The results of this study revealed that women in the lowest tertile of quadriceps strength were at elevated risk of whole knee and tibiofemoral joint space narrowing over 30 months compared with those in the highest tertile. However, in men, no associations were found between quadriceps strength or H:Q ratio and knee joint space narrowing. To our knowledge, this is the largest study to evaluate these relationships in both women and men.
The positive findings in women but not men are in concordance with previous findings for incident symptomatic knee OA from the MOST study.37
In that study women to a greater extent than men with low quadriceps strength were found to be at increased risk for incident
symptomatic whole knee OA. However, low quadriceps strength was not identified as a risk factor for incident
radiographic knee OA in either women or men. In men, the current study findings are also consistent with the lack of a significant change in joint space narrowing or grade following strength training, in comparison with a control program in subjects with pre-existing knee OA.20
Similarly, a review of observational studies of risk factors for progression of knee OA found no evidence to support a significant association with quadriceps weakness.38
One potential explanation for the association between quadriceps weakness and incident symptomatic as well as progressive worsening knee OA in women, but not in men, may relate to strength capacity being closer to a threshold for risk.39, 40
For example, men have higher strength and this may provide greater reserve, so that even with some loss of strength, the degree of weakness may be insufficient to result in risk for these outcomes. In contrast, women have less strength (57% that of men in this study despite no significant difference in BMI), so weakness may result in sufficient cause for knee joint damage.
Our findings differ from those of Brandt, et al18
and Thorstensson, et al41
who found no significant differences in mean quadriceps strength between women with and without tibiofemoral OA worsening. It is possible that the lack of agreement between those and the present study may have been due to different methods for measuring knee OA progression. In the present study, progression was defined as an increase in joint space narrowing in participants having a JSN score <3 at baseline. In the prior studies, progression was based on increasing KL grade. The development of KL grade 2 is characterized by osteophyte formation while progression from KL grade 2 is mainly characterized by joint space narrowing and it is plausible that these may result from different mechanisms. In addition, the study by Thorstensson, et al
used more functional measures of strength than the present study (single-leg rise/stance and 300 m walk time). Differences in results also may exist due to larger sample sizes and a greater number of subjects with progression in the present study.
We found a significant trend for a relationship between patellofemoral joint space narrowing and quadriceps strength in women (p=.0400) but not in men (p=0.7106). This result in women is consistent with a recent study that found greater quadriceps strength protective against cartilage loss on the lateral patellofemoral facet.21
In addition, cross-sectional data from a study evaluating the relationship between quadriceps weakness and prevalent tibiofemoral, patellofemoral, and mixed OA (presence of both patellofemoral and tibiofemoral OA) also showed a relationship between lateral patellofemoral OA and quadriceps strength, though this relationship lost significance after analyses were adjusted for knee pain.42
These authors suggested that quadriceps strength may prevent excessive lateral movement of the patella during knee flexion, which in the presence of weakness could lead to disproportionate compressive forces across the lateral patellofemoral joint.
In contrast, we did not find that a H:Q ratio of either <0.6 or <0.8, indicating greater quadriceps relative to hamstrings strength, protected against joint space narrowing. This result suggests that overall quadriceps strength may be more important than the balance of knee flexor to extensor strength for preventing knee joint space narrowing. As mentioned previously, a concentric:concentric H:Q ratio cutoff of 0.6 was prospectively selected, as this has been reported to be normal in studies of younger subjects.30, 43, 44
Recognizing that this cutoff may not be appropriate for older adults, we conducted secondary analyses using 0.8 as a cutoff and did not find a strong effect at this cutoff either.
We are not aware of normative H:Q radio data for adults age 50-79 years. However, based on the reported more rapid decline in isokinetic hamstring strength in comparison with quadriceps strength with aging,45
we might expect the H:Q ratio to slightly decrease in older adults. Extrapolating from that report of mean strength data for adults in the same age range as MOST participants, using similar methods, suggests H:Q ratios in men and women of 0.55 and 0.58, which declined to 0.52 and 0.57 respectively over 10 year follow-up. The HealthABC study also reported grouped mean±SD isokinetic hamstring and quadriceps strength data for high functioning eighth decade adults.46
In that study, the ratio between these was 0.74 for men and 0.79 for women. However, since H:Q ratio data were not presented, it is not possible to determine the H:Q ratio for individuals or whether those data would represent normative data for older adults in other decades or who were not selected for high mobility function. In the absence of normative H:Q ratio data for older adults, we assessed whether there was any cutoff or range of values that appeared to be associated with more severe knee OA or JSN outcome. As seen in , there did not appear to be any relationship between the median or interquartile range of H:Q ratio and a) sex, b) decade, c) knee OA (KL) grade, or d) JSN outcome. Although it was not feasible to assess peak eccentric strength in older adults with knee OA, an Hecc
ratio may be a more functional means of assessing muscle balance.30, 44
H:Q Ratio median (IQR) by sex, decade, KL grade and outcome
Results from the present and the aforementioned study of the MOST cohort indicate that quadriceps weakness is a risk factor for progressive and incident symptomatic but not incident radiographic knee OA for women in this cohort.47
Though a mechanism that could explain this finding was not the focus of this study, knee OA risk factors previously have been acknowledged to differ between women and men due to biochemical and biomechanical differences that, in this case, could also result in differences in risk for knee OA progression (hormonal and anthropometric factors and/or ligamentous laxity). One potential limitation was the inability to correct for gravity during the isokinetic testing, which could have reduced the raw quadriceps strength data for participants with heavier calves. However, the inter-participant differences in calf mass were found to be small and would be unlikely to alter the tertile of strength to which participants were assigned.
Several aspects of our longitudinal study design strengthen our findings. First, the MOST cohort is made up of individuals with radiographically-documented knee osteoarthritis or known risk factors for knee osteoarthritis such as elevated BMI or history of knee surgery or injury. Therefore, if quadriceps weakness plays a significant role in the pathogenesis of knee OA, it is likely to be detected in this cohort. In support of this, we found men and women with KL grade ≥ 2 in MOST had lower quadriceps strength cross-sectionally than those without radiographic knee OA (KES=123.8 vs. 131.3 N•m in men and KES=69.3 vs. 75.6 N•m in women). Additionally, rates of knee joint space narrowing in the study population were similar to or higher than those reported for progression in other studies with similar inclusion criteria. Thus, there was a sufficient amount of time for worsening to occur in an adequate number of study participants, suggesting that our findings may be generalizable.
Knee OA is one of the leading causes of disability in the United States 48
and identification of modifiable risk factors is a significant public health concern. Though this study identified quadriceps weakness as a risk factor for knee joint worsening in women only, lower limb function in older men and women is highly dependent on quadriceps muscle strength.49, 50
Quadriceps strength is known to decline with age.51
Muscle atrophy from inactivity or knee pain may further contribute to this decline in older adults with knee OA. Therefore, quadriceps strength is functionally significant in men despite the lack of association with joint space narrowing and may have a dual importance in women to prevent worsening knee OA as well as minimize physical functional limitations. Therefore, we would not interpret the results as an indication that only women with weakness should participate in lower limb strengthening activity, but rather that in addition to the known benefits on minimizing impairments, functional limitations and disability in men and women, women may attain an additional protection against worsening of joint space narrowing through maintaining higher quadriceps strength. Longitudinal studies are necessary in order to better understand whether strengthening of weak quadriceps muscles is protective.