proske and 
 gandevia described proprioception as collective information from specialized peripheral 
 neural endings , located at the muscle spindle , golgi tendon , or joints , which are capable of 
 alerting the central nervous system as to joint position , muscle force , and movement via the 
 dorsal roots of the spinal cord1 . in 
 addition , the sense of force , as one of the submodalities of proprioception , has also been 
 described as tension during the contraction that plays an important role in the quality of 
 motor function2 . disruption in proprioception would be regarded as a risk factor to the neuromuscular 
 system , and may result in a variety of injuries . for instance , age - related deficits in 
 proprioception of the knee joint reportedly play an important role in initiating and 
 advancing osteoarthritis of the knee3 , 4 . based on the literature , there are two main approaches used to measure the sense of force . 
 the first attempts to reproduce a target force as a certain level of maximum voluntary 
 isometric contraction ( mvic ) of a muscle group in one limb or a reference side , and is known 
 as the ipsilateral remembered method ; the second aims to reproduce the same target force in 
 the opposite limb , and is known as the contralateral method5,6,7 . evaluation of such a reproduced force sense requires a precise device with the capability 
 of measuring muscle tension in both limbs either at the same time , or within a very short 
 period of time . for the lower limb , the available instruments require the subjects to change 
 their position during the force sense survey . this would interfere with the accuracy of 
 results , as the crucial time frame allocated to each measuring test might be inadvertently 
 exceeded . furthermore , as the majority of knee studies have been devoted to other areas such 
 as sense of position , pathokinesiology , kinesthesia , or compromised electromyography ( emg ) 
 patterns of knee muscles8,9,10,11 , few academic studies have addressed all major methods of 
 force sensing measurement , including ipsilateral remembered ( ir ) , contralateral concurrent 
 ( cc ) , and contralateral remembered ( cr ) . because the knee is one of the joints most vulnerable to sports injuries , and due to the 
 lack of a comprehensive measuring device for use in force sense reproducing tasks , a new 
 device was designed , and its reliability for measuring sense of force was thoroughly tested . the need for a practical , low - cost device that can target both knee joints simultaneously or 
 separately , with ease of use and a high rate of accuracy , unlike currently available 
 isokinetic devices , motivated us to design such a clinically applicable instrument ; this 
 device does not require highly specialized operators or application of any invasive method 
 to achieve results . eighty students from tabriz university of medical sciences were included in our initial 
 group of participants , and 54 met our inclusion / exclusion criteria . thirty participants ( 8 
 female and 22 male students ) were selected randomly ( simple randomization ) from this group 
 of 54 eligible candidates by using randlist 1.2 software ( registration code : 1500247846 ) . the sample size was calculated by considering =0.05 , power=80% , and r=0.70 , with a 10% 
 difference . inclusion criteria were : no history of professional sports activities , age between 2030 
 years , and body mass index ( bmi ) between 2225  kg / m . the exclusion criteria 
 were : a history of knee pain and limited range of motion within the last six months , 
 deformities , fractures , arthritis , and any systemic or neurological disorder . then , a voluntary consent form was 
 signed by each participant to be officially considered for this study . the study protocol 
 was approved by the ethics committee of shahid beheshti university of medical sciences 
 ( tehran , iran ) . the device consisted of an adjustable chair , two dynamometric mechanical arms equipped with 
 hinged plates , two korean manufactured load cells ( cp 500 newton model from www.dacell.com ) , 
 one usb port for intercommunication between analogue and digital components of the device 
 using the c++ software program ( microsoft matlab ) , math works ( a sony laptop ) , vaio care 
 software , e series laptop ( 4192 - 71011 ) , and connecting cables . matlab software ( microsoft matlab , 
 math works ) was utilized for trend data analysis , mean calculation , and data classification 
 purposes ( fig . illustration of the designed device and reference position during the measurement 
 process before starting the actual tests , the accuracy of the load cells was checked by using sets 
 of paired standard weights including 1 , 2 , 3 , 5 , 10 , 15 , 20 , 25 , and 30  kg . the results are shown in table 1table 1.results of one - sample t - test for standard weights , using two load cellsstandardweightsloadcellsload cell 1load cell 2(n)(n)1  kg=9.8 n9.8  0.0049.8  0.0042  kg=19.6 n19.6  0.00519.6  0.0043  kg=29.4 n29.3  0.00529.4  0.0035  kg=49 n49.0  0.0449  0.0210  kg=98 n98.0 0.06196  0.0625  kg=245 n245  0.007245  0.0430  kg=294 n294  0.007294  0.04data are shown as mean  sd , n : newton . data are shown as mean  sd , n : newton to minimize tactile feedback12 caused by 
 direct contact between the lower leg and the plates , the distal portion of the lower leg was 
 protected by a thick foam pad and a piece of thin leather used to cover the surface of each 
 hinged leg plate . the device chair and dynamometric mechanical arms were adjusted to support 110 trunk 
 backward extension and 60 knee flexion , respectively . each hinged leg plate was positioned just above the middle 
 malleolus of the corresponding leg . the plates were attached to the lower leg with velcro 
 straps , and excessive movement was prevented by two straps fastened around the thighs and 
 across the trunk ( fig . the 
 dominant limb was determined by the subjects ; the reference position was demonstrated ; 
 verbal commands and visual feedback were explained . the study protocol included detection of maximum voluntary isometric contraction ( mvic ) and 
 two main measuring approaches known as ir and contralateral methods . the latter approach 
 contained two subgroups known as cc and cr methods . in order to detect mvic , three single , strong voluntary isometric contractions , with 
 60-second rest intervals , each verbally prompted contraction lasted for 5 seconds , and the highest value 
 was taken as the mvic . later , subjects were asked to exert only 50% of their recorded mvic 
 as a target force through the producing and reproducing phases of each contraction set4 , 12 , 13 . each set of the ir method included a visually - assisted produced target force for up to 8 
 seconds in the reference knee joint , and a 5-second rest period , followed by a similar 
 contraction quality of reproduced target force within the same side minus a visual cue . the contralateral method was subdivided in two parts , i.e. , cr and cc methods . while each 
 set of the cr method consisted of a visually - assisted produced target force in the reference 
 limb , a 5-second rest period , and a non - feedback assisted reproduced target force on the 
 opposite side , each set of the concurrent method followed a similar protocol , but with no 
 rest period . both methods asked subjects to sustain their produced / reproduced target force 
 for the entire 8 seconds4 , 12 , 13 . subjects were to alert the assessors when reaching their assumed target 
 force , and had to sustain this to the final second . although subjects had to complete every 
 method in the study protocol , this process was not accomplished in any particular order , and 
 methods could be covered randomly by each participant . participants received 5-minutes rests 
 between any two methods , and a 3-minute rest before initiating the next set of contractions 
 within the same method . to assess the test reliability in frequent measurements of force sense reproduction tasks , 
 subjects were instructed to complete all study procedures twice , with only a 72-hour gap 
 between test dates . in total , three strong contractions were enough to determine the mvic in 
 extensor and flexor muscle groups of the knee joint separately before executing 36 
 contractions , each equal to 50% of the mvic . therefore , the subjects were expected to cover 
 3 methods including [ ir , cc , and cr ] 3 sets 2 contractions 2 different muscle groups . in order to ensure accuracy and specificity of the device , a team including a 
 physiotherapist , designer , and biomechanical and electronic engineers met to evaluate any 
 force sense matching error through the study period . the accuracy of the usb port was also as high as possible , with 16-bit resolution , and a 
 sampling rate of 64 per second . the device calibration was also measured by linear regression and a one - sample 
 t - test , as shown in table 1 . the 
 data distribution was evaluated using the shapiro - wilk test , and deemed normal after an 
 acceptable minimum score of 0.112 was achieved . the difference between each produced and 
 reproduced target force was recorded for any set of contractions , and considered as an 
 absolute value to disregard the direction of the error . the average of these values for 
 every three sets of contractions in any method ( flexor / extensor ) was later calculated to 
 determine the absolute error of the mean for that method . in order to normalize the error values as a fair percentage of the recorded mvics for all 
 participants , the error of the mean for each method was divided by the mvic and the result 
 was multiplied by 100 [ the error of the mean / mvic 100]6 , 13 , 14 . the values of the intraclass correlation coefficient ( icc ) and actual units of standard 
 error of measurement or , shown in table 3table 3.the mean standard deviation , interclass correlation coefficient ( icc ) , standard 
 error of measurement ( sem ) , and smallest detectable difference ( sdd ) for three force 
 sense measuring methods of knee flexor and extensor muscle groupsvariablemeasuring methodsfirst sessionsecond sessioniccsemsddmean  sdextensor musclesir2.06  0.890.81  1.920.760.381.05cc2.10  0.902.20  0.990.80.481.32cr2.97  1.413.13  1.520.80.711.96flexor musclesir2.50  1.212.76  1.240.730.451.24cc3.01  1.382.83  1.340.810.371.02cr3.15  1.463.20  1.560.730.742.04ir : ipsilateral remembered , cc : contralateral concurrent , cr : contralateral 
 remembered , indicate the relative absolute reliability of this study . to determine the 
 actual changes in force reproducing error , ir : ipsilateral remembered , cc : contralateral concurrent , cr : contralateral 
 remembered a paired samples t - test was conducted to compare the mean variables obtained from different 
 dates of measurement , and spearman s correlation coefficient test was used to investigate 
 the existing correlation . demographic characteristics of participants and mvic mean value of flexor and extensor 
 muscle groups are shown in table 2table 2.demographic characteristics and mean maximum voluntary isometric 
 contraction variablevariable levelsmean  sd age ( years)23.4  2.5bmi ( kg / m ) 2.1  9.22mvic ( knee flexors)day 1173.3  33.7 nday 2167.8  25.4 nmvic ( knee extensors)day 1336.2  73 nday 2348.7  65 nmvic : maximum voluntary isometric contraction ; mvic : maximum voluntary isometric contraction ; n : newton in evaluating the calibration with a linear regression test , r square=1 and r square=0.999 
 were used for load cells one and two , respectively . the highest confidence rates were attributed to methods 
 cc=0.81 and cr , and cc=0.80 for knee flexor and extensor muscle groups , respectively ( table 3 ) . the pooled data did not show a statistically significant difference between the first and 
 the second date of the study ( paired sample t - test and spearman s correlation coefficient 
 test ) . however , the average value of mean  sd and sem for the cr method was higher for both 
 muscle groups in this experiment . the least value for sdd was attributed to the cc method in 
 the flexor muscle group , at 1.02 , while the cr method in the flexor muscle groups had the 
 highest sdd rate , at 2.04 . absence of meaningful differences across values of mean      sd for the 
 reproduced target force ( p<0.05 ) were convincing enough ; that no systemic bias could 
 interfere with the results . the device reliability for assessing force sense reproduction 
 across the different study test methods , 0.73 to 0.81 , was consistent with what munro 
 previously described as a high class of reliability16 . in this classification , icc ranges from 0 to 0.25 and 0.26 to 0.49 
 are known as poor and weak , respectively , while a moderate icc must show a range of 0.50 to 
 0.69 . within this classification , the high and very high ranges of reliability are 
 identified as 0.70 to 0.89 and 0.90 to 0.100 , respectively16 . in order to evaluate the device reliability in this study , and for measurement of the force 
 sense reproduction tasks , the mean  sd was calculated in a manner similar to that used by 
 docherty , dover , and their associates throughout their force sense experiments7 , 12 , 17 . when errors associated with frequent measurements are considered , sem will assist in 
 determining the range within which the subject s true score is expected to lie18 . this provides the value of measurement 
 error in the same units as the measurement itself . sem is also more applicable for daily 
 clinical decision - making than icc itself , which can sometimes be difficult to interpret15 , 19 . in fact , sem is inversely correlated with absolute reliability , and 
 thus the accuracy of the obtained measurements20 , 
 21 . in this study , the sem recorded 
 through a sequence of frequent measuring methods in reproducing the target force was 
 relatively small , at 0.37 to 0.74 . sdd , indicating the smallest statistically significant 
 change in measurement results , would enable us to estimate the actual error in force 
 reproducing tasks . in fact , the smaller range of sdd would allow those true changes in 
 different tests to be detected more precisely , or those larger than the defined sdd to be 
 considered as true errors in force reproducing methods15 . in general , regulating muscle tension and the sense of force should vary among individuals . 
 the variations in those who perform precise work , such as musicians , are much smaller , 
 whereas the reverse is true for those who perform powerful , coarse tasks22 . the term , just - noticeable difference ( jnd ) , quantifies how different two stimuli must be 
 before subjects reliably report them as distinct23 . a 510% jnd was previously reported by weber for producing and 
 controlling muscle force when participants were asked to hold a 975-g weight in their 
 hands24 , 25 . thus , some noticeable differences in measurement of force sense 
 reproducing tasks through two study dates could be attributed to variations in this sense 
 rather than accuracy of the device . however , as no specific research has measured the 
 accuracy of force sense reproducing tasks of the knee joint based on this study , no direct 
 comparison can be determined between our findings and previous studies . based on the results of this study , all test methods demonstrated acceptably high 
 reliability with a small standard error in frequent measuring tasks . therefore , for 
 measurement of force sense reproduction in knee joint flexor / extensor muscle groups , this 
 device can be applied for diagnostic and therapeutic procedures with a great degree of 
 reliability . the cost - effectiveness of this device in comparison with commercial isokinetic devices , and 
 its high precision to a level of one - hundredth of a newton , along with its ease of use and 
 comfort for operator and subjects , are major advantages . added advantages would include 
 simultaneous bilateral force sense evaluations on the lower limbs , which are equally 
 applicable to upper limb measurements with a small adjustment in the dynamometric arms of 
 the device . the device was only able to measure the sense of force and reproduced force through 
 isometric contractions , and the sample was exclusively composed of normal subjects .