood to lead to clinically meaningful strength gains, pain relief and physical function improvement in older adults at elevated risk for symptomatic or progressive KOA.

Inclusion Criteria:

1. Female

2. Age 40-85 years

3. One or more of the following:

   1. Knee symptoms (pain, aching, or stiffness) on most of the last 30 days; categorically defined, so all severity of symptoms ok, but must have knee symptoms on most days
   2. History of knee injury or surgery
   3. Body Mass Index (BMI) greater than or equal to 25 kg/m2

4. BMI less than 45 kg/m2

Exclusion Criteria:

1. Knee injection within 6 weeks prior to the study
2. Resistance training at any time in the last 3 months prior to the study
3. Bilateral knee replacement
4. Lower limb amputation
5. Lower limb surgery in the last 6 months that affects walking ability or ability to exercise
6. Back, hip or knee problems that affect walking ability or ability to exercise
7. Unable to walk without a cane or walker
8. Inflammatory joint or muscle disease such as rheumatoid or psoriatic arthritis or polymyalgia rheumatica
9. Multiple sclerosis or other neurodegenerative disorder
10. Known neuropathy
11. Self-report of Diabetes
12. Currently being treated for cancer or having untreated cancer
13. Terminal illness (cannot be cured or adequately treated and there is a reasonable expectation of death in the near future)
14. Peripheral Vascular Disease
15. History of myocardial infarction or stroke in the last year
16. Chest pain during exercise or at rest
17. Use of supplemental oxygen
18. Inability to follow protocol (e.g. lack of ability to attend visits or understand instructions)
19. Staff concern for participant health (such as history of dizziness/faintness or current restrictions on activity)
20. Unable to attend 12 or more sessions during the study
21. Implanted cardiac pacemaker, spinal cord stimulator, baclofen or morphine pump or other implanted electrical device.
22. Dermatitis or skin sensitivity.
23. Pregnancy