Quadricep strength and speed of force affects knee osteoarthritis

The lower limb muscles, especially the quadriceps, play an important role in the development of knee osteoarthritis (OA). Muscle weakness has already been identified as a potential risk factor for developing knee OA (due to increased knee loading), and researchers recently set out to see if SFP -- how fast the quadricep muscle is able to generate force, pushing the leg out -- would have an effect on physical function in people with, or at risk for, knee OA.

"As far as we know, there have not been studies on people with or at risk for knee OA that comprehensively investigated quadriceps SFP as a risk factor for worsening physical function -- such as walking, rising from a chair or climbing stairs," explains Neil Segal, MD, MS; director of clinical research and medical director of Musculoskeletal Rehabilitation in the Department of Rehabilitation Medicine at University of Kansas Medical Center. "Better understanding the relationship of speed of muscle force development and worsening physical function could inform design of rehabilitation interventions to protect people from suffering loss of physical function."

Through the Osteoarthritis Initiative -- a study of nearly 5,000 adults with or at risk for knee OA -- Dr. Segal's team followed 3,996 participants for 12 months, 3,820 for 24 months and 3,623 for 36 months (those with knee injuries or who developed rheumatoid arthritis, a type of inflammatory arthritis, did not continue through all 36 months of the study).

Participants' SFP was measured using a special chair with a cable that measures muscle strength when pushing the leg out. Physical functioning was measured using timed 20- and 400- meter walks as well as chair stand tests, which measure the speed at which participants stand from a seated position in a chair. Finally, the researchers noted participants' self-assessments of their functioning (e.g., walking, getting dressed, bathing, etc.) using the WOMAC-PF scale, which is the most common survey of self-reported physical function in people with knee OA.

Dr. Segal's team looked for any worsening in the areas of physical function, including: an absolute decrease in 20-meter walk speed of .14 meters per second; an increased time in the 400-meter walk of at least 20 seconds; an increased time to stand from a chair of 4.18 seconds; and/or a reported nine-point or greater worsening on the WOMAC-PF scale.

SFP was categorized as low, medium and high. Participants with medium SFP were found to be at lower risk for worsening self-reported function at 24 months when compared to those with low SFP. Interestingly, participants with high SFP didn't experience the same decreased risk as those with medium SFP. Additionally, at 36 months, those with medium and high SFP had a lower risk of worsening self-reported function than those with lower SFP.

Next, Dr. Segal's team looked specifically at women, and the results were similar. In comparison to those with lower SFP, those with the highest SFP had lower risk for worsening 20-meter walk, and those with medium SFP showed lower risk for worsening self-reported physical function at 24 and 36 months.

"The results of this study indicated that interventions to maintain physical function in people with knee OA should focus both on quadriceps strength and the SFP of the muscle," says Dr. Segal of the findings. "Compared to people who can push faster, we found people with slower muscle responses are more likely to suffer from worsening physical function in the future. We know that maintaining quadriceps strength is important for protection against painful knee OA. Now, we know the ability to move the muscle quickly is important for keeping people able to walk, stand from a chair and do other functional activities."

As a next step, Dr. Segal notes the importance of understanding why women seem to show more positive effects of SFP over men. He also suggests interventional studies should be completed to confirm the results of this observational study.