Quadriceps weakness contributes to pain and physical disability6,23
in patients with knee OA and has recently been implicated as a risk factor for the onset of symptoms in persons with radiographic evidence of the disease.11
Therefore, improving quadriceps strength is warranted in persons with OA. Whether persons in the early stages of the disease, when symptoms are nonexistent or mild, suffer from quadriceps weakness is unknown and requires future study, so that rehabilitation professionals can introduce strengthening interventions at an appropriate stage. Using a cross-sectional study design, we ascertained whether quadriceps strength differed among women with and without radiographic evidence of OA and with and without cartilaginous defects characteristic of OA as seen on MRI scans. Furthermore, we examined whether quadriceps strength varied based on radiographic and MRI OA severity.
Consistent with our hypothesis, women in our study classified as not having radiographic (K-L 0–1) and cartilaginous evidence (Noyes 0–1) of OA were stronger than women with clinical indications of the disease. Our findings are in agreement with those of others who have found osteoarthritic patients to have lower isometric quadriceps strength than healthy adults.24,25
The osteoarthritic women in our population were ~22% weaker than women with healthy knees, similar to previously reported isometric and isokinetic knee extension torque deficits, which have ranged from 20% to 40%.4,5,24,26
In general, women with early signs of OA (K-L 2 and Noyes 2) did display quadriceps weakness when compared with healthy controls (K-L 0–1) from the same population. Women with radiographic evidence of mild OA were ~18% weaker than the women without OA, whereas women with mild cartilage defects were ~15% weaker than women without cartilage defects. These data support our hypothesis that women with early radiographic and cartilaginous evidence of OA do indeed suffer from quadriceps weakness. Our findings illustrate that quadriceps weakness is not only present in the later or more advanced stages of OA, but also it does indeed seem to be present earlier in the disease process when radiographic and cartilaginous disease is classified as “mild.” On the basis of our results, we can infer that exercises and interventions aimed at improving quadriceps strength may prove beneficial in persons with evidence of early OA. Strengthening exercises introduced early on in the disease process may prove useful in promoting continued physical function and possibly may aid in preventing the onset of symptoms11
and thus seem to be warranted.
The lack of difference between mild (K-L 2) and moderate-to-severe radiographic OA (K-L 3–4) in our work is in agreement with the findings of Liikavainio et al.,24
who failed to identify any distinction in the quadriceps strength of men with K-L scores of 2, 3, and 4. These radiographic findings, however, seem to contradict our MRI data, which revealed differences in quadriceps strength between women with mild (Noyes 2) and moderate-to-severe (Noyes 3–5) cartilage defects in the medial tibia and femur. If the radiographic data were to be considered alone, one might surmise that OA severity does not impact quadriceps strength. However, when taking into consideration our MRI findings, it seems that quadriceps strength may indeed be affected by cartilaginous disease severity, with women displaying more severe cartilaginous defects and also having greater magnitudes of weakness. The inability of radiographs to directly quantify cartilage loss27
may limit its usefulness when establishing the overall health of knee joint tissues considered critical in OA and could help to explain the apparent disagreement in findings. The contradiction between our radiographic and MRI findings could also reflect that cartilage is more sensitive to deficits in quadriceps strength when compared with bone. The relationship between lower limb strength and MRI measures of cartilage loss (e.g., cartilage volume) has been previously examined by Ding et al.28
Their results showed that greater quadriceps/hip flexor weakness was associated with greater loss of medial and lateral femoral cartilage volume. Their findings when considered along with ours suggest that quadriceps strength may affect cartilage loss or vice versa.
It is of interest to note that less than half of the women with OA in our sample were symptomatic (i.e., complained of persistent knee pain) ( and ); however, these women were still weaker than women without evidence of OA. This suggests that the quadriceps strength deficits noted in the women with OA may be unrelated to pain. Quadriceps weakness associated with injury29,30
is often attributed to pain, which, in turn, causes arthrogenic muscle inhibition (an inability to fully activate the quadriceps musculature due to a failure to recruit alpha motoneurons). Despite these claims, several others have reported no relationship32,33
or only a small-effect relationship34,35
between pain and quadriceps weakness, suggesting that factors other than pain are primarily responsible for the quadriceps strength deficits associated with OA. Hurley et al.32
have suggested that degenerative changes to knee joint structures alter sensory signals arising from joint mechano-receptors, diminishing alpha motoneuron output, thereby causing arthrogenic muscle inhibition, which, in turn, leads to quadriceps weakness. The quadriceps weakness present in the women in our study may also be explained by atrophy that results from aging or disuse.36
Although women were classified into groups based on left leg K-L and Noyes scores for the purposes of this study, it should be mentioned that many of our women (~83%) presented with bilateral radiographic and cartilaginous evidence of OA. The presence of bilateral OA may have influenced our findings, because women with bilateral knee OA could have strength deficits of a greater magnitude then those with unilateral OA. Along similar lines, there were some women who were grouped as not having OA based on their left leg K-L scores but did in fact present with OA in the right limb (15% of the K-L 0–1 group). The presence of OA in the contralateral limb may have influenced the magnitude of quadriceps strength in these women, making them weaker than those without OA bilaterally. Because the strength data were collected as part of a longitudinal data set, not primarily focused on muscle strength and OA, only strength for the left limb was available and this is the reason why women were classified based on data recorded from the left limb only.
Although the use of a cross-sectional study design was reasonable to answer the questions proposed for this study, it does have limitations. Our data cannot speak to when in time (i.e., before or after the onset of radiographic evidence of OA) the muscle weakness occurred and also does not provide information as to why muscle weakness presented in our patients. To answer these questions, longitudinal research studies should be conducted. Another limitation to our study is that data were only collected on women, and thus we cannot confirm whether similar findings would be observed in men. On the basis of available research,24
however, we contend that the results noted in women would be comparable in men.