The results of our study showed that in asymptomatic subjects with OA risk factors, light exercise was associated with lower tibiofemoral T2 relaxation times, which suggests that lower water content and more intact collagen architecture are associated with light exercise compared to sedentary lifestyle and moderate-strenuous exercise. In women, moderate-strenuous exercise was associated with higher MFC T2 values, whereas this relationship was not observed in males. Interestingly, we did not detect any significant differences in the T2 values or WORMS grades when our normal cohort was grouped by exercise level; thus, the cartilage and meniscus of subjects without OA risk factors may not be affected in the same manner by exercise level. Finally, as frequent knee-bending activities were associated with higher tibiofemoral T2 values in all subjects, and higher patellar T2 values and WORMS grades in those with OA risk factors, such activities appear to be associated with greater cartilage degeneration.
As T2 values are a measure of early biochemical cartilage degeneration (15
), it could be postulated that light exercise may prevent deterioration of the cartilage collagen architecture and associated increased water content in asymptomatic individuals at risk for knee OA. Therefore, it is possible that lower impact exercise has a potential protective effect on tibiofemoral knee cartilage in this patient population. As everyone in the light exercise group walked ≥3 days/week for <2 hours/day, but only some did various other light recreational activities – darts, croquet, bowling – it is possible that frequent walking is the activity that best maintains cartilage biochemical homeostasis. These findings are in agreement with previous studies that have shown that aerobic walking can decrease pain and disability in subjects with knee OA (29
), but to the best of our knowledge, its effects have not been evaluated in asymptomatic subjects at risk for OA.
In women with OA risk factors, moderate-strenuous exercise was associated with greater MFC cartilage degeneration at the molecular level – based on T2 relaxation times. Thus, it is possible that higher impact exercise further increases these women’s risk of developing knee OA. In contrast, MFC T2 values did not differ between exercise groups in male subjects. One possible explanation for this discrepancy between sexes is the difference in loading behaviors inherent to men and women (31
). However, it is unclear if these differences during various activities are directly related to changes observed in the MFC in the current investigation. Interestingly, women do have a higher incidence of knee OA than men, and the most common location is the medial compartment (32
). Other contributing factors could be the specific sport/recreational activities performed by each sex, the strength of knee stabilizing muscles, or the thinner cartilage and smaller joint surfaces in women (34
In contrast to this study, Stehling et al., who evaluated a similar group of incidence cohort subjects in two previous studies, found that increased physical activity levels were correlated with higher patellar T2 values and WORMS grades (21
). However, their subjects were grouped by total PASE scores. Total PASE scores take into account not only exercise, but also household and occupational activities, which the scoring system weighs more heavily than exercise. It is possible that an increased amount of household and occupational activities may be associated with increased knee-bending activities, which we found to have a potential negative impact on knee cartilage. This could explain our conflicting findings. Our study evaluated exercise and knee-bending activities separately. We derived an exercise classification that would allow us to compare the knee cartilage of sedentary individuals, frequent low impact exercisers, and frequent higher impact exercisers, and we analyzed T2 values in both the tibiofemoral and patellofemoral joints rather than only in the patella. From our knowledge, this is the first study investigating the association of exercise on knee health in asymptomatic subjects with risk factors for OA using quantitative and morphological MR parameters.
Similar to previous studies (4
), we did not detect any differences in the cartilage of our normal cohort when analyzed by exercise levels. Recently, Chakravarty and colleagues examined 45 long distance runners and 53 controls and found that runners did not have more prevalent or severe OA when compared to controls (4
). However, other studies have shown a protective (5
) or detrimental (3
) effect. It should be noted that the majority of previous studies did not consider OA risk factors in their analysis. Kujala and colleagues found that former soccer players were at increased risk of developing premature OA, but concluded that previous knee injuries contributed to this finding (39
). Additionally, many studies used radiographs (4
), which cannot depict cartilage, menisci and internal knee structures, while only a few used MRI (37
). To the best of our knowledge only one other small study has examined the habitual effects of physical activity on quantitative T2 and/or T1rho values. Similar to our findings, Stahl et al. found no significant difference in T2 and T1rho values between 13 active and 7 sedentary asymptomatic healthy subjects, although they did not note whether any of the subjects had OA risk factors (41
Frequent knee-bending activities were associated with higher tibiofemoral T2 values in both the incidence and normal cohorts, and higher patellar T2 values and more severe cartilage lesions – based on WORMS grades – in those with OA risk factors. Thus, it is possible that such activities are associated with greater cartilage degeneration in all individuals, but potentially to a greater degree in subjects already at risk for knee OA. Our data is consistent with several studies that have shown that knee-bending activities are risk factors for knee OA (42
). During deep knee flexion, the stresses and loads in the knee dramatically increase (47
). It has been reported in cadaver studies that deep knee flexion to 90 degrees can result in tibiofemoral joint stresses of 26.6 MPa, exceeding the threshold for which cartilage lesions are known to occur (47
). Interestingly, the medial compartment experienced approximately 70% greater peak pressures during deep knee bending, which is consistent with the increased T2 values observed in the medial compartment in subjects who performed knee-bending activities in the current study. Additionally, with deep knee-bending activities, the patella also experiences high loads and may be subject to greater shear stresses during squatting than the femoral surfaces (48
There are several limitations in the current study that should be noted. First, we did not have information about the specific activities performed by each subject. The PASE scale combines weight-bearing and non-weight-bearing sports into the same categories. Soccer and cycling would load the knee joint quite differently. Similarly, all knee-bending activities were grouped together. Second, given our inclusion and exclusion criteria, a relatively small sample of subjects (n=33) from the OAI database qualified as normal subjects. This limitation may have affected our ability to detect differences in the normal cohort. Third, the predictive value of T2 quantification to project future cartilage deterioration remains unclear. Fourth, our study only took into account current activity level rather than lifelong activity history. Clearly cartilage composition is influenced by recent and habitual loading behaviors.
Future studies will need to investigate which specific activities increase or decrease a person’s risk for developing knee OA. Populations with and without known OA risk factors should be evaluated separately in longitudinal studies that utilize MRI-based quantitative imaging methods (e.g. T2, T1rho, dGEMRIC), which detect cartilage damage prior to irreversible morphological changes. Knowledge of this early degeneration would allow for preventive measures to be explored. Furthermore, our results indicate that the cartilage of men and women may respond differently to physical activities; however, these findings warrant further investigation to better understand the causal relationships.
In summary, the cartilage of individuals with and without knee OA risk factors appears to respond differently to physical activity. In subjects at risk for OA, light exercise was associated with lower tibiofemoral T2 relaxation times in both sexes, whereas moderate-strenuous exercise was associated with higher T2 values in women. Therefore, if subjects have OA risk factors, light exercise – possibly frequent walking in particular – may aid in preventing biochemical cartilage degeneration, while moderate-strenuous exercise, particularly in women, may further increase their risk of developing knee OA. In our normal cohort grouped by exercise level, we did not detect any significant differences in T2 measurements or WORMS grades; however, this cohort was smaller. Finally, frequent knee-bending activities appear to be associated with greater cartilage degeneration in all individuals, but possibly to a greater degree in subjects already at risk for knee OA.