osteoarthritis ( oa ) , the most common form of arthritis , is also the most common type of 
 musculoskeletal disorder .
while oa can affect virtually all joints , knee oa is of particular 
 interest since it has the potential to severely limit mobility1,2,3,4 .
severity of knee oa can be 
 determined radiographically using the kellgren - lawrence criteria5 .
current treatment strategies fail to prevent progression of 
 knee oa , and therefore the current therapeutic goal is to limit pain and disability6 . for assessing functional disability in oa 
 an objective measurement method is desired .
accelerometry is a practical , cost - effective and 
 repeatable method of reliably evaluating human movement of individual body parts or the 
 whole body without any radiation exposure7,8,9 . 

accelerometers can be used for observing a range of different movements such as postural 
 sway and falls , sit - to - stand transfer and gait9 .
equal values of gait variables on both sides of the body are the 
 definition of gait symmetry10 , 11 . in this context ,
pathological gait patterns are reflected 
 by a certain amount of gait asymmetry . however , there is no general agreement about when 
 gait asymmetry should be considered pathological , although it is frequently assessed and 
 described in the pre- and postoperative clinical evaluation of patients , as well as in 
 general rehabilitation .
for example , vogt et al.11 pointed out that gait asymmetry is often present in patients with 
 hip oa .
an arbitrary cut - off value of 10% difference from perfect symmetry has been reported 
 as a criterion of asymmetry in gait12 , 13 .
this 10% criterion , however , has been 
 considered invalid because of its non - parameter - specific nature13 .
visual gait analysis is used in the context of functional 
 capacity evaluation ( fce ) .
it is known as a clinical tool used internationally for the 
 purpose of predicting a safe return to work .
fces are standardized batteries of tests in 
 which a participant s functional ability is tested , analyzed and compared to the required 
 physical job demands11 , 13 , 14 . for 
 quantitative analyses of gait symmetry or asymmetry , statistical differences between groups 
 or limbs are used11 .
however , gait 
 symmetry or asymmetry should be evaluated in footfall measures and the movement of the upper 
 body .
accelerometer - based gait analyses of knee oa has to be established as a valid 
 criterion of gait asymmetry that is easily measured and interpreted , making it possible to 
 use in the rehabilitation of individual patients in clinical practice as well as in 
 research . a wide variety of accelerometer tools are used to estimate human movement , but 
 there are no adequate data relating to gait symmetry parameters in the context of knee oa . 

the aim of our study was to assess and describe 3d - accelerometer/3d - gyroscope based 
 parameters that are able to differentiate between pathological and normal gait patterns as 
 well as distinguish gait symmetry from gait asymmetry .
twelve patients ( 2 female , 10 male ) with knee oa diagnosed by a physician , confirmed 
 radiographically15 , 16 , and graded with the kellgren - lawrence scale ( 1 to 4 ; grade iii ( 7 
 patients ) and grade iv ( 5 patients ) ) participated in this study .
patients were excluded if 
 they had neurological , vestibular or musculoskeletal disorders , fracture of the lower 
 extremity , rheumatoid arthritis , generalized osteoarthritis , limping gait or any condition 
 that may have influenced a treadmill walking evaluation .
the characteristics of the included 
 patients were ( mean  sd ) : age , 44.4  7.6  years ; body weight , 75.5  11.1  kg ; height , 171.1 
  6.5  cm ; and body mass index , ( bmi ) : 26.9  3.2  kg / m . a control group of 7 
 healthy persons was also tested ( 1 female , 6 male ) .
their characteristics were ( mean  sd ) : 
 age , 41.7  8.8  years ; body weight , 73.7  9.8  kg ; height ,
175.6  7.2  cm ; and body mass 
 index , 26.1  2.9  kg / m .
this study was approved by the ethics committee of the 
 university medical center of goettingen .
the method of this study conformed to the principles of the 
 declaration of helsinki .
first , kinematics data were obtained using a six - camera optoelectronic system ( vicon 460 , 
 oxford metrics ) . using kistler force plates ( type 9287a , kistler instrumente ag , 
 switzerland ) , kinetic data was obtained .
when the magnitude of vertical ground reaction 
 forces exceeded 2% of a subject s body weight , gait cycles were normalized ( 0100% ) between 
 two successive foot contacts .
the motion sensors 
 ( freescale semiconductor , type mma76260q ) were three tri - axial accelerometers and three 
 gyroscopes ( adxrs 300 , analog devices ) connected to a portable data logger ( glonner 
 electronic noraxon oy ) .
body acceleration and angular velocity were recorded 
 three - dimensionally ( 3d ) in the anteroposterior , vertical and mediolateral directions .
the 
 accelerometers are piezo - electric sensors that were coupled to amplifiers (  2  g ) .
angular 
 velocity is measured by a rate gyroscope through the coriolis force and can be measured 
 directly11 , 15 .
the measurement sensors , the accelerometer and gyroscope , were placed 
 inside a container that was firmly attached to the individual by means of a flexible belt 
 around the waist over the middle part of the lower back over the l3 process so that it was 
 attached close to the centre of gravity of the human body .
participants had to feel free to 
 move , with respect to the skin . according to the marker positioning of the optoelectronic 
 system ,
the other two sensors were fixed to rigid bodies which were attached by belts to the 
 lateral malleoli of both legs , as described in previously published studies16 , 17 .
the 31 reflective markers of the optoelectronic system were attached 
 to anatomical locations according to the vicon plug - in - gait marker placement protocol . 

additionally , six reflective markers were attached to the rigid plates of the combined 
 accelerometers and gyroscopes for correlation and evaluating accuracy with the gold standard 
 video analysis .
after calibration of the systems , all participants walked 500  m on a 
 treadmill ( hp cosmos , germany ) at self - selected speed while acceleration and angular 
 velocity were measured using the setup described above .
after the experiment , all obtained 
 data ( 500  m walking distance ) were downloaded to a computer and converted into earth 
 acceleration units ( g ) following a standard calculation .
subsequently , data processing was 
 conducted with noraxon software ( noraxon myoreasearch 2.02 , noraxon , germany ) .
data 
 acquisition and post - processing for the optoelectronic system was done on a vicon - pc using 
 vicon software ( vicon body - builder 3.5 ) .
the generated acceleration data was post - processed 
 using a lowpass filter , fast fourier transformation ( fft ) , and spectral analysis to analyze 
 differences in trunk and locomotor limb acceleration .
the output of a stationary accelerometer s 
 sensing axis aligned with the global vertical must correlate to 9.81 m / s . in 
 this context
, a two - point linear calibration can be performed that transforms the raw output 
 to units of acceleration once a raw accelerometer output has been obtained for the static 
 condition of 9.81 m / s .
calibration procedures were similar to the two - point 
 method with a slope - intercept and zero - span , assuming linearity between the raw output and 
 acceleration .
for the gyroscopes , dynamic calibrations were performed by sensor rotations of 
 90 and 180 within a known time .
drift correction and calibration was done according to the 
 literature17 , 18 , 20 .
for further details on 
 calibration , see veltink et al20 . 
 according to previous studies11 , 13 , 14 ,
symmetry parameters were calculated for footfall and trunk 
 measurements . the angular velocity (  ) was recorded . by integrating the angular velocity 
 over time ,
accelerometers primarily measure 
 accelerations ( a ) , but integration over time allows determination of 
 velocities [  =  a   ( m / s ) ] and paths [ s =   a   
 =     ( m ) ] . following these principles , we measured accelerations and angles as 
 described above .
spatiotemporal walking speed ( distance per walking time ) and cadence 
 ( number of steps per walking time ) were determined .
the step time asymmetry was calculated 
 to determine differences between left and right leg movements .
using fft , front acceleration 
 data as well as mediolateral angular velocities were evaluated as a power spectrum on a 
 spectral frequency scale ( 1/sec ) .
it is a known fact that many gait variables change with 
 walking speed14 . to differentiate between 
 gait abnormality and the effect of different walking speeds , variables need to be controlled 
 for the effect of speed .
for this reason , every participant was asked to walk back and forth 
 at three steady - state speeds ranging from slow to fast ( a total of six walks ) , allowing 
 individual linear trend lines to be calculated over the speed range demonstrated by the 
 individual participants18 .
this data 
 normalization allowed inter - individual comparisons among the study participants , even though 
 the actual walking speeds were self chosen .
the patient group ( n = 12 ) and the control group ( n = 7 ) were 
 compared using unpaired t - test with a significance level of p = 0.05 .
statistical analyses 
 were performed using commercially available software ( spss 17 for windows , spss inc . , 

the fast fourier transform function within the analysis package of matlab 
 ( matlab 7.0 , mathworks , inc .
, concord , ma ) was used in the evaluation of power spectra . to 
 detect linear correlations between the recorded parameters ,
1 . the comparison of 3d - accelerometry ( blue lines ) with video analysis 
 ( vicon , red lines ) shows a significant correlation over time . shows the comparison of the velocities measured by accelerometers in the lumbar 
 region ( l3 ) with those derived from video analysis measured by the vicon system . 

2 . the comparison of 3d - gyroscopes ( blue lines ) with video analysis ( vicon , 
 red lines ) shows a significant correlation . shows the comparison of mediolateral lumbar ( l3 ) angles measured by gyroscopes with 
 those derived from measurements with the vicon video analysis system .
the root 
 mean square difference was used to evaluate the closeness of the results of video analysis 
 ( vicon ) with those of the accelerometer / gyroscope - based analysis .
linear regression analysis 
 revealed a significant correlation between video analysis results and the 3d - gyroscopes and 
 accelerometers analysis results ( r=0.69 ; p=0.01 ; fig . 1 and fig . 
 2 ) .
as shown in figs . 1 and 2 , errors increased at the highest walking speed for 
 the accelerometer data and peaks of angular velocities measured by gyroscopes , probably due 
 to the sensors being jarred or vibrated by heel strike .
the patients in the knee oa group 
 were older , heavier and shorter than the subjects in the control group , but no significant 
 differences in the significance level ( p values > 0.05 ) were found between the two 
 groups .
preferred gait velocity was significantly lower in the knee oa group ( 0.74  0.08 
 meters per second ) than in the control group ( 1.27  0.14 meters per second ) ( p < 0.001 ) . 
 controlling for differences in age ,
the patients walking speeds were significantly reduced 
 at higher ages ( p < 0.01 ) .
cadence was significantly ( p < 0.001 ) reduced in the knee oa group ( 97.8  11.5 steps per 
 minute ) compared to the control group ( 116.3  7.4 steps per minute ) . for trunk symmetry , 
 the mediolateral ( ml ) angles of the lumbar region ( l3 ) determined by gyroscopes were 
 significantly ( p < 0.001 ) higher in the knee oa group ( 9.4  2.2 ) than in the control 
 group ( 4.3  1.9 ) . for locomotor limb symmetry of the footfall ( ff ) measures , angular 
 differences between both shanks
were significantly ( p < 0.001 ) higher in the knee oa 
 group ( 11.7  1.6 ) than in the control group ( 3.1  2.3 ) . despite this , ff time 
 differences between the limbs were significantly ( p < 0.001 ) higher in the knee oa group 
 ( 5.7  2.8  msec ) than in the control group ( 1.2  0.9  msec ) . gait asymmetries were 
 significantly higher ( p < 0.001 ) in the knee oa group than in the controls . using the 10% 
 criterion ,
the knee oa patients walked with 
 lower cadence ( p < 0.001 ) , showed more asymmetric trunk acceleration ( ml ) ( p < 0.001 ) 
 and the stance phase of the affected limb was longer than that of the unaffected limb ( p 
 < 0.001 ) , as shown by the ff time differences between both legs .
the summation of an infinite number of 
 sine and cosine waves of different frequencies and amplitudes created from the movement of 
 different anatomical structures can be thought of as a frequency signal resulting from 
 ground reaction forces .
spectral visualization of normal gait ( control group , a 46-year - old male ) .
the blue 
 lines show 3d - accelerometer measurements in the anteroposterior direction , and the red 
 lines show 3d - gyroscope measurements in the mediolateral direction .
the plot shows two 
 peaks , one each for gyroscope and accelerometry . shows the spectral visualization of normal human gait derived from measurements by 
 3d - accelerometry and gyroscopes .
it shows two peaks , whereas the trace of the knee oa group , 
 fig .
spectral visualization of abnormal gait ( knee oa group , a 50  year - old male ) .
the 
 blue lines show 3d - accelerometer measurements in the anteroposterior direction , and 
 the pink lines show 3d - gyroscope measurements in the mediolateral direction .
there are 
 three peaks , two measured by 3d - accelerometry , and one measured with a 3d - gyroscope , 
 indicating abnormal values due to an asymmetric gait pattern .
the comparison of 3d - accelerometry ( blue lines ) with video analysis 
 ( vicon , red lines ) shows a significant correlation over time .  
the comparison of 3d - gyroscopes ( blue lines ) with video analysis ( vicon , 
 red lines ) shows a significant correlation .  
spectral visualization of normal gait ( control group , a 46-year - old male ) .
the blue 
 lines show 3d - accelerometer measurements in the anteroposterior direction , and the red 
 lines show 3d - gyroscope measurements in the mediolateral direction .
spectral visualization of abnormal gait ( knee oa group , a 50  year - old male ) .
the 
 blue lines show 3d - accelerometer measurements in the anteroposterior direction , and 
 the pink lines show 3d - gyroscope measurements in the mediolateral direction .
there are 
 three peaks , two measured by 3d - accelerometry , and one measured with a 3d - gyroscope , 
 indicating abnormal values due to an asymmetric gait pattern .
the aim of this study was to explore 3d - accelerometer and 3d - gyroscope parameters that have 
 the potential to differentiate between normal gait and pathological gait .
spectral analysis 
 provides a simple evaluation method , which is able to differentiate between normal and 
 pathological gait .
this sensor - based method showed good measurement reliability of the five 
 parameters that exhibited significant differences between the knee oa and control 
 groups . according to previous studies16 , 17 , body - mounted accelerometers and 
 gyroscopes give results in kinematics analyses that are comparable to those of vicon  video 
 analyses .
as shown in figs . 1 and 2 , errors increased at the highest speed for the 
 accelerometer data or peaks of angular velocities measured by gyroscopes , probably due to 
 the sensors being jarred or vibrated by heel strike . this should be considered when 
 designing future applications for accelerometers and gyroscopes . however , using body - mounted 
 accelerometers and rate gyroscopes is as accurate as the gold standard video analysis17 , as a physical method for collecting 
 kinematic data of healthy subjects as well as patients with knee oa . in addition , 
 accelerometers and gyroscopes used as body - mounted sensors are inexpensive and , combined 
 with a portable data - logger , fulfill all criteria for a fully portable system that can be 
 used in almost any environment .
comparable systems have been used in prior biomechanical 
 studies16 , 17 , 19 , 21 .
our body - mounted system could also be used in sports situations 
 with accelerations similar to those of walking because of its portability .
furthermore , this 
 system could be attached to participants undergoing fce observations to obtain quantitative 
 data .
huisinga et al.22 reported that 
 acceleration of the trunk during walking by patients with multiple sclerosis ( ms ) had larger 
 frequency dispersion in the ml direction , than in the ap direction .
this agrees with our 
 result that ml differences from normal gait are indicative of a pathological gait pattern . 

unsteady walking speeds can lead to an increase in trunk acceleration . in the present study , 
 however , participants walked at a constant speed during the testing . according to the study 
 by turcot et al.17
, high accelerations in 
 the ml direction may be a consequence of joint instability due to knee oa and an unstable 
 limb alignment .
ogata et al.23 reported 
 lateral acceleration generated by initial foot contact was caused by varus deformity in 
 medial knee oa patients .
therefore , ml differences in patients with unilateral knee oa 
 measured with 3d gyroscopes and represent compensatory posture , as observed in the present 
 ( table 1table 1 .
comparison of gait parameters between the study groups parameterknee oa group ( 12 patients)control group ( 7 participants)gait velocity ( m / s)*0.74  0.081.27  0.14cadence ( steps / min)*97.8  11.5116.3  7.4ml trunk symmetry ( angular degree)*9.4  2.24.3  1.9limb symmetry ( angular degree)*11.7  1.63.1  2.3limb symmetry ( time , msec)*5.7  2.81.2  0.9 * p<0.001 ) .
linear accelerations observed in this study were estimated at the same 
 functional location for each participant rather than an arbitrary or changing location on 
 the segment as in previous studies11 , 17 , 23 .
estimation of linear accelerations is less affected by angular 
 components induced by the movement of segments during gait when transposing accelerations 
 close to joint contact surfaces17 . 

estimation of internal acceleration may be affected by the vector joining sensor to bone . to 
 verify the sensitivity of the method , we compared the data of the sensor - based , body - mounted 
 system with vicon video analysis .
the results show a difference in 
 3d - acceleration and gyroscope measurements at peak magnitude and angles less than 5 ( figs . 1 and 
 2 ) .
these results are in line with the results of mayagoitia et al16 . in the present study , between - limb 
 differences in ff measures
furthermore , our results showed a certain amount of gait asymmetry was also 
 present in the controls ( table 1 ) , due to the 
 fact that values of either the 3d accelerometer or 3d gyroscope measurements would tend to 
 zero representing perfect symmetry16 , 17 , 23 .
sadeghi et al.24 
 showed that a certain amount of gait asymmetry is also present in healthy subjects and may 
 reflect functional between the limbs differences . for differentiating subjects with 
 pathological gait asymmetry from subjects without pathological gait asymmetry , the reported 
 cutoff criteria of gait asymmetry are too strict . in our present study ,
a general 10% 
 criterion was used to assess differences between groups gait asymmetry . this is preferred 
 because it is easily derived in clinical and research settings16 , 24 .
the lack of 
 standardization of the placement location of accelerometers can explain the variability of 
 the results reported in previous studies16 , 17 , 23 , 25 , 30 .
benoit et al.25 
 reported that skin movement artifacts originate from the skin moving over the 
 musculoskeletal structure .
for this reason , the movement of a subject s skeleton may not 
 exactly correlate with the measured movement on the skin where the sensors are attached .
applying an algorithm , as used in the 
 present study , to the measured movement data during post - processing , can remove most of the 
 skin movement25,26,27 .
sensors can be applied 
 in different ways to the participants using medical tape , wrapping bandages , velcro , plastic 
 plates , elastic straps and bone pins28 . 
 in line with previous studies
, we used double - sided medical tape to attach the sensors to 
 plastic plates and then fixed them to the subjects with tape17 , 27 , 28 .
the relaxation and contraction of the muscles underneath the 
 straps can cause their positions to change during locomotion when sensors are fixed to 
 elastic straps or velcro29 .
previous 
 studies8 , 16 , 
 17 have used accelerometers to measure 
 the acceleration experienced at impact at sampling rates of 10 khz .
these high sampling 
 rates are not required when the measurement of the impact transmitted by a subject s limb is 
 done proximal to the point of contact . in line with the study by henriksen et al.30 we used a sample rate of 1 khz , 
 significantly less than 10 khz . for data post - processing of longer distance
walks as used in 
 the present study , of about 400  m and more , a considerable amount of data is collected . in 
 this context , acceleration data can be post - processed using a low pass filter , fft , and 
 spectral analysis to observe differences in trunk acceleration to distinguish normal from 
 pathological gait ( figs . 3 and 4 ) .
our study data support the hypothesis that differences in ff 
 measurements occur due to unilateral knee oa , and the consequent adoption of a compensatory 
 posture .
the 3d - kinematic system is objective , inexpensive , accurate , and portable , and 
 allows long - term recordings in clinical , sport , ergonomic and fce settings .
for fast 
 post - processing , spectral analysis of the obtained data is recommended .
   [ purpose ] a wide variety of accelerometer tools are used to estimate human movement , but 
 there are no adequate data relating to gait symmetry parameters in the context of knee 
 osteoarthritis . 
 this study s purpose was to evaluate a 3d - kinematic system using 
 body - mounted sensors ( gyroscopes and accelerometers ) on the trunk and limbs . 
 this is the 
 first study to use spectral analysis for data post processing . 
 [ subjects ] twelve patients 
 with unilateral knee osteoarthritis ( oa ) ( 10 male ) and seven age - matched controls ( 6 male ) 
 were studied . 
 [ methods ] measurements with 3-d accelerometers and gyroscopes were compared 
 to video analysis with marker positions tracked by a six - camera optoelectronic system 
 ( vicon 460 , oxford metrics ) . 
 data were recorded using the 3d - kinematic system . 
 [ results ] 
 the results of both gait analysis systems were significantly correlated . 
 five parameters 
 were significantly different between the knee oa and control groups . to overcome time 
 spent in expensive post - processing routines , 
 spectral analysis was performed for fast 
 differentiation between normal gait and pathological gait signals using the 3d - kinematic 
 system . 
 [ conclusions ] the 3d - kinematic system is objective , inexpensive , accurate and 
 portable , and allows long - term recordings in clinical , sport as well as ergonomic or 
 functional capacity evaluation ( fce ) settings . for fast post - processing , spectral analysis 
 of the recorded data is recommended . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION