
 the promotion of physical activity ( pa ) is a major public health initiative in many countries due to its protective effect on numerous major health problems , including canada and the us where public health bodies recommend 30 to 60 minutes of moderate - to - vigorous activities per day .
however , there has long been a concern that such promotion could lead to a rise in hip and knee oa , the major public health problem in musculoskeletal medicine and a leading cause of chronic disability .
while there is a broad agreement that pa is an important determinant of joint health , it is unclear what amount and type of pa are beneficial or pose a risk . in short , despite numerous studies , the association between pa and joint health is complex and poorly understood .
while different study designs , case definitions , sampling frames , and size play a role , the wide variation in how pa is defined is the most probable reason for the uncertainty .
there is a lack of valid , reliable , and standardized instruments across studies , substantial measurement error , variation in the period and nature of pa measured , and failure to measure the most relevant aspect of pa - joint load .
where accurate and precise measures are available , they are impractical for use in population - based research . the goals of this study were to address these gaps by ( 1 ) measuring historic pa , a key variable given the long latency and asymptomatic induction period of oa that , while potentially imprecise , is likely more important than accurate measure of an irrelevant variable such as current activity , ( 2 ) examining loads applied to the joint using quantitative joint loading units that allowed for assessment of cumulative force from pa , including dose response , and ( 3 ) including household activity ( in addition to occupation and sport / recreation ) to capture all three primary domains of activity .
the purpose of this study was to evaluate the association between levels of lifetime knee joint force and knee oa in a large canadian sample of community dwelling adults .

 data source and study populationthe source population was community - dwelling members of the canadian association of retired persons , canada 's largest 50-plus advocacy group with 350,000 members .
briefly , recruitment was via direct email to 28,000 members and to 100,000 additional members via an advertisement in an online newsletter , appearing in two consecutive newsletters .
all e - mails and newsletters contained hyperlinks or banner advertisements directing subjects to the study website . through these methods ,
after completing an electronic consent form , subjects were given password access to the questionnaire .
all data collection was web - based and used skip logic technology that allowed subjects to follow individualized paths through the survey , moving forward based on responses to previous questions .
extensive pre- and pilot - testing was carried out to ascertain best recruitment methods , survey duration , navigation , and to ensure that respondents could understand items , retrieve information , and make appropriate estimations .
the baseline questionnaire , carried out from june to september 2005 , took 60 to 90 minutes to be completed.response rates have been previously described   and are presented in figure 1 .
individuals who completed the baseline survey were contacted by email and letter for follow - up surveys at approximately one ( may 2006 ) and two years ( june 2007 ) .
follow - up surveys inquired about knee joint health using the same questions as the baseline survey . 
 
the source population was community - dwelling members of the canadian association of retired persons , canada 's largest 50-plus advocacy group with 350,000 members .
briefly , recruitment was via direct email to 28,000 members and to 100,000 additional members via an advertisement in an online newsletter , appearing in two consecutive newsletters .
all e - mails and newsletters contained hyperlinks or banner advertisements directing subjects to the study website . through these methods ,
after completing an electronic consent form , subjects were given password access to the questionnaire .
all data collection was web - based and used skip logic technology that allowed subjects to follow individualized paths through the survey , moving forward based on responses to previous questions .
extensive pre- and pilot - testing was carried out to ascertain best recruitment methods , survey duration , navigation , and to ensure that respondents could understand items , retrieve information , and make appropriate estimations .
the baseline questionnaire , carried out from june to september 2005 , took 60 to 90 minutes to be completed .
individuals who completed the baseline survey were contacted by email and letter for follow - up surveys at approximately one ( may 2006 ) and two years ( june 2007 ) .
follow - up surveys inquired about knee joint health using the same questions as the baseline survey . a period prevalence study using a cross - sectional design was utilized , as there were an insufficient number of incident knee oa cases to power the study to meet the objectives . to compile the dataset ,
subjects reporting oa at any of the three time points ( figure 1 ) were identified as cases .
while the inclusion of prevalent cases could potentially limit the ability to delineate cause and effect , the approach was justified on several grounds , and steps were taken to guard against reverse causality .
first , increasing the number of cases by combining prevalent and incident cases allowed for greater study power to assess gender - specific relationships between pa and oa while including a number of covariates , and assessing dose - response relationship of pa with oa .
second , a new measure of exposure  quantitative lifelong joint force from work , sport , and household activity measured in joint loading units  was investigated for the first time .
third , the relationship between the historic pa measured in this study ( up to age 50 , as discussed in methods ) and the time of diagnosis for the vast majority of cases is separated in time , providing some protection against reverse causality .
in addition , the highest levels of lifetime pa for most people occur prior to the age of 50 .
lastly , since oa is not a curable condition , prevalence is a relatively stable reflection of disease frequency .
lifetime physical activity was assessed using the lifetime physical activity questionnaire ( l - paq ) , whose development and validation has been described previously . in summary ,
the l - paq is a self - administered , web - based questionnaire that was based on existing instruments [ 810 ] , developed using the principles of construct validation , adapted for self - administration over the internet , incorporated skip logic technology , and expanded to capture more detailed information including bodily movements involving the knee . using a subsample of the current study , intraclass coefficients for reliability ranged from 0.65 to 0.89 ; convergent validity testing against two validated lifetime questionnaires resulted in spearman correlation coefficients ranged from 0.41 to 0.71 .
the lpaq measures lifetime pa across three domains : sport / recreation , occupation , and household and had been described previously [ 4 , 5 ] .
sample questions in each domain are shown in appendix a. briefly , in the sports / recreational section , respondents were provided with a list of 64 possible sports and were permitted to add other sports .
data collected included the duration , frequency , and average length of time per occasion , and time spent per hour ( none , 15 , 515 , 1530 , 3045 , and 4560 minutes per hour ) in eight bodily positions or movements . the occupational section used an open format in which respondents , for each occupation ever held , provided details on job title or type , duration , average hours per week , and whether the job was full time , part time , or seasonal and time spent per eight - hour segment in nine bodily positions or movements .
household activity covered four areas : ( 1 ) caring for children ; ( 2 ) caring for elderly or disabled individuals ; ( 3 ) gardening ; ( 4 ) housework .
for each household area of activity , participants were prompted to provide detailed information on duration and average number of hours per week of participation and were required to report time spent in an 8-hour period for each of the bodily movements as per the occupational section ( appendix b
 list of activities and assigned joint force ) . to obtain a measure of cumulative joint force at the knee , a cumulative peak force index ( cpfi )
score was estimated at the knee and has been described previously   as the product of time spent in a specific activity ( total lifetime hours ) , bodyweight ( bw ) , and typical peak joint force for that activity ( % bw ) , ( i.e. , cpfi score ( bodyweight - hours ) = total lifetime hours  bodyweight  typical peak joint force , per each activity ) .
as previously described [ 4 , 5 ] , the cpfi is a newly proposed measure , and steps were taken to validate and/or ensure the greatest precision in the components that comprise it .
in addition to validation of the l - paq , self - reported bodyweight and height were utilized , measures with established validity properties   used in numerous epidemiologic studies .
we improved on a single self - report of bodyweight by asking about it at three time points ( baseline survey , age 20 , maximum lifetime ) , and deriving a lifetime bodyweight trajectory , interpolated using a lowess ( nonparametric smooth ) curve .
the third component of the measure was the typical peak joint force assigned to each activity ( appendix b ) .
these values were determined after an extensive literature [ 1240 ] ( full bibliography available on request ) that prioritized in vivo studies and incorporated judgments about data quality and study rigor .
data were synthesized and a consensus achieved by a panel of experts from biomechanical engineering , rheumatology , physiotherapy , and musculoskeletal epidemiology .
the main reason for this was to capture the primary pa exposure prior to the diagnosis of oa ( and first symptoms ) for the vast majority of cases , and to minimize the effects of subclinical , undiagnosed , or early oa on pa patterns .
support for this approach was also drawn from the previous study on lifetime trajectories   that revealed that peak exposure window for pa is prior to 50 , being at its highest lifetime levels from ages 3045 .
an algorithm was used to ascertain knee oa cases , requiring that subjects report both  health - professional - diagnosed knee oa  and  pain , aching , or stiffness on most days  at any of the 3 time points ( baseline , 1st and 2nd followups ) .
the questionnaire item on health - professional - diagnosed oa informed subjects that oa was distinct from other msk diseases ( e.g. , rheumatoid arthritis and osteoporosis ) , and a response confirmation to this question was required in a follow - up item .
the reliability and validity of self - reported knee oa was carried out in a substudy and has been previously published .
briefly , using the american college of rheumatology ( acr ) clinical classification criteria for knee oa   as a diagnostic referent , sensitivity was 0.76 , specificity 0.98 , positive predictive value 0.87 , and negative predictive value 0.95 .
our findings are consistent with other studies comparing self - report medically diagnosed oa and clinical oa [ 4446 ] .
the baseline questionnaire measured known knee health risk factors and included gender , age , body mass index ( bmi ) , ethnicity ( asian , black , caucasian , first nations , hispanic , and others ) , and education ( elementary , high school , postsecondary , and trade / technical ) . the following question from the baseline questionnaire
was used to determine the presence of significant knee injury :  have you ever had a knee injury that required you to use a walking aid ( e.g. , cane or crutch ) for at least one week ?    follow - up questions included the age at injury ( if more than one injury , the time of first injury was requested ) . only injuries that occurred before the diagnosis of oa
cpfi values for each activity were summed for sport , occupation , and household domains for each 5-year period of a person 's lifetime to factor in changes in bodyweight over time , and these domain values were then summed to give a total cpfi value . for the total force ( cpfi )
variable , subjects were categorized into quintiles of exposure for the overall distribution , prior to stratification by gender . for the domain specific analysis , the quintiles were based on the relative distribution within each domain ( occupation , household , and sport ) , again prior to stratification by gender ( e.g. , quintile 5 for occupation was at the same joint loading level for both sexes ) .
crude odds ratios were calculated for the relationships between knee oa and joint force variables and other study covariates .
covariates were selected based on scientific knowledge and the conceptual framework of causal pathways to knee oa .
factors associated with an increased risk of knee oa , which also could be confounders , such as age , previous injury , and bmi were adjusted for in all the analyses .
the potential effect of one domain on another ( e.g. , occupational activity when assessing the sport - oa relationship ) was also potentially confounding and included .
men and women were examined separately because of known gender differences in disease prevalence , physical activity profiles , injury rates , and bmi .
reference categories were the lowest cpfi , youngest age tertile ( < 58 ) , normal bmi ( 20.024.9 ) , and no previous injury .
test for trends was obtained by treating the categories ( quintiles ) of the cpfi variable as continuous and testing the slope for significance ; the models contained all relevant adjustment covariates .
multiple logistic regression was used to examine if levels of total knee cpfi were associated with a risk of knee oa , controlling for age , previous injury , and bmi .
an additional adjusted analysis was carried out investigating the separate effect of occupational , sport , and household cpfi ( adjusted for the other domains ) .
subject characteristics for the sample are presented in table 1 . frequencies of outcome , exposure , and covariates for this study are provided in tables 2 and 3 .
the prevalence of knee oa was 22.4% for the sample overall17.8% for men and 25.1% for women ( table 2 ) .
twenty - six percent of the sample were of normal bmi , with approximately 72% being either overweight ( 39.9% ) or obese ( 31.5% ) .
more men than women ( 47.7 to 36.1% ) were overweight , and more women than men were obese ( 33.4 to 29.5% ) .
twenty percent had a history of previous knee joint injury ( 24% in men and 18% in women ) .
the prevalence of subjects in occupational and household quintiles varied substantially by gender ( table 3 ) .
for example , in the male household strata , the largest prevalence ( 36.3% ) was in the lowest ( referent ) quintile of exposure , while the smallest prevalence ( 6.2% ) was in the highest quintile of exposure . for women , the largest prevalence ( 27.5% ) was in the highest quintile of exposure and the smallest prevalence in the referent quintile ( 11.1% ) . for occupational force , these relative proportions were reversed by gender , though the percentages were slightly different ( table 3 ) .
crude ors provided evidence that older age , previous knee injury , obesity in men , and overweight and obesity in women were associated with knee oa ( table 2 ) . compared with the lowest category of total lifetime knee force the highest category ( 5th quintile ) , in men and 3rd , 4th , and 5th quintiles in women
were crudely related to knee oa ( table 2 ) . for domain - specific lifetime force ,
the 5th quintile of occupational force in men and both occupational and household activity in and women were crudely related to oa ( table 3 ) . adjusted ors for risk factors on knee oa obtained from multiple logistic regression
oa and total lifetime force remained significant for the highest force quintile in both men ( 1.70 ; 95% ci1.06 , 2.70 ) and women ( 1.52 ; 95% ci1.15 , 2.02 ) . in domain - adjusted models , occupational force in men and household force in women were related to knee oa and tests for trend in these domains , as well as in total lifetime force ( both men and women ) , were significant ( p < 0.001 ) .
sport / recreational force was not related to knee oa . after adjustment , older age , injury , and high bmi remained
being overweight ( bmi 2529.9 ) increased the risk of knee oa for women ( or 1.69 ; 95% ci1.31 , 2.19 ) but was not related to knee oa in men .
obesity ( bmi > 30 ) was a greater than two - fold risk for men and three - fold risk for women .
this prevalence study on a large sample of canadian adults presents a newly proposed measure of lifetime mechanical knee joint force based on hours in pa , bodyweight , and typical knee joint force for specific activities and relates it to self - reported knee oa .
we provide evidence that while lifelong pa may generally be safe for the knee joint , that very high force from lifelong total force , and from high levels of total occupational force ( men and women ) and household activity ( women ) , is a potential risk for knee oa .
our results are consistent with previous studies that show overweight / obesity , age , female sex , and previous injury are significant risk factors for knee oa [ 4755 ] .
the results of this study must be compared cautiously with previous studies due to its cross - sectional design , how subjects were assembled , and how exposures and outcomes were measured .
for example , no other studies evaluating pa and knee oa have used the internet for data collection or have completely classified pa in terms of a joint loading variable over the long term , perhaps partly explaining inconsistent results from these past studies [ 44 , 5661 ] . of note , the cross - sectional design may have resulted in reverse causality potentially attenuating risk estimates .
conversely , subjects with oa may have overreported prior pa exposure because they perceive that pa caused their oa ( recall bias ) , potentially increasing the risk estimates .
since this type of bias threatens most prevalence studies , emphasis should be placed on recent high - quality cohort studies evaluating the association between pa and oa . a recent prospective cohort study with a 22-year - followup using physician - diagnosed oa , reported an adjusted or for the heaviest category of physical demands at work compared with the lightest category of 18.3 for knee
during 12 years of followup , recently reported that 463 of 1678 respondents ( 28% ) developed clinical knee oa , and that a high mechanical strain score was associated with an increased risk of knee oa ( hr 1.43 , 95% ci 1.151.77 ) after adjustment for a number of covariates .
, in a prospective cohort study of approximately 40,000 australians with an average 5 years of followup , reported a composite sport and occupational exposure ( past 6 months , measured at baseline ) and found a risk for total knee joint arthroplasty for the vigorous level of activity ( hr 1.42 , 95% ci 1.081.86 ) .
several studies from the framingham cohort suggest that job activities may cause as much as 15% to 30% of knee oa in men [ 65 , 66 ] .
felson et al .   reported that elderly persons ( average age 70 ) in the highest quartile of pa at a baseline examination had over three times the risk of developing radiographic knee oa nine years later , when compared with those in the lowest quartile .
mcalindon et al .   using longitudinal framingham data reported that the number of hours per day of heavy physical activity was associated with the risk of incident radiographic knee oa ( or = 7.0 for 4 + hours heavy physical activity / day ) .
in contrast , a study by hannan et al .   in the same cohort found no increase in the risk of knee oa with increasing physical activity . in the highest quartile of pa compared to the least active , the or was 1.3 for men and 1.1 for women ( both nonsignificant ) .
using data from the chingford study , followed 715 women ( mean age : 54 years ) for 4 years with no radiographic knee oa at baseline and included the pa categories of walking , occupation , and sport / recreation .
they found no relationship between incident knee oa and pa , while walking protected against joint space narrowing ( or = 0.4 , 95% ci 0.20.9 ) .
it is evident from these often - cited reports that , despite the longitudinal cohort designs , large samples and lengthy followups , and estimates for the risk of pa on knee oa vary extensively .
while differences in eligibility criteria , covariates included in multivariable models and small samples may account for some of the disparity , the most likely reason is the wide variation in pa exposure measurement . of note
, most studies have not measured the joint - force aspects of pa nor attempted to completely classify pa ( including historic pa ) from all three major activity domains .
apart from the verweij et al 's study , none of the above studies considered pa from all three major activity domains or attempted to estimate the effect of activities in terms of joint force .
was an or of 1.43 ( 95% ci 1.151.77 ) for a high knee mechanical strain score , close to our reported or 's from the highest quintiles of total knee force , occupational force , and household force in women .
results are not directly comparable since apart from differences in design , the mechanical strain score was a ranking ( 1 to 4 ) of certain physical activities over the past 2 weeks ( taken at baseline ) and did not look at sex - specific differences in occupational and household activity .
a number of longitudinal and case control studies [ 5557 , 6875 ] , we did not find a relationship between sport / recreational activity and knee oa . studies that have shown a relationship between sport and knee oa have generally been in populations of athletes in specific sports with high knee forces [ 54 , 7681 ] and not from population - based studies , or the association has been explained by joint injury . in population - based studies of lifetime activity ,
the highest sport / recreation rates typically occurs at a relatively young age , as it did in the current cohort , prior to the age of 25 .
thus , high forces from sport later in life , when the joint may be more vulnerable , were not well - represented in this sample and may contribute to the lack of association here .
the prevalence of knee oa in our study was 22.4% , 17.8% in men and 25.1% in women .
these gender differences in prevalence are consistent with previous large population - based north american studies for this age group [ 8284 ] .
even though there is probably some misclassification , our definition , which required a medical diagnosis and the presence of pain on most days , is important since pain is usually the most important aspect of disease to patients and may precede x - ray change , potentially capturing earlier disease .
we report the results of a validity study in a subsample of the current study comparing self - reported oa to clinical oa .
it was important to measure and simultaneously adjust for pa - related force from all three major activity domains .
most previous pa - oa studies have investigated one or two domains ( usually sport and/or occupation ) .
given the high levels of household and occupational pa reported in our previous paper , omitting one or both of these domains leaves these studies vulnerable to confounding from the unmeasured domain(s ) .   in studying all three domains separately by sex
, we also observed relationships of pa with very high occupational force in men and household force in women .
questionnaires used in many previous studies did not assess the frequency , duration , and intensity of pa actually performed by women .
the majority of women 's exposure to pa , particularly in older cohorts such as the current one , is due to accumulation of regular household activities [ 8588 ] .
while household activity may generally not be considered vigorous from an energy expenditure perspective and is often ignored in epidemiologic study of oa , there are many repetitive motions ( e.g. , stair climbing , squatting , and kneeling ) and activities ( e.g. , gardening , lifting , and carrying ) that are associated with high knee joint forces [ 20 , 21 , 28 , 36 ] but have low energy expenditure .
this is only the second study to measure lifelong household load at the knee joint and relate it to knee oa , and the first to quantify household knee joint force from historic activity for the assessment of dose response . in the previous study by sandmark et al .
, exposure to physically demanding tasks at home was significantly associated with knee oa among women ( but not men ) and was the strongest risk factor for women among the physical load variables that were investigated in that study . given that women have been shown to have higher pa than men when including household together with occupational and sporting activities , and that reasons for the higher prevalence of knee oa in women are not clearly elucidated , our findings provide preliminary evidence that the role of historic household pa requires further investigation .
the role of occupational activity has received much more study , and is better understood in men than in women , in part because previous studies were based historically on male - dominated workforce cohorts .
even though women often spend forty or more hours a week at a full - time job and from twenty to forty - five hours per week working in the home , questionnaires used in many previous studies do not assess the frequency , duration , and intensity of pa actually performed by women .
it has been shown that , when the definition of regular physical activity measured in surveys is expanded to include household activity , pa levels rise and associations with health outcomes are more evident  including protective relationships with cardiovascular disease and myocardial infarction , cancer , and an inverse relationship with all - cause mortality [ 85 , 90 ] .
our finding of an increased risk of knee oa for the top quintile of occupational joint force is generally consistent with previous studies [ 62 , 64 ] and several systematic reviews [ 9294 ] .
the cpfi , a quantitative joint force measure , together with a large sample allowed for evaluation of a dose - response relationship between lifelong force and knee oa . in the models where a significant relationship with knee oa was found ( total force , occupation , and household ) , there was an increasing , significant trend in the ors from lower to higher levels of cpfi , though only the ors for the highest ( 5th ) quintile reached statistical significance .
while this requires confirmation and further delineation in future studies , the presence of dose response strengthens evidence for a causal relationship .
this study had several strengths , including a large sample drawn from the population and a sufficient number of cases to adjust for a number of covariates , the separate analyses of men and women ( equivalent to including interactions with gender for all variables ) , and assessment of dose - response .
another strength was the use of detailed information on the duration , frequency , and joint loading aspects of historic activities , from all three main physical activity domains , allowing for relatively complete classification of the total volume of pa .
historic pa is a potentially important exposure in oa etiology given the lengthy induction and asymptotic latency period .
measuring current or recent levels of pa does not capture long - term joint forces , may miss etiologically important periods of exposure and is a poor proxy for cumulative lifetime exposure .
lastly , many studies have used advanced disease markers , such as total joint arthroplasty or moderate - to - marked radiographic change as the outcome in assessing the role of pa .
it is not clear whether the relationship with pa for early , symptomatic cases is the same as that observed for advanced or radiographically defined oa .
this may be important in understanding modifiable risk factors that could play a role in a prevention strategy for oa , something not currently available .
there are a number of limitations that are important in interpreting the results of this study .
self - report of knee oa may lead to misclassification . in our examination of the measurement properties of our case definition
this is critical for studies of risk factors , since low specificity ( inclusion of many false positives among cases ) causes a greater attenuation of effect than low sensitivity .
ppv was also high , another important measure indicating that the vast majority of the cases identified in the survey were true cases .
nevertheless , we did not use radiography as part of the classification criteria for knee oa .
radiographic oa in the presence of symptoms is thought to represent the best definition of oa .
however , x - ray change is associated largely with moderate - to - advanced disease , and there is only moderate agreement between pain and symptoms and x - ray changes .
wu et al . , in a study using a validated outcome instrument for knee oa based on arthroscopic visualization , suggest that the acr clinical classification criteria can be used to identify patients with early articular cartilage loss , before any radiographic changes are evident .
however , it is probable that the false positives include not only subjects with early oa not captured by the acr criteria , but also other causes of knee symptoms
the subjects were fairly well - educated , predominantly caucasian canadians with access to public health care and internet users .
this method of data collection may not be as effective in low - income populations , and those with decreased access to medical care  important since we asked about medically - diagnosed osteoarthritis as part of the case definition .
further , since recruitment and enrolment of subjects was via the internet , subjects were largely self - selected .
self - selection implies that the nature of the bias can not be known with certainty .
studies of subjects who participate in online research reveal that they are more likely to be older , females and have higher socioeconomic status .
many more individuals potentially view invitations to participate in research , with most declining to participate , making validity of results more challenging to interpret .
in online surveys , there is no single response rate - multiple metrics for calculating a response rate have been defined such as the participation rate and completion rate . however , the goal of this study was not to describe characteristics of the population at large , but to assemble subjects to test hypotheses about pa - related knee joint force and knee oa in a large sample of individuals who met criteria for a disease and those who did not , sampled in the same way ( internal validity ) . the limitations of our measure of self - report of pa measures and construction of the cpfi variable have been discussed elsewhere [ 4 , 5 ] . in short , self - reported pa measures require cautious interpretation because of large within - person variability and problems with recall [ 102104 ] , that may lead to nondifferential misclassification and attenuation of the effect size in analytic studies using the exposure .
in particular , this attenuation may have contributed to a lack of a significant finding from the sport domain , since the highest sport levels occurred in the distant past ( prior to age 25 ) for most subjects , are not part of the generic memory pattern ( shown to have better recall )   and thus may been imprecisely recalled . despite these limitations , it has been repeatedly shown that pa questionnaires are both practical and valid when used appropriately for large - scale epidemiologic studies [ 86 , 104 , 106 , 107 ] .
the cpfi , a time - force - bodyweight product , was a stronger predictor of knee oa than any of its component parts alone and is a new measure of pa - related force measured in joint loading units .
however , the cpfi does not separately and specifically capture elements of activity - related force that may be most injurious such as shear , rapid deceleration , or high - impulse loads .
activities where those elements of force occur ( e.g. , cutting and pivoting sports , jumping sports , and carrying heavy loads ) were captured , indirectly measuring these harmful types of load , but the strength of a potential signal from these forces may have been blunted .
another potential limitation related to recall is the possibility of recall bias , where the ability to recall past exposure is dependent on outcome status . of note , subjects with oa at baseline
this could lead to increased risk estimates , and while justified for the reasons outlined previously , the results remain vulnerable to this type of bias .
however , risk estimates for sport and occupational exposure as well as other covariates were generally in the expected direction and consistent with the literature including prospective data [ 4755 ] , lending validity to the findings .
regardless , the possibility of this bias must be acknowledged , and study results interpreted in light of this .
although this study provides evidence of an association between high levels of lifelong joint force , overweight / obesity , previous injury , bmi , and knee oa , the cross - sectional design makes the determination of a cause and effect more challenging .
however , the time window used for the main pa exposure ( prior to age 50 ) captures the ages [ 3045 ] with the highest level of lifetime force and is separated in time from knee oa diagnosis for the vast majority of cases . supporting this , most of the risk estimates for covariates reported in this study were in the expected direction and effect sizes consistent with the literature [ 4755 ] , and
lastly as this study is the first attempt to examine the effect of a new exposure measure ( quantitative lifelong joint load from all three primary activity domains ) on knee oa , a cross - sectional approach is reasonable .
the finding that most pa - related force is not related to knee oa , but that the highest levels of joint force are , is biologically plausible and fits within the conceptual framework of causation . under normal physiological conditions ,
the transmission and distribution of joint loads can occur for decades with little or no wear .
however , when normal joint physiologic mechanisms are overwhelmed via excessive local mechanical force , biologic events are triggered which destabilize the normal coupling of degradation and synthesis of articular cartilage and subchondral bone .
animal studies clearly illustrate that high joint force from pa affects cartilage metabolism and plays a role in the development of oa [ 110112 ] . in summary , a newly proposed measure of lifetime mechanical knee force was used to estimate the risk of self - reported knee oa .
while it must be interpreted cautiously because of the cross - sectional design and the possibility of recall bias , this study suggests that lifelong physical activity is generally safe .
high levels of lifetime knee force from occupational activity in men and women , and household activity in women were associated with knee oa .
prevention efforts may best be directed at occupations requiring high physical demands , at weight - control programs and injury prevention .
future research should further investigate the potential role of household activity , improve the estimation and validity of knee force measurement in new populations , and apply these measures in longitudinal studies .
 
 purpose . to investigate the association of cumulative lifetime knee joint force on the risk of self - reported medically - diagnosed knee osteoarthritis ( oa ) 
 . methods . exposure data on lifetime physical activity type ( occupational , household , sport / recreation ) and dose ( frequency , intensity , duration ) were collected from 4,269 canadian men and women as part of the physical activity and joint heath cohort study . 
 subjects were ranked in terms of the  cumulative peak force index  , a measure of lifetime mechanical knee force . 
 multivariable logistic regression was conducted to obtain adjusted effects for mean lifetime knee force on the risk of knee oa . 
 results . 
 high levels of total lifetime , occupational and household - related force were associated with an increased in risk of oa , with odds ratio 's ranging from approximately 1.3 to 2 . 
 joint injury , high bmi and older age were related to risk of knee oa , consistent with previous studies . 
 conclusions . a newly developed measure of lifetime mechanical knee force from physical activity 
 was employed to estimate the risk of self - reported , medically - diagnosed knee oa . while there are limitations 
 , this paper suggests that high levels of total lifetime force ( all domains combined ) , and occupational force in men and household force in women were risk factors for knee oa . 
 
1. Introduction
2. Methods
3. Results
4. Discussion