the strength of the knee 
 extensor and flexor muscles has been extensively studied and is a relevant clinical 
 indicator of health status and functional capacity in coronary artery disease8 , neuromuscular diseases9 , the elderly10 , renal disease11 , and fibromyalgia12 . in sports medicine ,
knee extensor and flexor muscle strength is 
 often monitored with the aim of preventing or treating orthopedic as well as muscular 
 injuries13,14,15,16,17,18,19 .
therefore , practitioners 
 need to have reliable tools for easy and reproducible assessment of muscle strength .
the 
 isokinetic concept of exercise20 is 
 currently considered the best method for the assessment of knee extensor and flexor strength 
 because of its well - established validity and reproducibility21,22,23,24,25 .
however , the acquisition and maintenance costs , the time required 
 to perform an isokinetic assessment , as well as the absence of portability of isokinetic 
 dynamometers can limit their use .
conversely , hand - held dynamometers ( hhd ) are characterized 
 by specific features such as low cost , portable design , and rapid data acquisition .
an hhd provides reliable 
 and reproducible results when used for the evaluation of muscle groups that produce little 
 or moderate amounts of force .
however , the results obtained for strong muscles , such as the 
 knee extensors , are less convincing26,27,28 . 

a systematic review comparing muscle strength assessment by hhd and isokinetic testing29 showed a positive correlation between 
 these methods .
however , the heterogeneity of the protocols and devices evaluated made the 
 interpretation of the results difficult . therefore , the aim of this study was to : 1 ) assess 
 the validity of hhd vs. isokinetic dynamometry in the evaluation of knee extensor and flexor 
 muscle strength , 2 ) establish the reproducibility of hhd measurements of knee extensor and 
 flexor muscle strength , and 3 ) compare the flexor / extensor ratios obtained with the 2 
 methods .
the study was carried out at the muscle evaluation unit at the clemenceau rehabilitation 
 university institute ( strasbourg , france ) and involved 30 healthy volunteers ( table 1table 1.characteristics of the population ( n=30)mean  sdrangeage ( years)32.8  11.52059height ( cm)176.9  10.3160197weight ( kg)74.5  15.852109gendermales ( n=20)females ( n=10)lateralityright ( n=27)left ( n=3)test sequence iso then hhd ( n=16)hhd then iso ( n=14)iso : isokinetic dynamometer ; hhd : hand - held dynamometer ) . iso : isokinetic dynamometer ; hhd : hand - held dynamometer subjects were recruited by means of advertisement in the university network from january 
 2015 to november 2015 .
inclusion criteria were as follows : age older than 18  years , and 
 without cardiovascular disorders , osteoarticular diseases of the knees , or previous knee 
 osteoligamentous or thigh muscle injuries .
all participants were fully 
 informed about the risks and procedures associated with the experiment and provided written 
 informed consent to participate in the study .
each participant completed a 10-min warm - up on a cycle ergometer ( power : 1  w / kg body 
 weight ; pedaling rate : 7080  rpm ) . the testing method sequence ( hhd vs. isokinetic testing ) 

was established according to a randomization table prepared by the biostatistics department 
 of the strasbourg university hospitals using r software version 3.2.2 ( r core team ( 2015 ) .
the 
 randomization of the subjects was generated by a draw using a bernoulli distribution with a 
 parameter of 0.5 to maintain a mean balance in the test order .
a recovery period of 20  min was implemented between testing 
 methods . for both methods ,
muscle strength was first measured in the dominant leg , and after 
 a 10-min rest , in the non - dominant leg . to avoid inter - examiner variations , all measurements 

were carried out by a single investigator , specialized in physical medicine and 
 rehabilitation ( male , 31  years old , 180  cm , and 90  kg ) .
the strength of the extensor and flexor muscles of both knees was assessed using an 
 isokinetic dynamometer ( con - trex mj ; cmv ag , dbendorf , switzerland ) .
each participant was 
 positioned on an adjustable chair with the back set at 90 of posterior inclination .
the knee range of 
 motion was set at 90 ( from 100 to 10 of flexion ; 0 corresponding to the complete 
 extension of the knee ) after having aligned the leg segment to anatomical zero .
finally , a 
 measurement of gravity for the leg segment to be tested was carried out over the entire 
 range of motion .
the protocol of isokinetic evaluation included 6 series of contractions on 
 the dominant and non - dominant sides including concentric , isometric , and eccentric 
 evaluations of the knee extensor and flexor muscles ( table 2table 2.protocol for muscle strength measurement with the isokinetic dynamometerseriesspeedmusclesmoderepetitionsrom / positionwarm - up180/secext / flexconcentric1090warm - up120/secext / flexconcentric1090series 160/secext / flexconcentric390series 2180/secext / flexconcentric690series 30/secextisometric190 of flexionseries 40/secflexisometric190 of flexionseries
560/secexteccentric390series 660/secflexeccentric390ext : knee extensors ; flex : knee flexors ; rom : range of motion ) . a recovery period of 60  s was observed between each maximal series . before 
 each
, participants received standardized oral 
 information on the experimental procedure and were verbally encouraged in a standardized 
 manner by the operator during the test .
following 10  min of rest after the evaluation of the 
 dominant leg , muscle testing was performed in exactly the same way for the non - dominant 
 side .
ext : knee extensors ; flex : knee flexors ; rom : range of motion the strength of the extensor and flexor muscles of both knees was assessed with hand - held 
 dynamometry ( microfet2 , hogan health industries , inc .
this 
 dynamometer is a portable digital instrument than can be held in the palm of the hand giving 
 muscle strength measurements in kilogram - force ( kgf ) . as with manual muscle testing
, this 
 dynamometer is placed between the leg segment to be evaluated and the examiner s hand . 

participants were asked to sit with their legs dangling over the end of a standard , 
 adjustable examination table , with hips and knees flexed to 90 , in order to have a distance 
 of 12  cm between the popliteal fossae and the table end .
the height of the examination 
 table was adjusted to have a distance of about 10  cm between the participant s feet and the 
 floor .
participants had to hold the side - edges of the table with their hands and carry out a 
 maximal isometric voluntary contraction for 5 s. the examiner positioned a knee on the 
 floor , with the arm fully extended in front of the lower limb to be tested . to evaluate the 
 knee extensor muscles ,
the dynamometer was placed on the anterior part of the lower leg , 
 above the talotibial joint line ( fig .
1.position of an individual subject for the evaluation of knee - extensor ( panel a ) and 
 knee - flexor ( panel b ) muscles using the hand - held dynamometer ) .
the examiner placed one of his feet against the wall to better resist muscle 
 contraction . to evaluate the knee flexor muscles ,
the dynamometer was placed on the 
 posterior part of the leg , 12  cm above the lateral malleolus ( fig .
in both cases , the examiner produced a resistance force in 
 the horizontal direction to counter the force developed by the participant and maintain an 
 isometric contraction of the knee extensor and flexor muscles .
the evaluation started with 
 the extensor muscles of the dominant leg , followed by the flexors of the dominant side . 

after a recovery period of 10  min , the knee extensor and flexor muscles of the non - dominant 
 leg were assessed using the same experimental design . for each muscle group , participants 
 carried out 3 isometric maximal voluntary contractions of 5 s. a 60-s recovery period was 
 observed between each muscle contraction .
the position of the leg to be tested was verified 
 initially with the help of a standard goniometer . before the evaluation
, participants 
 received standardized oral information on the test procedure and were encouraged orally 
 during the test .
position of an individual subject for the evaluation of knee - extensor ( panel a ) and 
 knee - flexor ( panel b ) muscles using the hand - held dynamometer for each isokinetic series , the measurement criterion was the peak torque ( pt ) obtained 
 during each series , expressed in newton - meters ( nm ) .
the maximal force developed during each contraction , expressed in kilogram - force ( kgf ) , was 
 recorded . for each muscle group ,
the maximal force was averaged over the 3 isometric 
 contractions and used as the measurement criterion .
thus , the 2 obtained variables were the peak torque ( nm ) and the maximal force ( kgf ) . by assuming a correlation of 0.6 between the 2 measurements and a total magnitude of the 
 confidence interval of 0.45 around the estimate , a sample size of 28 subjects was needed . 

the reproducibility of the hhd measurement was established using the coefficient of 
 variation over 3 consecutive measures .
according to stokes , reproducibility was considered 
 acceptable when the coefficient of variation was 15% or below30 .
the statistical analysis was performed independently by the 
 biostatistics department of the strasbourg university hospitals .
all analyses were carried 
 out using r version 3.2.2 , with all the required packages , in the most updated version 
 available at the time of the data analysis .
the gaussian character of the variables was 
 assessed with the shapiro - wilk test .
when 2 variables followed a normal distribution , the 
 pearson correlation coefficient was used ; if not , the spearman correlation coefficient was 
 employed . concerning the correlations , the  test sequence  and
the  laterality  effects were 
 evaluated by ensuring that the confidence intervals ( ci ) at 95% for each variable were 
 overlapping in the 2 modalities .
the results obtained with the 2 
 evaluation modes on the whole population are summarized in table 3table 3.results of the muscle strength measurementsmean  sd ecart - typerangemedianisokinetic ( nm)concentric 60/secext d179.1  52.2*$#100.3275.8180.7ext nd165.4  52.0 * 90.5260.3155.3flex d91.9  28.5$51.1164.793.8flex nd85.1  25.551.0143.878.1concentric 180/secext d132.3  39.4*$75.4207.8136.8ext nd130.4  39.6 * 71.5204.2134.1flex d72.2  22.9$32.1115.372.8flex nd71.8  24.1$32.0141.669.2isometric 0/secext d177.2  57.3 * 89.5291.8181.2ext nd167.4  56.6 * 88.0281.0162.2flex d84.0  25.248.0153.784.7flex nd79.3  25.945.0144.273.1eccentric 60/secext d201.9  61.9 * 110.9357.5191.5ext nd195.5  69.8 * 111.8357.0169.9flex d123.1  44.169.5252.3111.6flex nd116.7  36.865.9210.3103.8hhd ( kgf)ext d52.9  16.7 * 22.787.457.6ext nd51.1  17.2 * 22.284 .
351.8flex d26.7  6.017.440.927.2flex nd26.9  5.318.238.927.1ext : knee extensors ; flex : knee flexors ; d : dominant side ; nd : non - dominant side ; sd : 
 standard deviation .
$ p<0.05 vs. 
 eccentric 60/s in similar muscles . # p<0.05 concentric 60/s vs. concentric 180/s 
 in similar muscles .
the strengths measured for the knee extensor muscles were greater than the 
 corresponding values for the knee flexor muscles , whatever the mode and velocity of the 
 muscle actions ( p<0.05 ) .
ext : knee extensors ; flex : knee flexors ; d : dominant side ; nd : non - dominant side ; sd : 
 standard deviation .
# p<0.05 concentric 60/s vs. concentric 180/s 
 in similar muscles the knee flexor / extensor strength ratios did not change according to the evaluation mode , 
 with no difference between the dominant and non - dominant sides ( table 4table 4.flexor/extensor ratios in isokinetic and hand - held dynamometrydominant sidenon dominant sideconcentric 60/sec52  7.0%53  9.7%concentric 180/sec55  7.0%55  8.4%isometric 0/sec49  8.7%49  11.0%eccentric 60/sec61  8.1%61  10.9%hhd53  12.9%57
hhd : hand - held dynamometer the knee extensor muscle strength was significantly greater than for the knee flexor 
 muscles , both on the dominant and non - dominant side ( p<0.05 ) .
there was no difference 
 between the dominant and non - dominant side in a given muscle group ( table 3 ) .
the ratios of knee flexor / extensor muscle strength were 
 similar in dominant and non - dominant sides and were not significantly different compared to 
 isokinetic ratios ( table 4 ) . the coefficient of variation of the 3 isometric muscle strength measurements using the 
 microfet 2 hhd ranged from 3.2 to 4.2% according to the muscle group evaluated .
mean 
 difference ( absolute and relative values ) as well as limits of agreement were similar 
 between muscle groups ( table 5table 5.reproducibility of muscle strength evaluation with microfet 2 hand - held 
 dynamometercoefficient of variationmean differencelimits of agreement(%)(kgf)(%)(kgf)(%)dominant knee extensors3.20.1  3.70.1  7.07.37.613.814.0non - dominant knee extensors3.30.0  3.70.4  7.47.57.515.114.4dominant knee flexors4.20.4  2.51.4  8.94.65.416.419.2non - dominant knee flexors3.70.3  2.10.9  7.73.94.614.416.3 ) .
the correlation coefficient between the isokinetic and hhd muscle strength values ranged 
 from r=0.87 ( 0.750.94 ) to r=0.72 ( 0.480.86 ) ( p<0.01 ) , indicating that the correlation 
 between testing methods was generally good ( table 
 6table 6.correlation coefficients and 95% confidence intervals for knee muscle strength 
 assessments obtained by hand - held ( hhd ) and isokinetic dynamometers ( con - trex)con - trexeccentricisometricconcentricconcentric60/sec0/sec60/sec180/sechhdextensors dr=0.87r=0.85r=0.87r=0.85(0.750.94)(0.70 
0.92)(0.730.93)(0.710.93)extensors ndr=0.84*r=0.87r=0.86r=0.87(0.690.92)(0.740.94)(0.720.93)(0.740.94)flexors dr=0.80*r=0.75r=0.81r=0.85(0.620.90)(0.530.87)(0.630.91)(0.700.93)flexors ndr=0.75*r=0.72*r=0.83*r=0.82(0.530.87)(0.480.86)(0.670.92)(0.660.91)*spearman correlation coefficient ; d : dominant side ; nd : non - dominant side .
* spearman correlation coefficient ; d : dominant side ; nd : non - dominant side .
all 
 coefficients of correlation are significant ( p<0.01 ) . the flexor / extensor ratios were significantly correlated between the hhd and isokinetic 
 dynamometer in concentric 180/sec and isometric 0/sec modes on the dominant and 
 non - dominant sides ( p<0.05 ) .
the flexor / extensor ratio measured in concentric 60/sec 
 mode was correlated to the hhd only for the non - dominant side .
no correlation was observed 
 for the flexor / extensor ratios between hhd and isokinetic eccentric 60/sec mode ( table 7table 7.coefficients of correlation for knee flexor / extensor muscle strength ratios 
 obtained with hand - held or isokinetic dynamometrydominantnon dominantconcentric 60/sec0.230.46 * concentric 180/sec0.39 * 0.45 * isometric 0/sec0.37 * 0.46 * eccentric 60/sec0.040.13*p<0.05 ) .
the results of the present study show : 1 ) good reproducibility of the hhd for knee 
 extensor / flexor muscle strength assessment , 2 ) clinically acceptable agreement between the 
 values obtained with the hhd versus the isokinetic dynamometer , and 3 ) the need for caution 
 when interpreting the flexor / extensor ratios using hhd compared to isokinetic 
 dynamometry . the muscle strength values using isokinetic dynamometry recorded in this study are in good 
 agreement with those found in the literature31 , particularly in the study by maffiuleti et al . in 200723 . for a similar population of healthy 
 individuals ( age : 30  5 vs. 33  11  years old ; height : 175  8 vs. 17710  cm ; weight : 70  
 13 vs. 74  16  kg ) , the strength measurements using the con - trex isokinetic module were very 
 close , with a peak torque for the knee extensors on the dominant side of 178  46 vs. 179  
 52 nm ( concentric at 60/s ) ; 132  38 vs. 132  39 nm ( concentric 180/s ) , and 202  68 vs. 
 202  62 nm ( eccentric 60/s ) when compared with our values . similarly , the muscle strength values recorded with the hhd are in agreement , albeit with 
 greater absolute data , with literature data32 , 33 and with the recent study by douma et al . , 
 who established reference values for knee extensor and flexor muscle strength in the active 
 dutch population by using the microfet 2 hhd34 .
the magnitude of difference between our values and previously 
 published data using hhd is likely related to the characteristics of the participants , who 
 were younger and fitter in our study compared to previous studies .
greater limits of agreement were previously observed for reproducibility of muscle strength 
 using hhd in strong muscle groups ( i.e. , knee extensors ) compared to weaker muscle groups 
 ( i.e. , knee or elbow flexors)34 . 

consequently , hhd was not considered as suitable methodology for muscle strength assessment 
 in knee extensors35 .
conversely , our 
 results are at odds with this view as we report that the reproducibility of muscle strength 
 assessment is not altered by the level of muscle strength developed by the subjects . indeed , 
 the variation coefficients were lower for the 3 measures of knee extensors compared to knee 
 flexors , despite higher levels of absolute strength in the former .
similarly , we did not 
 observe greater limits of agreement ( loa ) in the knee extensors compared to the knee 
 flexors . of note ,
absolute values of loa in the present study are somewhat lower than those 
 previously published34 .
we suggest that 
 if hhd devices are manipulated by experienced operators , hhd can be used for the evaluation 
 of muscle strength , even in strong muscles such as the knee extensors .
it should also be 
 kept in mind that the strength of the operator might be key in stabilizing the hhd device 
 during the measurement procedure , and therefore the operator s own strength is likely to be 
 of great importance for the accuracy of the measurements36 . when compared with previous results reporting the coefficient of variation ( cv ) of 
 isokinetic measurements using a contrex device ( 1.93.4% for knee extensors and 2.73.6% for 
 knee flexors ) ,
the cvs reported in the present study are very similar for hhd ( 3.23.3% for 
 knee extensors and 3.74.2% for knee flexors ) .
this result suggests that the reproducibility 
 of the strength measurements using hhd is in close agreement with that observed with 
 isokinetic testing23 . in a previous literature review comparing hhd with isokinetic dynamometry , stark et al
. 
 showed that correlation coefficients ranged between 0.43 and 0.99 for the knee extensor and 
 flexor muscles29 .
however , the 
 heterogeneity of the protocols as well as the different models of dynamometers make the 
 results difficult to interpret .
the present study is the first to analyze the correlation 
 between these specific dynamometers ( contrex isokinetic vs microfet 2 ) , and we report a good 
 correlations between the 2 methods ( r=0.720.87 ) . of note , the present data were gathered 
 using a simple protocol , easily reproducible without any additional equipment , in order to 
 be established in routine clinical practice and to take the greatest advantage of the 
 practical aspects of hhd .
when looking at subjects engaged in regular sports practice , only one study previously 
 explored the correlation between hhd and isokinetic measurements37 .
the study reported lower correlation coefficients 
 ( r=0.334  0.110.617  0.7 ) compared to our results , but it should be emphasized that the 
 number of subjects , the type of dynamometers , and the protocol employed were different .
lastly , the present study is also the first to provide a correlation analysis between hhd 
 and isokinetic testing for the evaluation of knee flexor / extensor muscle ratios , an index 
 used routinely in the field of sports medicine .
our results show correlation coefficients 
 ranging from 0.37 to 0.46 ( all p<0.05 ) .
although statistically significant , these results 
 indicate a moderate level of correlation between the 2 methodologies , and even no 
 correlation at all when comparing flexor / extensor ratios obtained using hhd with that from 
 isokinetic testing in eccentric mode .
therefore , flexor / extensor muscle strength ratios 
 measured using hhd should be interpreted with caution until further investigation is 
 performed in this area . a limitation of our study is the fact that our population consisted of healthy , relatively 
 young volunteers without osteoarticular or neuromuscular pathologies .
compared 
 muscle strength measurements by using hand - held and isokinetic dynamometers in a population 
 of individuals with various unilateral orthopedic pathologies of the knee5 .
they found a significant correlation 
 between both methods ( r ranging from 0.57 to 0.80 ) .
however , the evaluation by hhd did not 
 highlight a statistically significant difference between sides , whereas the isokinetic 
 evaluation found a force deficit on the injured side .
the pain that the patient may 
 experience during the evaluation could be a confusing factor and must be taken into account 
 when interpreting the strength results in the presence of pathologies .
another limit of this 
 work is that hhd and isokinetic devices do not provide strength assessment with similar 
 units of measurement .
this makes direct comparison of absolute strength values difficult and 
 limits the present comparison to correlation analysis . to obtain strength values expressed 
 in the same units between the 2 devices ( newtons or kgf ) would have required an evaluation 
 of the lever arm , which is likely to have introduced more variability in the data .
each evaluation method has its specific advantages and limitations . for a practitioner 
 looking for rapid and regular muscle strength testing
knee extensor and flexor muscle strength 
 recorded with an hhd is reproducible and significantly correlated with the isokinetic 
 values , indicating that this method may in some cases be a useful replacement for isokinetic 
 strength measurement , even for the assessment of strong muscles .
however , for strength ratio 
 assessment , and when judged against the isokinetic standard , hhd is not a valid option .
 [ purpose ] to compare measurements of knee extensor and flexor muscle strength performed 
 using a hand - held dynamometer and an isokinetic dynamometer in apparently healthy 
 subjects . 
 [ subjects and methods ] thirty adult volunteers underwent knee muscle strength 
 evaluation using an isokinetic or a hand - held dynamometer . 
 [ results ] strong positive 
 correlations were found between the 2 methods , with correlation coefficients r ranging 
 from 0.72 ( 95% confidence interval [ ci ] , 0.480.86 ) to 0.87 ( 95% ci , 0.750.94 ) , depending 
 on the muscle group and the isokinetic evaluation mode . 
 the reproducibility of the 
 hand - held dynamometer findings was good , judged by a coefficient of variation of 3.24.2% . 
 however , the correlation between the 2 methods for the assessment of flexor / extensor 
 ratios ranged from 0.04 to 0.46 . [ conclusion ] 
 knee extensor and flexor muscle strength 
 recorded with a hand - held dynamometer is reproducible and significantly correlated with 
 the isokinetic values , indicating that this method may in some cases be a useful 
 replacement for isokinetic strength measurement . 
 however , for strength ratio assessment , 
 and when judged against the isokinetic standard , a hand - held dynamometer is not a valid 
 option . 