Detection of Submaximal Effort in Isometric and Isokinetic Knee Extension Tests

Transcription

1 Detection of Submaximal Effort in Isometric and Isokinetic Knee Extension Tests Copyright All rights reserved. Pao-chun lin, MS, PT' Michael E. Robinson, PhD * john Carlos, )r., PhD, PT3 Patrick O'Connor, MS, PT4 M usculoskeletal injuries are the most frequent cause of chronic or permanent impairment (2). Thc total economic cost of musculoskeletal injuries has been estimated at approximately $70 billion dollars a year, second only to cardiovascular disorders (2). One method used to evaluate the status of the musculoskeletal system is the measurement of force produced by voluntary contraction. The measurement of maximal voluntary contraction is based, however, on a presump tion that the patient is motivated to give a sincere, maximal effort. Failure to assess accurately a patient's effort level can result in unnecessary procedures due to lack of response to treatment, and this can, in turn, lead to an increase in health care costs. Therefore, the therapist is often faced with the need to detect suh maximal efforts. It has been suggested that the variability of force production may be an indicator of effort level. For isometric exercise, Kishino et al (9) proposed that when a patient is willfully attempting to produce a submaximal effort, the obtained force measurement$ will show increased variability and reduced reliability. Although the authors speculated about this hypothesis, they offered no data to support it. Bohannon (1) compared the variability of maximal and submaximal The use of torque variability and slope measures to detect submaximal efforts has been studied in isometric tests, but not fully investigated in isokinetic tests. The purpose of this study was to investigate differences between maximal and submaximal efforts in isometric and isokinetic knee extension using torque variability and slope measures obtained from 32 volunteers (age = years). The coefficient of variation of average toque, coefficient of variation of peak torque, and slope to peak torque were obtained from maximal and submaximal torque curves during isometric and two isokinetic tests (60%~ and 180 /secj. Significant differences between effort levels (maximal and submaximah were shown for all variables in isometric and isokinetic tests. An optimized cut-off value to determine submaximal efforts without false positives was determined for each variable. The coefficient of variation of average torque detected 75% of submaximal efforts at 180 /sec and slope to peak torque detected 63% of submaximal efforts at 60 /sec. For both speeds, combining the coefficient of variation of average torque with slope improved the submaximal detection rate to 84%. No variable provided a satisfactory detection rate for the isometric test. Therefore, submaximal isokinetic knee extensions are detectable with toque variability and slope measures, but submaximal isometric exercise is not detectable. Key Words: muscle strength, knee, effort rating ' Physical Therapist, Associate in Department of Physical Therapy, University of Florida, Gainesville, FL Associate Professor, Department of Clinical and Health Psychology, P.O. Box HSC, University of Florida, Gainesville, F ' Assistant Professor, Department of Physical Therapy, University of Florida, Gainesville, FL Physical Therapist, Shands Hospital at the University of Florida, Gainesville, FL tests of isometric elbow flexion by calculating the coefficient of variation (CV). He reported that the CV of force measures was significantly greater during submaximal trials than during maximal trials, which sup ported Kishino et al's hypothesis. Later studies, however, questioned the validity of this hypothesis. In studies of isometric lumbar extension ( ), the reproducibility of torque production was examined by test-retest correlations. For both normal subjects and low back pain patients, no differences were reported in testretest reliability between effort conditions. It was also found that there were no significant differences between the variances for maximal peak torque and submaximal peak torque in isometric lifting test. (7). In a recent study, Robinson et al (1 1) examined the possibility of using the coefficient of variation to determine effort level of grip force. The suh maximal effort condition showed significantly more variability than the maximal effort condition. However, the classification of effort level based on the CV cut+ff value was found to have unacceptably large errors. Another method that has been proposed to detect submaximal efforts in isometric tests is the slope to IOSFT Volume 24 Number 1 July 1996

2 Copyright All rights reserved. peak torque. Kroemer and Marras (10) hypothesized that maximal contraction only requires the central nervous system to send out the commands to recruit all the muscle fibers available at their greatest firing rates, while submaximal contraction requires continual feedback signals for regulation of both the muscle fiber firing rate and recruitment. Experimentally, their results indicated that submaximal isometric contraction involved a slower build-up of torque than did maximal contraction. They suggested that the slope of the torque-time curve during the torque build-up period might be used to assess the effort level. In the grip force test, Gilbert and Knowlton (6) analyzed the torque-time curve and also found that maximal contraction had a higher value on the slope to peak torque. However, this method was only studied in isometric tests and has not been generalized in the case of dynamic (eg., isokinetic or isotonic) tests, which more closely a p proximate muscular action in functional activities. Little research has investigated the discrimination of effort level in an isokinetic task. An earlier study (4) reported a correlation coefficient of 1.0 between perceived exertion and the produced torque at submaxima1 level. This precise perception was shown to be reproducible by a testretest correlation of 0.91, indicating that submaximal efforts could be repeated precisely. Hazard et al (7,8) developed several measurement models to assess torquedistance curve variability in isokinetic trunk and lifting force tests and reported satisfactory classification rates of effort level. When analyzing the variability between curves, one measurement they used was called the "maximal difference anywhere," which was the maximal difference between curves at any distance. This appears to represent a nonstandardized method of selecting points and, therefore, the methods they used are not generally accepted. The absence of uniform criteria for force variability assessment may contribute to the controversy in the studies mentioned above. To address this limitation, the coefficient of variation, which has been widely accepted as a measure to assess variability (3,l I), was used to evaluate differences in force variability. The CV represents a "relative standard deviation" and is calculated by dividing the standard deviation of the o b tained force measures by their mean. As compared to Hazard et al's studies (7,8), this relatively simple approach has a control for differences between a group of force curves. This measurement was named "CV of average torque" in this study. Since the peak torque is also often used in isokinetic testing, the CV of peak torque as well as the CV of average torque were both chosen for analysis. The purposes of this study were to apply the isometric effort detectors (eg., CV, slope to peak torque) to isokinetic testing and to assess the difference in torque variability between slow and fast isokinetic movement. The first hypothesis of this study was that the CV values of torque (CV of average torque and CV of peak torque) in submaximal effort would be greater than maximal effort in isometric (OO/sec) and isokinetic (60 and 180 /sec) exercise. The second hypothesis was that the slope to achieve peak torque in maximal effort would be greater than submaxima1 effort at all three velocities. Determining the specificity and sensitivity of these analyses will be applicable to the clinical setting and should have an impact on patient care and disability determination. Subjects Thirty-two healthy students (21 females and 11 males) from physical therapy classes at a major southeastem university were studied to determine the extension torque of the knee during isometric and isokinetic exercise. Exclusion criteria consisted of any history of neuromuscular disease or any injury to the lower extremity that required medical or surgical intervention. The ages of the subjects ranged from 19 to 38 years (X = 25.2, SD = 4.7). Informed consent was obtained from the participants in accordance with the Institutional Review Board. Instrumentation Extension torque of the knee was measured by the Biodex dynamometer (Biodex Medical Systems Inc., Shirley, NY), which can be set for both isometric and isokinetic testing. This device is an isokinetic dynamometer interfaced with an NEC Powermate-2 microcomputer. The validity and accuracy of the Biodex dynamometer has been investigated under static and dynamic conditions with satisfactory results (17). The subject was secured on the Biodex by a pelvic stabilization strap passing through two openings in the backrest and a thigh stabilization strap attached to the seat. The rotating arm was attached to subjects by means of a padded cuff placed over the distal portion of the calf just superior to the malleoli. On every testing day, a 251b weight was attached to the calibration arm to calibrate the strain gauge tension transducer enclosed within the dynamometer following the manufacturer's standard protocol. Procedures The procedure of this study was to assess isometric and isokinetic torque of the knee extensors. The leg tested was the leg touching the ground when one kicks the ball. Each subject was seen on two occasions, separated by at least 48 hours. During each occasion, the subjects were first secured to the testing device with straps around the pelvis, thigh, and ankle. The axis of the knee was placed in line with the axis Volume 24 Number 1 July 1996 JOSPT

3 RESEARCH STUDY Copyright All rights reserved. of the rotating arm. The limb was weighed before testing by the Biodex's automatic limb weighing system to correct for the gravitational effect on torque value. Each estimate of the gravitational effect was added to the quadriceps torque. The subjects were then instructed to perform 25 maximal knee extension trials (five at OO/sec, five at 60 /sec, and 15 at 180 /sec) as the warm-up. Each isometric trial lasted 5 seconds with the knee flexed at 60". In each isokinetic trial, subjects were positioned at 90" of knee flexion and extended their knee to full extension. Subjects' arms were folded across their chest during testing. After the warm-up, subjects rested for 5 minutes to reduce the effect of fatigue. Thereafter, the muscle performance test was started. On one occasion, subjects completed five maximal isometric repetitions and five sub maximal (50%) isometric repetitions with a!%minute rest in between. No performance feedback was given to subjects. The rest interval was 1 minute between repetitions. Subjects were instructed to kick as hard as they could in the maximal effort condition and were told to give a 50% effort in the submaximal effort condition. The testing order of maximal vs. submaximal conditions was randomized across subjects to eliminate a possible order effect caused by fatigue. On the other occasion, after the warm-up, half of the subjects completed five maximal isokinetic repetitions at slow speed (60 /sec) followed by 15 maximal isokinetic repetitions at fast speed (180 /sec). After resting for 5 minutes, they performed five slow and 15 fast submaxima1 isokinetic repetitions. The other half of the subjects were instructed to reverse this order, giving the submaximal (50%) effort for the first set and maximal effort on the second set. Commands with respect to effort levels were the same as in the isometric condition. The order of the two conditions was counterbalanced. Data Analysis Since this was a within-subject design, the data were not analyzed by gender. The dependent variables to be analyzed were the CV of average torque (Wave), the CV of peak torque (CVpeak), and the slope to peak torque. All CVs were withinsubject calculations. For the isometric test, the only independent variable was the effort level, which was maximal and submaximal. In the isokinetic test, the independent variables were effort level and speed (60 and 180 /sec) when analyzing the CV values. The slope of isokinetic torque was compared only across effort levels since the torque curves of slow and fast isokinetic movements are obviously different. The CV of average torque was obtained from the software package of the Biodex computer, which was camed out on repetitions 2, 3, and 4. The peak torque was determined from each torque-time curve of repetitions 2-5 in isometric and isokinetic tests. The mean and standard deviation of the peak torque values were calculated to obtain the CV of peak torque. Data were analyzed separately for isometric and isokinetic tasks. In the isometric exercise, the Student's t test was used to assess the difference between effort level in these two CV values. In the isokinetic test, the twolevel repeated measures analysis of variance was used when comparing these variables across effort level and speed. The slope to peak torque was taken as the peak torque divided by the time to peak torque on repetitions 2-5. The mean of slope value was used for analysis. A t test was done to compare slope between maximal and submaximal effort for each velocity. In order to test the practical utility of using these variables to differentiate effort levels, the optimized cut-off values were determined for all variables with significant differences between maximal and submaximal effort. The optimized cut-off value was the value that separates maximal from submaximal effort, with the restriction that no maximal efforts be misclassified as submaximal. The suh maximal detection rates for all cut-off values were calculated. RESULTS Before analyses regarding the variability and slope of different effort levels (maximal and submaximal) could be done, it was essential to determine that the torque production between the two conditions differed at each effort level for each of the three velocities (0, 60, and 1 80 / sec). The peak torque measures on repetitions 2-5 were first averaged for each condition of these three velocities. In the isometric test, the peak torque was kg-m (X f SD) for maximal exercise compared with 7.54 kg-m for submaxima1 exercise. For isokinetic movement at 60 /sec, the peak torque was kg-m for maximal exercise compared with 8.21 kg-m f 3.59 for submaximal exercise. For isokinetic movement at 18O0/stc, the peak torque was kg-m for maximal exercise compared with 4.99 kg-m for submaximal exercise. The Student's t test using the torque measures as the dependent variables for each velocity indicated that the subjects produced significantly less torque in the submaxima1 effort condition: OO/sec: t(31) = 11.26, p <.001; 60"/sec: t(s1) = 9.50, p <.OOl; 180 /sec: t(31) = 9.50, p <.001. These results indicated that the manipulation of effort level waq successful. lsokinetic Test The descriptive statistics for the dependent variables in isokinetic tests are shown in Tables 1 and 2. For movement at 60 /sec, the CV of average torque was % for maximal exercise and % for submaximal exercise. For movement at 180 /sec, the CV of average JOSFT Volume 24 Number 1 Julv 1W6

4 RESEARCH STUDY Maximal Submaximal X SD X SD > - Wave 'Wpeak Slope to Peak 180 /sec > 10% > 10% <20.62 kg-* 6O0/sec >19% >16% kg-rnhe OO/sec >18% >11% <3.74 kg-m/se Copyright All rights reserved. Si,gn~i~c,~ntlv rliii(w~r ironl ni.nin~,~l thmc-tc~i\tics (p <,051. V1 = Coefficient of variation of average torque. V2 = Coefficient of variation of peak torque. V3 = Slope to peak toque. TABLE 1. Descriptive statistics for the dependent variahles in isokinetic tests at 60 /sec. torque was % for maximal exercise and % for submaximal exercise. For the CV of average torque, there was a significant main effect of effort level (F = 62.66, rlf 1 31, P <.001) and speed (F = 1i.97, (f 1,31, P < AO.5). The average torque of submaximal isokinetic movement was significantly more variable than the maximal movement, and the average torque of movement at 60 /sec was more variable than 1 80 /sec. For movement at 60 /sec, the CV of peak torque was % for maximal exercise compared with t % for submaximal exercise. For movement at 180 /sec, the CV of peak torque was % for maximal exercise compared with % for submaximal exercise. For the CV of peak torque, there was a significant main effect of effort level (F = 36.53, (f 1,31, P <.OO1). The main effect of effort level 6.04) for maximal exercise compared ' S~gn~i~c,~ntlv dirier~nr iron1 nl,~u~ni,~l cli,~r,~ctenst~cs (p <,051. Vl = Coefficient of variation of average torque. V2 = Coefficient of variation of peak toque. V3 = Slope to peak torque. TABLE 2. D~ccriptive statistics for the dependent variahles in isokinetic tests at 1800/sec. * Si,qnitic,lnrl\, d~tii~rc~nt rroni nl,~tin~,~l c-/i,~r,~c rc~ri\rrcs (p <.05). V1 = Coefficient of variation of average torque. V2 = Coefficient of variation of peak toque. V3 = Slope to peak toque. TABLE 3. Descriptive statistics for the dependent variables in isometric tests. showed that the peak torque of suh maximal isokinetic movement was significantly more variable than the maximal movement. For movement at 180 /sec, the mean slope was kg-m/ sec for maximal exercise compared with 21.72? kg-m/sec for submaximal exercise. In movement at 60 /sec, the mean slope was 39.53? kg-m/sec for maximal exercise compared with kg-m/ sec for submaximal exercise. At both speeds, the slope values were significantly higher in the maximal exercise compared with the submaximal exercise [180 /sec: t(31) = 7.30, P <.001; 60 /sec: t(31) = 7.08, P <.O01]. Isometric Test The descriptive statistics for the dependent variables in isometric tests are shown in Table 3. The CV of average torque was 6.47% (SD = 4.05) for maximal exercise compared with 10.66% (SD = 6.58) for submaximal exercise. The CV of peak torque was 4.38% (SD = 2.17) for maximal exercise compared with 9.97% (SD = 5.50) for submaximal exercise. The mean slope was kg-m/sec (SD = with 8.39 kg-m/sec (SD = 3.75) for submaximal exercise. The analyses revealed significant differences in the two CV values between maximal and submaximal exercise [CV of average torque: t(31) = 2.65, P =.013); CV of TABLE 4. Cut-off values to determine submaximal efforts. peak torque: t(31) = 5.16, P <.001]. Both the average torque and peak torque of submaximal isometric efforts were significantly more variable than the maximal ones. The difference between the slope for maximal exercise and the slope for submaximal exercise was also significant [t(31) = 2.23, P =.033]. The slope value for maximal isometric effort wm significantly higher than the submaximal one. Classification of Effort Level Optimized cut-off values were determined for all variables. Table 4 shows the optimized cut-off values which could separate maximal from submaximal without misclassifymg any maximal efforts as submaximal, ie., without any false positives. For example, at 180 /sec, the CVave values of all maximal efforts were less than 10%. This 10% cut-off can detect more submaximal effort. than any other cut-off greater than it. Therefore, all the efforts with CVave greater than 10% were classified as submaximal effort.. while all the others were determined as maximal. The submaximal detection rate of the cut-off value for each variable, which indicated the proportion of submaximal effort.. detected by the cut-off value, is demonstrated in Table 5. A 10% CVave cut-off correctly classified 75% (24/32) of submaxima1 efforts at 180 /sec without any false positives (maximal efforts classified as submaximal). The best submaximal detection rate at 60 /sec was 63% with a kg-m/sec' cutoff of slope variable. None of these variables provide satisfactory submaxi- Volume 24 Number 1 -July 1W6 JOSPT

5 Copyright All rights reserved. VI = Coetficient of var~ation oi average torque. V2 = Coefiicient of variation of peak torque. V3 = Slope to peak torque. Vx + Vy = The combination of Vx and Vy. TABLE 5. Submaximal detection rates for individual variables and multiple variables (the correct classification rates of maximal efforts were 100%). ma1 detection rates for the isometric test. The submaximal detection rates of variable combinations are also demonstrated in Table 5. When combining CVave and slope at 180 /sec, it means only the effort with both Wave below 10% and slope above kg-m/sec2 was considered a maximal effort. This combination at 180 /sec yielded no false positives and correctly classified 84% (27/32) of submaximal efforts. At 60 /sec, this combination yielded the same submaximal detection rate of 84%. Adding CVpeak to this combination did not improve the detection rates. In the isometric test, the highest sub maximal detection rate (from the combination of Wave, CVpeak, and slope) was still not satisfactory (44%). DISCUSSION Results from this study, which are consistent with previous findings (1 1, 15,16), indicate that the submaximal (50%) effort condition is significantly more variable than the maximal effort condition in isometric testing. However, effort level classification based on the CV method was found to have unacceptably large errors, in that over 50% of submaximal efforts were misclassified with an optimized cut-off. As has been suggested by earlier studies (7,ll-15), the variability method may lack sensitivity in determining isometric effort level since it fails to detect the majority of sub maximal efforts. In this study, there were no significant differences in slope value between maximal and submaximal efforts in the isometric knee test. Nevertheless, it was found that isometric effort.. of higher amplitude had significantly greater slope values (5,6), which is contrary to what was found in this study. Perhaps this difference is due to the force tests used in these studies. Grip force testing was used in Gilbert and Knowlton's study (6), and hand muscles were tested in the study of Freund and Budingen (5), while in the current study knee extension was tested. As compared with the isometric results of this study, the use of torque variability and slope measures with an isokinetic task shows more promise. Increased sensitivity is achieved in addition to detecting significant differences between maximal and sub The use of torque variability and slope measures with an isokinetic task shows more promise. maximal efforts. In our study of a healthy sample, the optimized cut-off values with no false positives (maximal efforts classified as submaximal) were determined. Wave successfully detected 75% of submaximal efforts at 18O0/sec; only eight of 32 submaxima1 efforts were misclassified. Slope successfully detected 63% of submaxima1 efforts at 6Oo/sec. Subject by sub ject analysis showed that the three dependent variables were often picking up different submaximal efforts. Therefore, a combination of discriminators could thus increase the detection rates. Combining Wave with slope improved the detection rate to 84% for both speeds. These results indicate that the isokinetic task may represent an approach that improves upon the discrimination of submaxima1 from maximal efforts. This a p pears to confirm the view of Robinson et al (15) in isometric and isotonic knee extension tests that the sensitivity to detect submaximal efforts was much greater for the dynamic condition. It seems that movement is the key difference between methods that better discriminate (iso kinetic and isotonic) and methods that are less successful (isometric). It may be that subjects are less able to reliably reproduce torque and velocity of submaximal efforts because of the dynamic nature of the task. This study could be beneficial to clinicians and professionals involved in treatment planning and disability determination. The dynamic testing approaches established in this study and the previous study (15) could assist clinicians in interpreting the evaluation results correctly and making appropriate decisions regarding treatment planning. These methods are crucial in the investigation of submaximal efforts and in the cost savings to the health care system. However, the limitations of using healthy subject.. should be mentioned. Further, differences between injured and uninjured populations need investigation to determine the applicability of these methods in clinical populations. Previous results (1 1) have raised doubt about the stability of using a low number of repetitions (usually three) to calculate the CV in isometric tests to evaluate the effort level. The calculation of this measure on such a small number of trials raises the possibility that the measure may be very unstable. However, this calculation is available commercially and is convenient for clinical use. Even though significant classification accuracy of effort level was obtained in this study using a healthy population, instability of the CV would compro mise the applicability of the method JOSPT Volume 24 Number 1 July 1996

6 RESEARCH STUDY Copyright All rights reserved. and certainly would decrease the clinical utility of the results. Therefore, it is important to further explore the stability of the CV in the isokinetic condition. Replication of the CV methods in future research with an increased number of repetitions for each calculation may improve the sensitivity of this approach. The long term stability of measures used in this study needs to be assessed in future research. The multiple variable combinations derived from a retrospective exploration of the data also demand further testing before application. Finally, variables such as fear, fatigue, pain, and perception of movement may prove fn~itful for future study. CONCLUSIONS The results of this study have shown significant differences between maximal and submaximal efforts in variability and slope values of isokinetic and isometric movement. The CV of average torque was the best detector at 180 /sec, and the slope to peak torque was the most accurate detector at 60 /sec. The best combinations of variables were "CV of average torque + slope to peak torquew and "CV of average torque + CV of peak torque + slope to peak torque," which yielded a submaximal detection rate of 84% at both 180 and 60 /sec. However, no single variable or variable combinations provide a satisfactory detection rate for submaximal efforts in the isometric test. It is concluded that for the young, healthy population, application of torque-time curve analysis allows for detection of 84% of submaximal isokinetic knee extensions at 180 and 60 /sec. However, isometric knee extension effort level is not detectable with these measures. The methods used in this study show considerable promise for the detection of submaximal isokinetic knee extension, but they have not currently been applied to a clinical population. Future research could emphasize the clinical application of these ap proaches and the determination of cut-off values for patients. JOSFT ACKNOWLEDGMENTS Special thanks to Denis Bnmt, associate professor and graduate coordinator, and Martha A. Clendenin, professor and chairman, Physical Therapy Department, University of Florida, Gainesville, FL, for their critical review of the article and their helpful suggestions. REFERENCES 1. Bohannon RW: Differentiation of maximal from submaximal static elbow flexor efforts by measurement variability. Am / Ph ys Med Rehabil 66(5): , Brena S, Meacham A: Evaluation of function and disability. In: Bonica 1, Loeser 1, Chapman CR (eds), The Management of Pain, pp Philadelphia: Lea and Febiger, Carlsoo S: With what degree of precision can voluntary static muscle force be repeated? Scand / Rehabil Med 18: 1-13, Cooper DF, Grimby G, Jones DA, Edwards RHT: Perception of effort in isometric and dynamic muscular contraction. Eur J Appl Physiol 4 1: , Freund HI, Budingen HI: The relationship between speed and amplitude of the fastest voluntary contractions of human arm muscles. Exp Brain Res 3 1 :1-12, Gilbert JC, Knowlton RG: Simple method to determine sincerity of effort during a maximal isometric test of grip strength. Am / Phys Med Rehabil62(3): , Hazard RG, Reeves V, Fenwick JW: Lifting capacity: Indices of subject effort. Spine 17(9): , Hazard RG, Reid S, Fenwick /, Reeves V: lsokinetic trunk and lifting strength measurements: Variability as an indicator of effort. Spine l3(1):54-57, Kishino ND, Mayer TG, Gatchel RJ, Parrish MM, Anderson C, Gustin L, Mooney V: Quantification of lumbar function. Spine 10: , Kroemer KHE, Marras WS: Evaluation of rnaximal and submaximal static muscle exertions. Hum Factors 23: , Robinson ME, Geisser ME, Hanson CS, O'Connor PD: Detecting submaximal efforts in grip strength testing with the coefficient of variation. / Occup Rehabil3(1):45-50, Robinson ME, Millan MM, O'Connor P, Fuller A, Cassisi JE: Reproducibility of maximal versus submaximal efforts in an isometric lumbar extension task. / Spinal Disord 4: , Robinson ME, O'Connor PD, MacMil- Ian M, Fuller A, Cassisi JE: Effect of instructions to simulate a back injury on torque reproducibility in an isometric lumbar extension task. I Occup RehabilZ(4): 1-9, Robinson ME, O'Connor PD, MacMil- Ian M, Shirley FR, Greene AF, Geisser ME, Fuller AK: Physical and psychosocia1 correlates of test-retest isometric torque variability in patients with chronic low back pain. / Occup Rehabil2(1):11-18, Robinson ME, O'Connor PD, Shirley FR, Riley JL: Variability of isometric and isotonic leg exercise: Utility for detection of submaximal efforts. / Occup Rehabil4: , Smith GA, Nelson RC, Sadoff S], Sadoff AM: Assessing sincerity of effort in maximal grip strength tests. Am J Phys Med Rehabil 68(2):73-80, Taylor NA, Sanders RH, Howick El, Stanley SN: Static and dynamic assessment of the Biodex dynamometer. Eur J Appl Physi0162(3): , Volume 24 Number 1 July 1996 JOSPT