A recent paper reviewing the muscle impairments associated with knee arthritis was published in Sports Health that I thought did a great job overviewing the current evidence on the subject. Alnahdi, Zeni, and Snyder-Mackler discussed several factors associated with muscle impairments and knee arthritis.
I thought I would take this article and combine it with some of my thoughts and recommendations from the “The MOVE Consensus” published several years ago in Rheumatology.
There are a lot of muscle impairments associated with arthritis, proper knowledge of these impairments should allow us to develop more appropriate rehabilitation and fitness programs for individuals with knee arthritis.
Quadriceps Strength
Quadriceps weakness and muscle impairment is well documented in the literature. Previously published papers report strength deficits of the quad ranging from 11-56% when compared to healthy controls. Even more disparity exists when assessing eccentric strength, with deficits up to 76% in some reported publications.
This loss of strength, especially eccentrically, can have several implications on functional deficits. Think about how many daily tasks involve concentric and eccentric control of the quad – standing from a chair, getting up off the ground, ascending and descending stairs – all of these activities (and more) become limited and contribute to overall dissatisfaction with arthritis patients.
Quadriceps Atrophy and Inhibition
There are two main factors associated with loss of quad strength in patients with knee arthritis – atrophy and muscle inhibition. The quad has been shown to exhibit a 12% reduction in cross sectional area, representing atrophy, in patients with knee arthritis. This atrophy obviously contributes to loss of strength, however inhibition of volitional control of the quadriceps has also been found.
Again, the exact mechanism is still unknown but some potential reasons that the altered ability to contract muscle probably relates to alterations in the afferent discharge of knee receptors. This could be altered due to degenerative changes in joint structures, effusion, pain, inflammation, and laxity.
Other Lower Extremity Strength Deficits
The loss of quadriceps strength has been one of the most commonly cited impairments associated with knee arthritis. Much emphasis has been placed on the quad, however impairment of other muscles have also been identified. Several papers have been published that demonstrate that patients with knee arthritis also have a:
- 4-38% reduction in hamstring strength
- 16% reduction in hip extension strength
- 26-40% reduction in hip flexion strength
- 27-40% reduction in external rotation strength
- 20-43% reduction in internal rotation strength
- 22-24% reduction in abduction strength
- 26% reduction in adduction strength
This are pretty big strength deficits that seem to occur in every plane of motion. I would imagine this again represents a general level of deconditioning associated with the development of knee arthritis. Muscular weakness and imbalances can have a significant impact on the ability to develop and withstand forces without compensatory movement patterns that increase force applied to the static joint structures.
Bilateral Deficits
Interestingly, strength deficits of the quad are not isolated to the involved leg. The contralateral leg has also been shown to exhibit a 16-26% deficit in quad strength compared to healthy controls. This deficit isn’t as severe as the involved side but shows that both extremities should be examined carefully. Volitional control has also been shown to be reduced bilaterally, with greater inhibition on the involved knee.
The reason behind this contralateral deficit is not completely known, however it could again represent general weakness and deconditioning of the patient.
The Chicken or the Egg?
If strength and volitional control is so poor in several muscle groups bilaterally in patients with knee arthritis, the classic “which came first, the chicken or the egg” question comes to mind. Does knee arthritis have such a dramatic impact on muscle impairments of the body or did these impairments precede, and potentially facilitate, the develop of knee arthritis?
There have some studies published that prospectively showed that weaker quadriceps strength was correlated to the development of knee arthritis. This makes sense to me, as it certainly appears that several of the above factors could be related to general deconditioning of the patient.
Perhaps there is a reason that we see bilateral deficits with the involved knee showing greater impairments. Maybe knee arthritis begins with a certain level of weakness, imbalances, and overall deconditioning. Then overtime, this deconditioning is superimposed with inhibition from the natural consequences of knee arthritis, such as effusion, pain, and inflammation.
Clinical Implications
After reviewing this well written article, I think we can summarize the following:
- Quadriceps strength is significantly impaired in subjects with knee arthritis
- Both activation deficit and atrophy contribute to this weakness
- Impairments also occur with the hamstrings and hip muscles
- Strength and activation impairments are seen bilaterally, though the involved side shows greater impairments
- Strength is a major determining factor for functional activities
- Strength is predictive of the development of knee arthritis
The authors also included a summary of the recommendations from Roddy et al and The MOVE Consensus, which I would summarize as:
- Both strengthening and aerobic exercise can reduce pain and improve function in patients with knee and hip osteoarthritis, with few contraindications, and are essential in the management of osteoarthritis.
- Improvements in strength and proprioception gained from exercise may reduce the progression of osteoarthritis, although adherence is the principle predictor of long-term outcome from exercise.
In addition to these recommendations, I would suggest that we also include the following principles for the development of rehbailitation and fitness programs for people with arthritis:
- Exercise and strengthening of the entire lower extremity, with emphasis on quadriceps strength and muscle imbalances, are an essential part of exercise programs for those with arthritis
- Any deficits and imbalances of the hip should also be addressed
- Exercise programs should be performed bilaterally, with emphasis on areas of greatest muscle impairment
- Any exercise program should focus on strengthening, dynamic stabilization, and neuromuscular control of the lower extremities
- Programs to should be developed to also enhance mobility in people with arthritis
- Any deficits in muscle impairments should be correlated to altered movement dysfunctions
- Programs should be developed that reduce specific muscle impairment, mobility concerns, and movement impairments
Hopefully we can all make a positive impact on people suffering from knee arthritis. Understanding and improving some of the muscle impairments, strength deficits, and muscle imbalances associated with knee arthritis is imperative. Keep these findings and recommendations in mind next time you are working with someone with knee arthritis, don’t just focus on pain control and quad strength, look at the bigger picture!
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