The promotion of physical activity is a major public health initiative in western countries worldwide. It is well recognized that physical activity is beneficial in the management of numerous major health problems, including cardiovascular disease, mental illness, and obesity (31,43). However, the influence of physical activity on the development and progression of osteoarthritis (OA), particularly on weight-bearing joints such as the knee, is unclear. Given the prevalence of OA is predicted to increase in the coming decades and physical activity is being highly promoted (48), it is important that we understand the effect of physical activity on the health of the knee joint.
Although a large number of epidemiological studies have examined the relationship between physical activity and knee OA, the results are conflicting. Not only is there evidence to suggest that physical activity is detrimental to the knee joint (12,40) but studies have also reported physical activity to have no effect (17,27) and even be beneficial to joint health (13,36). A previous systematic review by Vignon et al. (45) concluded that sport and recreational activities are risk factors for knee OA and that the risk correlates with the intensity and duration of exposure. Although this systematic review investigated a broad range of different types of activity, including daily life, exercises, sports, and occupational activities, only the results of six studies that examined sports activity were retained in the review after evaluation.
Moreover, although the knee joint is a complex, synovial joint consisting of a variety of different structures, and epidemiological studies have assessed the effect of physical activity on osteophytes (26,33), joint space width (as a surrogate measure of cartilage thickness) (27,41,42), and subchrondral bone (46), no systematic review has summarized the effect of physical activity on individual joint structures. Given that previous studies have reported the development of osteophytes with physical activity, but no effect on joint space narrowing (40), it may be hypothesized that physical activity may have different effects on structures within the knee joint. The aim of this systematic review was to examine the effect of physical activity on the health of specific joint structures within the knee joint.
Data sources and searches.
To identify relevant studies for this review, we performed electronic searches of MEDLINE, EMBASE, and CINAHL up to November 2008. Search terms used included MeSH headings "knee" and "osteoarthritis" and the free text word "physical activity." The search was restricted to studies of humans and those published in English. We also screened the reference lists of key articles and previous systematic reviews.
We included studies that met the following criteria: 1) investigated the association between physical activity and development and/or progression of knee OA and 2) reported radiographic or magnetic resonance imaging (MRI) evidence of knee OA when investigating OA progression and healthy knees when investigating OA incidence. Studies that examined sporting and recreational activity, which has been previously defined as activities pursued by professional athletes or physical educators and trainers, as well as amateur sports activities performed competitively or recreationally were included (45). We excluded studies if they investigated only the patellofemoral joint, subchondral bone, children, subjects after knee arthroplasty, osteotomy, or underlying pathology (e.g., rheumatoid arthritis) or examined activities of daily living, prescribed exercises (e.g., by a physiotherapist), and non-weight-bearing or occupational activities.
Data extraction and quality assessment.
Data on the characteristics of the included studies were tabulated. This included details of the study population, including the mean ± SD age and percentage of female participants, whether information on previous injuries was provided, the method of assessment of both OA and physical activity, and study results and conclusions.
The methodological quality of the studies was independently assessed by two investigators (F.H. and P.B.) using standardized criteria that examined internal validity and informativeness of the study (30). Not all items were appropriate for cross-sectional, case-control, and cohort studies; thus, only relevant criteria contributed to the total score for each study. The total score was calculated as a sum of the positive scores. If the methodological quality score was greater than the mean of the quality scores, the study was considered to be of high quality (30).
Data synthesis and analysis.
Because of the heterogeneity of the studies included in this review, we chose to perform a best-evidence synthesis rather than statistically pooling the data. Studies were classified according to their study design, with the prospective cohort study considered the preferred design, followed by the case-control study, and then the cross-sectional design. Studies were also ranked according to their methodological quality score using the levels of evidence adapted from Lievense et al. (30): "strong evidence"-generally consistent findings in multiple high-quality cohort studies; "moderate evidence"-generally consistent findings in one high quality cohort study and more than two high-quality case-control studies or more than three high-quality case-control studies; "limited evidence"-generally consistent findings in a single cohort study, one or two case-control studies, or multiple cross-sectional studies; "conflicting evidence"-inconsistent findings in <75% of the trials; and "no evidence"-no studies could be found.
Identification and Selection of the Literature
We identified a total of 1362 studies from our electronic database searches, of which 37 studies were potentially eligible for inclusion. Nine studies were excluded as they examined tibial plateau bone area (46), the patellofemoral joint (16,47), children (20), prescribed strength training (34), non-weight-bearing activities (35), and knee structure during a short period (9,24,38). Once we excluded these studies, 28 studies remained.
Characteristics of Included Studies
We identified 22 radiological studies and 6 MRI studies that examined the relationship between physical activity and knee OA (Table 1, A and B). Of the 22 radiological studies, 2 studies were cross-sectional (2,25), 6 studies were case-control (7,10,19,22,23,29), and 14 studies were longitudinal in design (3,6,11,12,17,18,26-28,32,33,40-42). Three of the six MRI studies were cross-sectional (4,8,15), two were longitudinal (13,14), and one study had both a cross-sectional and longitudinal component (36).
Of the 28 studies included in the review, 9 were undertaken in the United States (3,11,12,17,19,26-28,32) and 8 in Australia (4,7,13-15,36,41,42), with the remaining 11 studies from the United Kingdom, Hong Kong, North Africa, and several European countries, including Finland, Sweden, Denmark, Switzerland, and Germany (2,6,8,10,18,22,23,25,29,33,40) (Table 1, A and B). Most of the participants were either recruited from elite or community sporting clubs, including the Australian Football League (7) and the 50-Plus Runners Association (3), or from existing cohorts, such as the Chingford (18) and Melbourne Collaborative Cohorts (36). The age of the subjects ranged from 45.0 to 79.0 yr, and the percentage of women in the studies varied from 0% to 100%. Whereas 8 studies excluded subjects and/or controls with previous injury (4,7,8,10,14,15,22,36), 16 studies included subjects with injury (3,6,11-13,17-19,23,25-27,29,32,33,40), but only 10 made adjustments for this in their analyses (3,11-13,17,19,25,29,32,40). The remaining four studies provided no or limited information regarding previous injury (2,28,41,42).
A variety of methods was used to examine different joint structures in the assessment of radiological OA. The Kellgren and Lawrence scale (or a modified version), which predominately assesses osteophytes, was the most commonly used instrument, with 11 studies implementing this scale (2,3,6,10-12,17,19,25,29,32). However, measurement of joint space narrowing, a surrogate measure of cartilage thickness, was also used either in isolation or combined with other radiological measures. In contrast, the six MRI studies measured cartilage volume and/or the presence of cartilage defects (4,8,13-15,36). Most studies assessed physical activity using study-specific questions asked via an interview or questionnaire, with only seven studies using a validated instrument, such as the Allied Dunbar Health Survey or the Framingham Physical Activity Index (4,12,17,32,40-42).
Methodological Quality Assessment
The mean score for methodological quality of the included studies was 78%, with a range from 50% to 100%. A total of 16 studies were considered to be of high quality (3,4,11-15,25-27,29,32,36,40-42). Of the methodological criteria assessed, most studies scored well on criteria 9 and 16, which involved assessing OA identically in the studied population and adjusting for at least age and sex. However, several studies scored poorly on criteria 6, 8, and 12, which assessed whether the physical activity assessment was blinded and examined before the outcome and whether a prospective design was used respectively.
Cross-sectional and nested case-control radiographic studies.
Of the two cross-sectional and six case-control studies that examined the association between physical activity and radiographic knee OA (2,7,10,19,22,23,25,29) (Table 2), only one of the eight studies was of high quality. The study by Kujala et al. (25), which examined joint space narrowing as a surrogate for cartilage thickness, reported a greater risk of knee OA in soccer players compared with runners, weight lifters, and shooters (odds ratio = 5.21, confidence interval = 1.14-23.8).
Longitudinal radiographic studies.
Of the 14 cohort studies that examined the relationship between physical activity and radiographic knee OA (3,6,11,12,17,18,26-28,32,33,40-42) (Table 3), 9 were considered to be of high quality (3,11,12,26,27,32,40-42). Three high-quality studies used the Kellgren and Lawrence scale, which is heavily focused on the presence of osteophytes, and each found an association between physical activity and osteophyte formation (12,26,32). Moreover, four of the high-quality cohort studies that used a combination of both osteophyte and joint space measures found no association between radiographic OA and physical activity (3,11,27,41).
Cross-sectional and longitudinal MRI studies.
Of the three cross-sectional MRI studies (4,8,15), two longitudinal studies (13,14) and one cross-sectional/longitudinal study (36) that examined the relationship between physical activity and knee OA (Tables 4 and 5), all studies, with the exception of one (8), were of high quality. Of the three high-quality cross-sectional studies, one study of 45 healthy men reported an inverse relationship between physical activity and tibial cartilage volume (4), whereas the other two studies of healthy, community-based subjects found a positive association for tibial cartilage volume and an inverse relationship for cartilage defects (15,36). Moreover, although one high-quality longitudinal MRI study found no association between cartilage volume loss and levels of physical activity (14), there was one high-quality longitudinal MRI study that found a positive relationship between physical activity and tibiofemoral cartilage volume (36) and two high-quality cohort studies that found an inverse relationship between physical activity and cartilage defects (13,36).
If all studies in the review were collectively examined, we would conclude that there is conflicting evidence for the relationship between physical activity and knee OA. However, if we consider the relationship between physical activity and individual joint structures, we conclude that:
- there is strong evidence (from multiple high-quality cohort studies) that there is a positive relationship between osteophytes and physical activity;
- there is strong evidence (from multiple high-quality cohort studies) that there is no relationship between joint space narrowing, as a surrogate for cartilage thickness, and physical activity;
- there is limited evidence (from a cohort study and two cross-sectional studies) that there is a positive relationship between cartilage volume and physical activity; and
- there is strong evidence (from multiple high-quality cohort studies) that there is an inverse relationship between cartilage defects and physical activity.
This systematic review found that the relationships between physical activity and individual joint structures at the knee joint differ. Although we found strong evidence for a positive association between physical activity and tibiofemoral osteophytes, there was also strong evidence for no effect of physical activity on radiological joint space narrowing, a surrogate method of assessing knee cartilage. Moreover, we found limited evidence, particularly from longitudinal studies, for a positive relationship between physical activity and tibial cartilage volume, and strong evidence for an inverse relationship between physical activity and cartilage defects. Although further investigation is needed, these results suggest that osteophytes are a functional adaption to mechanical stimuli and, in the absence of cartilage degeneration, that physical activity is not detrimental to the knee joint but is actually beneficial to joint health.
On the basis of three high-quality cohort studies (12,26,32), we found strong evidence for a positive relationship between physical activity and knee joint osteophytes. We also found strong evidence, based on four high-quality longitudinal studies (3,11,27,41), for the absence of a relationship between joint space narrowing and physical activity. There are several possible explanations for the discordance in the relationships between physical activity and the presence of osteophytes and joint space narrowing. It has previously been suggested that this may be due to the lower reproducibility of joint space narrowing compared with osteophytes, which may result in nondifferential misclassification and reduce the likelihood of detecting an association (40). In addition, a large number of studies have previously used the Kellgren and Lawrence grading system, which is a composite measure commonly used to assess radiographic OA, which relies heavily on the presence of osteophytes for the identification of knee OA.
However, an alternative explanation may be that physical activity has different effects on osteophytes and joint space narrowing. Although osteophytes, bony outgrowths covered by fibrocartilage, are highly associated with cartilage damage, there is also evidence to suggest that osteophytes can develop without explicit injury to cartilage (44). This is consistent with the findings that osteophytes do not correlate with cartilage volume measured on MRI, but joint space narrowing, a surrogate measure of articular cartilage, shows a strong correlation (5). Moreover, joint space narrowing has been used as the primary outcome in studies of disease progression in OA (37) and in recent clinical trials investigating treatment strategies (21,39). Thus, in response to mechanical stimuli, such as physical activity, osteophytes may enhance the functional properties of the joint by increasing the joint surface area for the greater distribution of load or by reducing motion at a joint and improving joint stability (44). In contrast, articular cartilage may not be affected by mechanical stimuli or may actually enhance the loading properties of cartilage. Although it is possible the higher prevalence of osteophytes identified in people exercising may be detrimental to the knee joint, it could also be argued, in the absence of cartilage destruction, that physical activity is beneficial and osteophytes are simply a response to mechanical stimuli.
To further investigate the relationship between physical activity and knee joint cartilage, we identified six MRI studies that directly measured cartilage volume and defects within the knee joint. Although limited evidence was provided for a beneficial effect of physical activity on knee cartilage volume, there was strong evidence for a protective effect against cartilage defects. With respect to the three high-quality cohort studies, Racunica et al. (36) found vigorous physical activity to be positively associated with tibial cartilage volume and inversely associated with cartilage defects, and Foley et al. (13) found a reduced risk of tibial cartilage defects with strenuous exercise. Although Hanna et al. (14) found no association between physical activity and knee joint cartilage, the study had limited power to show an effect because it only included 28 male subjects with a limited range of ages, BMI, and physical activity scores. Although further MRI investigation is warranted, these findings indicate that physical activity has a protective effect on knee joint cartilage.
There are several limitations to our study. We were not able to perform a meta-analysis to summarize our results because of the heterogeneity of the studies included in this review and therefore undertook a best-evidence synthesis. Moreover, given there were a limited number of MRI studies that specifically examined the effect of physical activity on the tibiofemoral cartilage volume, some of the conclusions we could make from this review were limited.
In summary, this review found that the relationship between physical activity and specific knee structures differed, with strong evidence for a positive relationship between physical activity and tibiofemoral osteophytes, absence of an association between physical activity and joint space narrowing, and strong evidence for an inverse relationship between physical activity and cartilage defects. These findings highlight the need to examine the effect of physical activity on individual structures of the knee joint rather than the joint as a whole. Moreover, these findings suggest that physical activity may not have a detrimental effect on the knee joint but may be beneficial to joint health.
D. U. and J. F. L. T. are joint first authors. D. U., F. H., A. W., and C. D. were supported by National Health and Medical Research Council fellowships (grant Nos. 284402, 418961, 317840, and 490049, respectively).
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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