it is well accepted that levels of physical activity in older adults relate to physical and cognitive performance measures .
physical activity is promoted as beneficial for a variety of diseases affecting middle - aged and older adults .
activity levels , specifically locomotor activities , are also relevant to total joint arthroplasty ( tja ) surgery , both for the design of implants and for rehabilitation programs where postsurgical activity level is considered to be an important clinical outcome measure . in addition , differences in activity levels have been shown to affect the rate of polyethylene wear , a critical factor in implant longevity . specifically , walking cycles have been reported to influence wear rates of polyethylene implants [ 2 , 3 ] .  
activity levels in patients after total hip and knee arthroplasty have been studied extensively , and have been reported to average approximately 5,000 steps per day in these patients [ 46 ] , with one report suggesting that younger patients may attain an average of 6000 steps per day . in contrast , the currently recommended goal for daily physical activity in adults is 10,000 steps per day , or 30 minutes of brisk walking .
a meta - analysis examining step counts assessed by pedometer in adults found that activity levels are related to age with activity in adults 1865 approaching the 10,000-step recommendation while in those over 65 the mean daily step count was approximately 6500 steps .
while much can be gained from this meta - analysis it does not provide information specifically concerning activity levels in healthy older adults who are free from clinical evidence of osteoarthritis in the lower extremities .
such information would provide a realistic benchmark for activity levels in patients after joint replacement who often have expectations of returning to similar activity levels as their peers .  
this study sought to expand on existing literature regarding activity levels in adults by ( 1 ) characterizing the activity level of an older adult population without evidence of clinical osteoarthritis in the lower extremities , and ( 2 ) examining what factors might explain variance in activity levels in this population .
we specifically targeted healthy adults between the ages of 45 and 70 as future projections indicate that this age group will comprise a majority of total joint arthroplasty patients in the coming years .
in addition , we utilized an accelerometer - based activity monitor rather than pedometers which have been reported to vary in their accuracy   and monitored the subjects over a seven - day period so as to capture a representative daily step count that is derived from activity on both weekdays and weekends .
healthy , asymptomatic subjects aged 45 to 70 years , and with no history of fracture or surgery involving the lower extremities , were recruited for participation in the present study through advertisement and word of mouth with the approval of the institutional review board at rush university .
inclusion criteria were as follows : no clinical evidence of osteoarthritis in the lower extremities , no pain in the lower extremities , and no past history of surgery involving the hip , knee , or ankle bilaterally . to objectively assess pain in the hips and knees , subjects completed question no . 1 of the western ontario and macmaster universities osteoarthritis index ( womac )   which asks subjects to rate their pain in hip and knee joints on a visual analog scale ( vas ) .
subjects were included and classified as asymptomatic if they reported less than 10  mm ( of a maximum of 100  mm ) of lower extremity pain bilaterally at the hip and knee joints during level walking in response to this question .
41 women and 13 men ( out of 65 total recruited ) met the inclusion criteria for the study and were compliant with wearing the activity monitoring device for a seven - day period .
the subject 's strength and range of motion were assessed through gross manual muscle testing procedures and by goniometry by a trained physical therapist .
all subjects exhibited strength and range of motion in the lower extremities that were within normal limits .  
a standard gait analysis protocol was employed to obtain data on walking speed for each subject in the laboratory setting .
briefly , four optoelectronic cameras ( qualysis , gotenborg , sweden ) tracked the motion of six passive retroreflective markers , placed at the iliac crest , greater trochanter , lateral knee joint line , lateral malleolus , lateral aspect of calcaneus , and the head of the fifth metatarsal .
seven trials were collected for each subject , three trials at a self - selected  normal speed  , and two each at self - selected  fast  and  slow  speeds .
data presented in the present work correspond to the  self - selected  normal speed for each subject , so as to replicate normally walking as closely as possible in the laboratory setting .  
subjects donned an accelerometer - based activity monitoring device ( the amp 331 dynastream , innovations , cochrane , ab ) on their right ankle .
each subject wore the activity monitoring device for a period of seven consecutive days during all waking hours except when showering or bathing .
the device calculates the number of steps taken during level walking excluding stair - climbing activities .
the total number of steps taken during the one - week period ( 7 consecutive days ) was obtained , and a mean daily step count was computed .
the one - week time period was chosen to ensure that week days and weekend days were both included for all subjects .
step count accuracy of the activity monitor was validated against an optical tracking system ( qualisys , innovision systems , inc . ,
columbiaville , mi ) . in order to evaluate walking under normal conditions , eleven ( 5f/6  m ) healthy , non - tja volunteers age 28.8  5.5 ( range : 1941 ) years old and a bmi of 24.9  5.4  kg / m ( range : 19.939.0 
kg / m ) were asked to wear the activity monitor while walking on a treadmill ( motion analysis corp , california ) at a normal self - selected speed ( 1.1  0.2  m / s , 0.71.4  m / s ) .
in addition to the activity monitor , two reflective markers were placed on the toes and heel area of the left and right feet , allowing the tracking and determination of steps .
acceleration , top self - selected speed , deceleration , and travel distance of the treadmill were adjusted and programmed for each volunteer .  
intraclass correlation coefficients ( icc ( 2,1 ) ) were used to determine whether data from the activity monitor were in agreement with the optical tracking system .
an icc greater than 0.7 indicated the two measurements were statistically similar . in order to graphically depict the goodness of agreement , bland - altman plots were generated , in which the average difference ( =  bias  ) of the two methods is plotted against the mean of the two measurements .
the limits of agreement were computed as bias 1.96 standard deviation of the difference .   
the icc ( 2,1 ) between activity monitor and optical tracking system was 0.98 for step count and 0.76 for speed .
this suggested that the activity monitor was accurate enough to count steps under normal walking conditions for a variety of body mass indices . also speed was reasonably well correlated with the optical tracking system .
as shown in figure 1(a ) , the step monitor slightly overestimated the actual number of steps .
in contrast , the speed was underestimated by the monitor in most cases , on average by 9% ( figure 1(b ) ) .
a repeated measures anova was used to examine day - to - day variability in step counts for the group as a whole .
pearson correlations were utilized to examine the relationship between age , bmi , gait speed , and average daily step count for the study population as a whole .
in addition , a backwards elimination regression analysis was conducted with average daily step count as the dependent variable and age , gender , and bmi as independent variables to assess how these factors might simultaneously explain variance in activity level .
subjects were also subdivided into three groups based on their mean number of steps per day according to the classification scheme outlined by tudor - locke and bassett .
low activity group ,  those who averaged between 7500 and 9999 steps per day were considered a  somewhat active group ,  and those who averaged greater than 10,000 steps per day were considered to be an  active  highly active group  .
between - group comparisons of demographic data and gait parameters were made using an analysis of variance with bonferroni post hoc tests .
all statistical calculations were carried out using spss statistical software version 13.0 ( chicago , il ) .
activity data were collected on fifty - four subjects ( 41 women and 13 men ) who met inclusion criteria for participation and were compliant wearing the monitor for seven days .
the mean ( sd ) age of the subjects was 54.1  6.1 years , with a range of 45 to 68 years .
the mean ( sd ) body mass index ( bmi ) was 26.8  4.3  kg / m with a range of 17.4 to 38.6  kg / m . as a whole ,
the subjects averaged 61,691  25,274 steps over the seven - day period ranging from a minimum of 12,298 steps to a maximum of 148,180 steps .
the daily step totals translated into an average of 8813  3611 steps per day with a range of as few as 1757 steps to as many as 21,169 steps .
women averaged 8944  3899 steps / day and men averaged 9220  2330 steps / day ( p = 0.758 ) .
examination of the day - to - day variation in activity of the entire cohort revealed a sinusoidal pattern of activity level with step counts tending to rise from monday to tuesday then fall from tuesday to a low point on thursday ,  mid - week  slump , followed by a rise again to a peak on saturday with a fall again beginning from saturday to sunday .
this day - to - day variability , while not statistically significant ( p = 0.313 ) , reflects the characteristic pattern for the group as a whole ( figure 2 ) .   
pearson correlations revealed a significant positive correlation between gait speed and the mean number of steps per day ( r = 0.530 , p < 0.01 ) .
bmi was significantly negatively correlated with the mean number of steps per day ( r = 0.337 , p = 0.013 ) .
age was not significantly correlated with average daily step count ( r = 0.126 , p = 0.364 ) . to examine how age , gender , and bmi might explain variance in average daily step count when considered together , a backwards elimination regression analysis was conducted .
only bmi exhibited explanatory power in the regression model (  = 0.337 , adjusted r = 0.097 , p = 0.013 ) .   
evaluation of the activity levels of the group as a whole revealed an even distribution of the different activity groupings specified by tudor - locke and bassett   ( table 1 ) . when considered according to activity group , the  sedentary 
low - activity  group was younger than the  somewhat active  group ( p = 0.05 ) ( table 1 ) , and had a higher bmi ( p = 0.09 ) and walked significantly slower ( p = 0.044 ) than the  active  highly active  group ( table 1 ) .
the  moderately active  group trended toward slower walking speeds compared to the  active  highly active group 
the present work describes activity levels , in terms of average daily step count for a specific subset of adults , that is , older adults between the ages of 45 and 70 who are without any evidence of clinical osteoarthritis in the hips and knees . to the best of our knowledge ,
activity levels for this specific subset of adults are not available in the existing literature .
our rationale for selecting this specific group was to provide a realistic benchmark for activity levels in patients after joint replacement . in the population of the present work , the average number of steps taken per day was close to 9000 , which is approximately 4000 steps per day greater than has been previously reported in individuals after total joint arthroplasty [ 47 ] and approaches the recommended 10,000 steps suggested as a goal for physical fitness .
these data suggest that patients who undergo arthroplasty do not typically attain the same level of physical activity as age - matched adults without joint disease .
the present findings differ from previous questionnaire - based reports that did not find such differences in physical activity after total hip arthroplasty ( tha ) patients when compared to age and sex - matched healthy adults [ 14 , 15 ] .
two potential reasons for this difference are the method of data collection ( i.e. , accelerometer - based step counts versus questionnaire ) and the younger age of the population studied in the present work .
franklin et al . reported that older age and high bmi increase the likelihood of poor functional gains after total knee arthroplasty . in the present work , involving healthy adults between 45 and 68 years of age , those with a higher bmi tended to be less active ; however , age did not explain variance in average daily step count .
our results are consistent with existing literature that suggests that bmi is inversely correlated with physical activity in patients after knee and hip replacement [ 17 , 18 ] .  
accelerometer - based activity monitors have been used for the quantification of activity levels ( based on step count ) in healthy , osteoarthritis , and tjr populations [ 3 , 1823 ] .
the high cost of these monitors is one of the main disadvantages when compared to low - cost waist - mounted pedometers ; however , the availability of multiple locomotion parameters and their accuracy and the detailed information provided have to be considered in the overall cost - benefit comparison . in this investigation the results of the amp monitor validation showed a high accuracy ( icc 0.98 ) of step counts over a normal range of walking speeds ( 0.71.4  m / s ) . the lower - accuracy , but still acceptable , speed measurements ( icc 0.76 ) could be attributed to a not fully natural treadmill walking ; from which monitor acceleration discrepancies may have been generated , affecting speed estimations .
the limitations of treadmill walking are recognized ; however they allow the analysis of locomotion parameters in a controlled manner .  
one limitation in this study is the fact that subjects knew their activity was being monitored and this potentially may have caused an artificial increase in activity level . in an effort to address this issue , subjects were required to wear the activity monitor for a period of seven consecutive days as opposed to one or two days immediately following the gait test , in the belief that a longer monitoring period might negate this effect . a second potential limitation of this work is the skewed distribution of males and females within the study population . when analyzed separately ,
the average daily step counts for men and women were not significantly different and thus the data from both genders were pooled .
female gender is a risk factor for developing knee osteoarthritis and thus one could argue the data from the current work might be more applicable to knee than hip arthroplasty .  
it is projected that patients under the age of sixty - five will account for more than 50% of all tha and total knee arthroplasty ( tka ) patients by the years 2011 and 2016 , respectively .
specifically , the fastest growing age group for tka is individuals aged 4554 , with the demand projected to increase 17-fold by 2030 .
in addition , candidates for joint replacements are reporting increased participation in regular physical activity as compared with those in decades past .
research suggests patients ' preoperative expectations for participation in leisure activities and recreational sports do not match their postoperative capabilities at 1 and 5 years after total knee replacement surgery .
while patients need to be encouraged to have realistic expectations for surgical outcomes , it is imperative to understand the activity level of adults in this age group who have pain - free , healthy knee , and hip joints , in order to provide a benchmark when designing rehabilitation programs to promote postoperative functioning . ultimately , to reach the goal of returning such patients to a functional state comparable to their peers , as well as designing implants which can sustain such levels of physical functioning , a clearer understanding of activity levels in both the elderly and in patients who are progressively younger at the time of surgical intervention is required .
 this work evaluated activity levels in a group of healthy older adults to establish a target activity level for adults of similar age after total joint arthroplasty ( tja ) . with the decreasing age of tja patients , it is essential to have a reference for activity level in younger patients as activity level affects quality of life and implant design . 
 54 asymptomatic , healthy older adults with no clinical evidence of lower extremity oa participated . the main outcome measure , average daily step count , was measured using an accelerometer - based activity monitor . on average 
 the group took 8813  3611 steps per day , approximately 4000 more steps per day than has been previously reported in patients following total joint arthroplasty . 
 the present work provides a reference for activity after joint arthroplasty which is relevant given the projected number of people under the age of 65 who will undergo joint arthroplasty in the coming years . 
1. Introduction
2. Methods
3. Results 
4. Discussion 