Factors influencing the reproducibility of isokinetic knee ... - IOS Press

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Isokinetics and Exercise Science 22 (2014) 333–342 DOI 10.3233/IES-140555 IOS Press

Factors influencing the reproducibility of isokinetic knee flexion and extension test findings Tobias Alt∗ , Axel J. Knicker and Heiko K. Strüder Institute of Movement and Neuroscience, German Sport University, Cologne, Germany

Received 6 May 2014 Accepted 24 July 2014

Abstract. BACKGROUND: Although the reliability of isokinetic strength tests of knee flexors (flex) and extensors (ext) has been examined several times, statistical evidence about the influence of internal and external factors is missing. OBJECTIVE: This study aims to examine the impact of familiarisation, muscle group, contraction mode, angular velocity and test parameters on the reproducibility of findings derived from an isokinetic dynamometer (IsoMed 2000). METHODS: Thirty-three male subjects (mean age: 22.3 years) with no prior experience of isokinetic exercise participated in three identical test sessions (T1, T2, T3), each separated by 48–72 h. Peak moment (PM), angle of peak moment (APM) and contractional work (CW) were determined unilaterally (left and right) during maximum concentric (con) and eccentric (ecc) knee flexion (abdominal position) and extension (supine position) at 30, 90 and 150◦ /s, respectively. An ANOVA with repeated measures confirmed systematic bias. Reproducibility of consecutive tests (T1–T2, T2–T3) was assessed by calculating the intra-class correlation coefficient (ICC 2,1) (relative reliability) as well as the standard error of measurement (SEM) (absolute reliability). ICC values were averaged according to respective factors (Fisher’s z-transformation) and tested for significant differences by Steiger’s formulas. RESULTS: PM and CW demonstrated a high absolute reliability (SEM: 4.7–10.5%). Relative reproducibility varied considerably (p 0.05) between muscles (ecc flex > ecc ext), contraction modes (con ext > ecc ext) and test parameters (PM = CW > APM), but did not depend on angular velocity (30 = 90 = 150◦ /s). Due to familiarisation the reliability of PM obtained from eccentric knee extensions significantly increased (T2–T3 > T1–T2). CONCLUSIONS: These results improve the development and execution of reliable isokinetic strength testing protocols for unilateral knee flexion and extension together with the interpretation of different test parameters. Keywords: Reproducibility, intra-class correlation coefficient, standard error of measurement, maximum strength testing, familiarisation, IsoMed 2000

1. Introduction Isokinetic exercise describes a movement with constant angular velocity and accommodating resistance. ∗ Corresponding author: Tobias Alt, Institute of Movement and Neuroscience, German Sport University, Am Sportpark Müngersdorf 6, 50933 Cologne, Germany. Tel.: +49 221 4982 3322; Fax: +49 221 4973 454; E-mail: [email protected].

Since its inception in the late 1960s, it has become an essential part in sports sciences and physiotherapy being applied both in research and training studies as well as in rehabilitation (for review see [1–4]). Isokinetic devices are used for training and testing purposes due to their unique contraction conditions [5–8], rehabilitative and preventive application possibilities [9–14] as well as their predictive power in relation to several sport-specific capabilities [15–19]. Reliability rep-

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T. Alt et al. / Factors influencing the reproducibility of isokinetic knee flexion and extension test findings

resents an important quality criterion of measurement instruments for research and performance diagnostics. It should be maximized by highly standardized measurement protocols to improve the subsequent suitability of data for meaningful evaluation and interpretation [2,20,21]. The intra-class correlation coefficient (ICC) is a common measure to confirm relative reliability, whereas the standard error of measurement (SEM) does so for absolute reliability [20,22,23]. It is recommended to combine both these methods as estimates of correlation attenuation (ICC) and of measurement precision of individual subjects’ scores (SEM) to gain a comprehensive insight into the test-retest reliability of measurement instruments [22,24,25]. There are plenty of studies investigating the reproducibility of isokinetic data obtained from the hip, knee, ankle, shoulder, elbow, wrist and trunk joint. Among them the knee is the most heavily examined joint in isokinetic studies [21]. Their results, summarized by the reviews of Nitschke [26] or more recently by Caruso et al. [21], generally indicate a good to excellent reliability of different isokinetic devices (Cybex, Biodex, Kin-Com, Lido, Con-Trex, IsoMed). Throughout the past five decades much research has been conducted to identify the factors influencing reliability of isokinetic knee exercises, such as familiarisation, muscle group, contraction mode, angular velocity and test parameters. Familiarisation is a well-known side effect of isokinetic strength tests impeding accurate assessment of reliability because it induces a practice-based improvement, not only of the knee joint [2,27–29]. A single habituation session lead to a 10% increase of concentric knee extensions’ PM and CW [30]. To produce stable isokinetic data, Sawhill et al. [31] advised a minimum of four test sessions. In contrast to this recommendation, Lund et al. [32] detected no learning effect within six isokinetic tests of concentric knee flexors and extensors. In general, at least a single habituation session should be executed to minimize accommodationrelated improvements in isokinetic strength diagnosis and to enhance the reproducibility [27–29]. Test-retest reliability was generally higher when data were obtained from the lower compared to the upper extremity [21,33]. Differences between the reproducibility of the concentric knee flexors and extensors usually did not become apparent [32–36]. In contrast, Keating and Matyas [37] stated in their review that the reliability of concentric knee flexion was generally higher than of concentric extension because the magnitude of PM and variability demonstrated a strong pos-

itive relationship [38]. This appeared to be true for eccentric contractions as well [39]. However, due to cocontraction phenomena during reciprocal tests some studies revealed the opposite [40,41]. Contraction mode did not appear to influence the reliability of isokinetic knee flexion and extension [8, 42–45]. However, two studies supposed that concentric measurements were more reproducible than eccentric ones [40,46]. This feature was explained by the fact that unaccustomed subjects were unfamiliar with maximum eccentric force production resulting in a greater variability [47]. With regard to the angular velocity, the test-retest reliability seems to diminish with increasing velocity, representing an often-cited problem in isokinetic dynamometry [39,41,48–51]. This phenomenon might emerge due to both mechanical (e.g. valid velocity adjustment) and psycho-physiological reasons (e.g. pain tolerance, injury scare) [21]. On the other hand, many results indicated no effect of angular velocity on the reliability of isokinetic knee flexion and extension [2,33, 34,36,42,43,46,52], which seems to be unique to the knee joint [21]. One study even reported higher relative reliability indices at 120◦ /s than at 60◦ /s [40]. It is well-recognised that the test-retest reliability of different test parameters varies considerably. Across the majority of studies, PM and CW displayed high reliability indices (ICC > 0.9; SEM < 10%), while APM showed a questionable reproducibility (ICC < 0.7; SEM > 10%) [2,8,21,33,34,39,42,43,53]. In some cases relative and absolute reliability of PM even exceeded the scores of CW [40,54]. To avoid drawing misleading conclusions about possible muscular adaptations, APM should not serve as an independent parameter in assessing isokinetic knee flexor and extensor characteristics [55]. Nevertheless, a reason for the low reliability of APM has not been mentioned or detected yet. In spite of an extensive research, to our knowledge no study has so far investigated if the abovementioned effects of the different factors influencing test-retest reliability actually reached statistical significance. Previous studies estimated the effect of different parameters by regarding the confidence intervals while except for two studies [46,56], a seated subjects’ positioning was examined. The aim of the present study is hence to identify which factors have a significant bearing on the reproducibility indices of unilateral isokinetic knee flexion and extension. This insight is important for both clinicians and researchers for the development and execution of reliable isokinetic strength testing protocols of the knee joint together with the appropriate interpretation of different test parameters [2,20,21].


2. Methods 2.1. Subjects Thirty-three physically active healthy male subjects (mean ± SD, age: 22.3 ± 3.9 years, height: 183.4 ± 5.5 cm, weight: 79.0 ± 8.4 kg) gave their written informed consent to participate voluntarily in the study. All of them were habituated to lower extremity resistance training, but had no previous experience in isokinetic exercise or testing. The local ethics commission confirmed that the requirements of the Declaration of Helsinki were met. Within the testing period, all subjects maintained their normal physical activity level except lower extremity resistance training. All subjects were free of injuries, pain or other limitations that might have inhibited maximum force exertion. 2.2. Instruments All tests were carried out at the isokinetic dynamometer IsoMed 2000 (D&R Ferstl GmbH, Hemau, Germany). A double shin pad for unilateral knee flexion and extension was attached to the motor-driven axis of the active dynamometer. According to the manufacturer’s guidelines, the device was calibrated before and after each testing session. Data were recorded by the manufacturer’s computer software IsoMed analyze V.2.0. A self-developed software was used to compute relevant test parameters from the obtained momentangle curves. 2.3. Testing procedure Each subject performed three identical testing sessions separated by 48–72 h. To minimize inaccuracies due to circadian rhythm and examiner variability, the tests were carried out at the same time of the day (± 1 h) and by the same examiner. After determining their weight, the subjects performed an individual warm-up (jogging, stretching) for ten minutes. This general warm-up was followed by the subject’s positioning and fixation on the dynamometer. The mechanical axis of the dynamometer was aligned with the lateral femoral epicondyle of the subject’s knee. The lining up with this bony reference of the joint’s axis of rotation was facilitated by a laser pointer. To minimize accelerative inaccuracies the subjects were asked to take off their shoes. At a 90◦ flexed knee, the distal part of the double shin pad of the dynamometer’s lever arm was fixed by a strap approximately 2–3 cm proxi-

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mal to the lateral malleolus. The individual settings of each subject were registered and stored by the manufacturer’s computer software. This guaranteed identical positioning throughout the three testing sessions. At the first session, a gravity correction measurement by the integrated software was conducted for each condition and each leg. Therefore, the lever arm was oriented horizontally and the muscles of the involved leg were completely relaxed. After all preparations, the subjects conducted a subsequent warm-up at the isokinetic dynamometer consisting of 15 submaximal concentric and eccentric repetitions of the respective muscle group. The testing order of the four conditions (left flexion, right flexion, left extension, right extension) was randomized but always alternating side (e.g. left, right, left, right). For knee flexion (flex), the subjects were asked to lie prone (extended hip) and to push their trunk with the hands to the lounger. The range of motion was set at 110◦ starting with full knee extension (0◦ ). Knee extension (ext) was performed in a supine position (extended hip). This positioning was chosen in order to simulate muscle fiber lengths that occur during the mid stance phase of running and sprinting [56,57]. Hand grips beside the hip served to maintain sufficient stability [58]. Movement ranged from 0◦ to 90◦ of knee flexion. For each condition, both concentric (con) and eccentric (ecc) contractions were conducted at three different angular velocities. Testing slow velocities prior to fast ones (30, 90 and 150◦ /s) meets the recommendations of Wilhite et al. [45]. The order of these six sets remained the same: con30◦, ecc30◦ , con90◦, ecc90◦, con150◦ and ecc150◦. To attain the largest possible isokinetic range of motion, a very quick acceleration at the beginning and a hard deceleration at the end of the movement were chosen. Each set consisted of five repetitions (two practice, three with maximum effort) of which the last three ones were analysed. All tests were performed as discrete movements in a single direction (uni-directional) which is recommended for valid reliability studies [59]. The return of the involved leg into starting position always occurred passively with 120◦ /s. The inter-set rest interval was set at one minute ensuring sufficient recovery [60,61]. After each condition the dynamometer’s position was adapted to the following testing condition which lasted approximately three minutes. Within the test, strong verbal encouragement ensured maximum effort. 2.4. Statistical analysis Raw data were recorded by the manufacturer’s software at a sampling rate of 200 Hz. Data was stored as

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ASCII files which were processed by a self-developed software isolating the isokinetic range of motion. To reduce oscillations of the derived moment-anglecurves, a 5th order Butterworth low-pass filter with a cut-off frequency of 6 Hz was applied. For each testing condition (left and right flex and ext) and contraction mode (con and ecc), the trial with the highest gravity-corrected peak moment (PM) was selected. Additionally, the corresponding angle of peak moment (APM) and contractional work (CW) were determined. Due to general recommendations, normalization to body weight was not executed [62,63]. To compare the amount of CW performed during knee flexion and extension, their values were divided by the respective range of motion (110◦ and 90◦ ). A repeated measures analysis of variance (ANOVA) with Bonferroni post hoc test was conducted to reveal systematic bias. As the side (left, right) showed no significant difference their values were combined so that the actual sample size was 66. As the Levene’s test confirmed homoscedasticity for all remaining factors (test session, muscle group, contraction mode and angular velocity, test parameter), the intra-class correlation coefficient (ICC 2,1) as a measure of relative reliability and √ the standard error of measurement (SEM = SD * 1 − ICC) as an index of absolute reliability were calculated with SPSS V.22.0 (SPSS Inc., Chicago, Illinois, USA) [20,22,49]. The level of signi?cance was set at α 0.05. In agreement with general recommendations, an ICC of above 0.9 was regarded as high, values between 0.9 and 0.8 as moderate and those under 0.8 as low [64]. In addition to this three-layered approach of reliability studies (1. repeated measures ANOVA to determine systematic errors; 2. ICC to assess correlation attenuation; 3. SEM for the interpretation of scores from individual subjects) recommended by Weir [22], the influence of the factors test session, muscle group, contraction mode, angular velocity and test parameter on the relative reliability (ICC) was statistically verified. This method was adopted from a clinical study of Blekher et al. [65], employing Fisher’s z-transformation to identify significant influencing variables on ICC values. With respect to the relevant factors (test session, muscle group, contraction mode, angular velocity and test parameter), ICC values were averaged by Fisher’s z-transformation to normalize and variance stabilize their ICC values and to prepare them for subsequent inferential analysis. Significant differences (α 0.05) between the ICC values of the respective conditions were examined with the aid of Steiger’s formulas [66]

to compare dependent (test session) and independent correlations (muscle group, contraction mode, angular velocity and test parameter) measured on the same subjects. 3. Results Repeated measures ANOVA revealed a systematic increase of PM obtained from concentric knee flexion after the first test (range of T1 vs. T2: 2.9–3.6%; range of T1 vs. T3: 4.2–5.0) (Table 1). This statistical trend was less pronounced regarding the CW although the relative improvements were similar (range of T1 vs. T2: 2.5–3.7%; range: of T1 vs. T3: 3.3–5.6%). In comparison to T1, significantly higher values of concentric extension were observed at T3, both for PM (range: 2.5–4.2%) and CW (range: 1.7–3.8%). PM of eccentric knee flexion showed an increase between T2 and T3 at all angular velocities (range: 2.9–3.1%), whereas CW was just elevated at 90◦ /s (3.0%). Eccentric extensions revealed a continuous increase of PM (range of T1 vs. T2: 6.1–8.5%; range: of T2 vs. T3: 4.0–5.1%) and CW (range of T1 vs. T2: 4.9–7.4%; range of T2 vs. T3: 0.9– 4.4%) throughout all three test sessions accompanied by a significant shift of APM towards higher knee flexion (range of T1 vs. T2: 2.0–5.7%; range of T1 vs. T3: 3.8–10.4%) (Table 1). As all data were homoscedastic in nature, relative (ICC) and absolute (SEM) indexes of reliability were calculated. These indices are listed in Tables 2 and 3 according to the respective factors. The test parameters PM and CW offered moderate (0.8–0.9) to high (> 0.9) ICC values with corresponding SEM values of 4.7–10.5% for concentric and eccentric knee flexion and extension. These amounts exceeded by far the reliability of APM whose ICC values (< 0.7) were rated as questionable (Table 2) [64]. Familiarisation within the test sessions led to an elevated reliability of PM obtained from eccentric knee extensions. The significant influence of muscle group and contraction mode on the reproducibility of PM and CW became apparent as eccentric knee flexions and concentric knee extensions were significantly more reliable than eccentric knee extensions (Table 2). Angular velocity did not influence the reliability scores of any test parameter derived from unilateral isokinetic knee testing (Table 3). 4. Discussion The aim of the present study was to identify the factors influencing test-retest reliability of unilateral isok-

285.3 ± 45.6* 243.4 ± 39.0 220.5 ± 36.5*

ext con 30◦ /s 273.7 ± 41.7 280.0 ± 42.4 90◦ /s 237.4 ± 30.9 239.7 ± 36.4 150◦ /s 213.2 ± 28.2 215.5 ± 33.3 0.000 0.000 0.000

0.008 0.075 0.010

0.009 0.014 0.045

0.000 0.000 0.003

p-value T2

13.7 ± 11.4 19.4 ± 13.2 24.2 ± 12.6

T3

66.2 ± 8.0 61.8 ± 7.2 67.5 ± 9.7

70.4 ± 8.4 71.8 ± 9.0 65.3 ± 9.0 68.8 ± 8.3* 65.4 ± 10.6 69.2 ± 9.9

66.5 ± 7.7 62.0 ± 7.5 66.7 ± 9.1 0.720 0.671 0.666

0.493 0.010 0.004

0.415 0.501 0.261

p-value

73.1 ± 6.8* 0.037 72.1 ± 7.9* # 0.000 69.4 ± 8.0* 0.034

67.0 ± 7.4 61.2 ± 7.8 66.6 ± 9.6

17.3 ± 15.5 19.6 ± 17.6 20.0 ± 19.9 19.3 ± 15.0 23.9 ± 17.8* 22.3 ± 16.9 21.5 ± 16.7 28.6 ± 20.3* 24.5 ± 17.9

14.5 ± 7.5 13.1 ± 7.5 20.7 ± 11.6 21.3 ± 12.2 25.6 ± 13.2 27.0 ± 14.4

T1

APM [◦ ] T2

T3

3.23 ± 0.45* 3.10 ± 0.45* 2.88 ± 0.45

1.93 ± 0.38 2.10 ± 0.38# 2.22 ± 0.43

0.014 0.022 0.247

0.063 0.026 0.078

0.000 0.002 0.060

p-value

3.60 ± 0.55 3.78 ± 0.66* 3.81 ± 0.52* 0.004 3.87 ± 0.55 4.15 ± 0.66* 4.34 ± 0.66* # 0.000 4.14 ± 0.63 4.40 ± 0.71* 4.57 ± 0.81* # 0.000

3.11 ± 0.42 3.19 ± 0.46 3.03 ± 0.36 3.09 ± 0.46 2.83 ± 0.37 2.86 ± 0.42

1.87 ± 0.42 1.91 ± 0.44 2.05 ± 0.40 2.04 ± 0.43 2.17 ± 0.43 2.18 ± 0.47

1.62 ± 0.30 1.68 ± 0.34* 1.71 ± 0.28* 1.59 ± 0.31 1.63 ± 0.32 1.65 ± 0.30* 1.51 ± 0.34 1.55 ± 0.33 1.56 ± 0.32

T1

CW [J/◦ ]

0.913# 0.886

ecc T1–T2 T2–T3

# 0.000

6.8; 4.7 7.7; 5.3

7.0; 6.0 7.5; 6.3

0.663 0.829*

0.831‡ 0.881

ICC

*0.011

‡ 0.011

pvalue

SEM [Nm;%]

pvalue

35.2; 10.5 27.9; 7.8

18.1; 7.4 16.3; 6.6

SEM [Nm;%]

0.565 0.601

0.468 0.481

ICC

pvalue

flex

11.6; 53.2 11.7; 50.7

9.1; 44.5 9.4; 47.5

SEM [◦ ;%]

0.401 0.491

0.503 0.556

ICC

APM pvalue

ext

7.3; 10.7 6.1; 8.6

6.0; 9.2 5.8; 8.9

SEM [◦ ;%]

0.915# 0.906#

0.898 0.895

ICC

# 0.003

# 0.000

pvalue

flex

0.13; 6.4 0.13; 6.5

0.10; 6.6 0.10; 6.3

SEM [J/◦ ;%]

0.635 0.751

0.827‡ 0.886‡

ICC

CW

‡ 0.013

‡ 0.013

ext pvalue

0.41; 10.3 0.37; 8.7

0.44; 6.0 0.47; 5.2

SEM [J/◦ ;%]

Significant differences (p 0.05) are indicated with respect to T1-T2 (*), ext(# ) and ecc (‡ ), PM = peak moment, APM = angle of peak moment, CW = contractional work, flex = flexion, ext = extension, con = concentric, ecc = eccentric.

0.906 0.892

con T1–T2 T2–T3

ICC

ext

flex

PM

Table 2 Effect of familiarisation, contraction mode and muscle group on relative (ICC) and absolute (SEM) reliability of isokinetic knee flexion and extension

*Significant difference to T1, # significant difference to T2 (p 0.05), PM = peak moment, APM = angle of peak moment, CW = contractional work, flex = flexion, ext = extension, con = concentric, ecc = eccentric.

ecc 30◦ /s 337.1 ± 60.3 357.7 ± 70.6* 372.2 ± 63.9*# 90◦ /s 322.2 ± 45.1 349.5 ± 64.6* 367.2 ± 66.4*# 150◦ /s 313.1 ± 46.6 337.0 ± 63.2* 353.1 ± 72.8*#

147.5 ± 29.2*# 150.0 ± 28.7# 149.4 ± 29.9#

ecc 30◦ /s 142.2 ± 30.6 143.1 ± 27.9 90◦ /s 147.3 ± 28.6 145.4 ± 28.6 150◦ /s 145.8 ± 28.7 145.2 ± 31.2

T1 T2 T3 flex con 30◦ /s 124.8 ± 23.7 128.4 ± 24.5* 130.6 ± 23.0* 90◦ /s 112.8 ± 20.7 116.9 ± 19.4* 118.4 ± 21.6* 150◦ /s 105.7 ± 21.5 109.4 ± 18.9* 110.1 ± 20.6*

PM [Nm]

Table 1 Mean values and standard deviations of isokinetic data obtained from concentric and eccentric knee flexion and extension at three test sessions (T1, T2, T3)

T. Alt et al. / Factors influencing the reproducibility of isokinetic knee flexion and extension test findings 337

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inetic knee exercise obtained from inexperienced subjects. To gain a complete insight into the muscular capacities of the knee joint, both muscle groups (flexors and extensors) as well as both contraction modes (concentric and eccentric) were examined. This is recommended for meaningful assessment of muscular balance and subsequent purposeful training application and injury prevention [5–8]. After an initial habituation session (T1), concentric flexion and extension produced stable isokinetic data of PM and CW (Table 1). This is in accordance with recent research suggesting that a single habituation session is sufficient for inexperienced subjects to eliminate practice-induced improvements [2,27,28, 30]. However, this was not applicable to eccentric flexion and extension revealing still significant increases after T1. Due to substantial delayed onset of muscular soreness (DOMS) after T1, PM of eccentric flexion remained constant at T2. In the course of a repeated bout effect, a fast neural adaptation such as an increased motor unit synchronization might have lead to a significant rise of PM at T3 [67,68]. The fact that CW was not affected, supported the conclusion that just a small portion of the range of motion (ROM) was influenced by this effect. With regard to eccentric extension, both PM and CW demonstrated a continuous increase throughout all three test sessions accompanied by a significant shift of APM towards higher knee flexion (Table 1). This indicated that the subjects were able to sustain muscular tension during eccentric extensions longer because PM was attained in the latter part of the movement where high knee flexion occurred. This effect emerged at all angular velocities (30, 90 and 150◦ /s) and revealed that habituation caused increased pain tolerance and reduced injury scare [47]. Therefore, more than one familiarisation session is needed to receive stable PM obtained from eccentric knee flexion and extension. Consequently, the general advice of Sawhill et al. [31] that a minimum of four test sessions – equal to two familiarisation sessions – is necessary to produce stable isokinetic data, was confirmed by this study for eccentric, but not for concentric isokinetic knee exercise. Dirnberger et al. [28] did not detect such a different habituation pattern between concentric and eccentric knee extensions with maximum effort. This might be due to methodological differences with respect to subjects’ positioning (seated vs. prone/supine) and threshold value (50 Nm vs. none). To our knowledge, the only isokinetic studies examining the test-retest reliability of concentric and eccentric knee flexion and extension in prone or supine position

were performed by Tredinnick and Duncan [46] and Ayala et al. [56]. However, up until now, no information is available regarding the impact of joint configuration on the reproducibility of isokinetic knee exercise findings. There are a lot of assumptions about which internal and external factors might influence the testretest reliability of isokinetic knee exercise [for review see [21,26]. In spite of an extensive research, there is a lack of statistical evidence proving significant impact of familiarisation, muscle group, contraction mode and angular velocity on the reproducibility of different test parameters. 4.1. Familiarisation Although familiarisation is a well-known side effect of isokinetic knee testing impeding accurate assessment due to a practice-based improvement [2,27–31], its impact on test-retest reliability has not been statistically analysed yet. Dirnberger et al. [28] demonstrated high reliability of concentric and eccentric knee extensions as ICC values ranged from 0.907–0.955 and SEM indices from 7.4–19.1 Nm for T1–T2. T2– T3 showed an enhanced reproducibility both for relative (ICC: 0.968–0.984) and absolute reliability (SEM: 4.5–10.9 Nm) suggesting the benefit of a familiarisation session for reproducibility. However, this assumption is not statistically proven. The present data clarifies that habituation just had a significant impact on relative reliability of PM obtained from eccentric knee extensions (T2–T3: 0.829 > T1–T2: 0.663; p 0.05), whereas all other testing conditions and test parameters showed no enhanced reliability due to learning effects (Table 2). One potential reason for this effect is that the subjects were not used to perform eccentric knee extensions with extended hip joint. Within the three tests, increased pain tolerance and reduced injury scare caused more reliable data [47]. In contrast to previous reports [28], absolute reliability of isokinetic singlejoint knee exercise – as an indicator of measurement precision – generally did not display enhanced reproducibility throughout the three tests (Table 2). 4.2. Muscle group Inferential statistical analyses revealed a significant influence of the muscle group on the reproducibility of isokinetic knee joint exercise. PM and CW obtained from eccentric flexion were significantly (p 0.01) more reliable than data from eccentric extension, whereas concentric contractions showed no de-


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Table 3 Effect of angular velocity and test parameter on relative (ICC) and absolute (SEM) reliability of maximum isokinetic knee exercise ICC

PM p-value

SEM [Nm;%]

ICC

T1-T2 30◦ /s 90◦ /s 150◦ /s

0.851* 0.852* 0.839*

*0.000 *0.000 *0.000

39.4; 17.7 36.1; 17.4 36.3; 18.3

T2-T3 30◦ /s 90◦ /s 150◦ /s

0.874* 0.879* 0.868*

*0.000 *0.000 *0.000

38.1; 16.6 35.7; 16.6 36.0; 17.6

APM p-value

SEM [◦ ;%]

ICC

CW p-value

SEM [J/◦ ;%]

0.447 0.484 0.509

21.2; 50.5 17.7; 41.6 17.4; 37.8

0.822* 0.853* 0.856*

*0.000 *0.000 *0.000

0.41; 15.8 0.40; 14.9 0.43; 15.8

0.590 0.491 0.517

18.7; 43.9 18.2; 41.7 17.4; 37.2

0.833* 0.888* 0.882*

*0.000 *0.000 *0.000

0.40; 15.3 0.37; 13.5 0.41; 15.0

Significant differences (p 0,05) to APM are indicated with *(‡ ), PM = peak moment, APM = angle of peak moment, CW = contractional work.

pendence on muscle group (Table 2). This evidence supports the finding of Steiner et al. [39], but disagrees with several other studies which found no difference of reliability between knee flexors and extensors [32–36]. Keating and Matyas [37] stated in their review that the relative reliability of concentric knee flexion was higher than of concentric extension because there was a strong positive relationship between the magnitude of PM and variability [38]. However, this justification cannot be applied to the present data as they were all homoscedastic in nature. A speculative and not verifiable explanation for the significantly lower reliability of PM and CW during eccentric extension might be a greater misalignment between axis of dynamometer and anatomical knee axis of rotation during maximum force exertion resulting in unpredictable measurement errors. The already mentioned increased pain tolerance and reduced injury scare might have had an additional impact [47]. 4.3. Contraction mode Contraction mode appeared to influence reliability of PM and CW as concentric knee extensions were significantly (p 0.05) more reproducible than eccentric ones (Table 2). This is in accordance with available experimental data [40,46]. This effect might emerge due to the fact that unaccustomed subjects were unfamiliar with maximum eccentric force production resulting in greater variability [47]. Concerning maximum knee flexions, no systematic effect of contraction mode became apparent supporting results from recent research [8,42–45]. However, these studies reported comparable reliability indices of knee extensors, too. According to the present data, the statement that concentric isokinetic contractions are more reproducible than eccentric ones can be confirmed for knee extensors, but denied for knee flexors.

4.4. Angular velocity Angular velocity had no bearing on the test-retest reliability of maximum isokinetic knee tests (Table 3). This result supports the existing opinion that the reliability of isokinetic knee flexion and extension is velocity-independent [2,33,34,36,42,43,46,52], representing apparently a unique feature of the knee joint [21]. As neither mechanical (e.g. valid velocity adjustment) nor psycho-physiological reasons (e.g. pain tolerance, injury scare) appear to interfere the results [21], PM (ICC: 0.839–0.879; SEM: 16.6– 18.3%) and CW (ICC: 0.822–0.888; SEM: 14.9– 15.8%) showed a similarly moderate reliability across the three examined angular velocities when concentric and eccentric flexion and extension reliability indices were averaged. 4.5. Test parameters The present study supports the majority of studies, demonstrating that PM and CW were moderately reliable, but APM displayed a questionable reproducibility [2,8,21,33,34,39,42,43,53] (Table 3). Potential reasons for the weak reliability of APM might be differences in building up muscular tension as well as the interaction between the contractile and the series elastic components of the muscle-tendon unit [69]. Although the highly significant effects of test parameters on testretest reliability were derived from data listed in Table 3 (PM = CW > APM, p 0.01), reliability scores in Table 2 offered interesting insights, too. Absolute reliability indices (SEM) did not differ between the three test parameters (PM: 4.7–10.5%; APM: 8.6–10.7%, excluding flexion values; CW: 5.2–10.3%), whereas the relative reproducibility (ICC) was moderate to high for PM (0.831–0.913, excluding eccentric extension) as well as for CW (0.827–0.915, excluding eccentric

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extension) and only questionable for APM (0.401– 0.601) (Table 2). This indicates that no difference in measurement precision of individual subjects’ scores (reliability of absolute values), but a correlation attenuation (reproducibility of subjects’ order in the analysed cohort) has taken place between these three common isokinetic test parameters [22,24,25]. The high relative SEM [%] of APM obtained from concentric and eccentric flexion is misleading. Absolute SEM [◦ ] was not substantially different between flexion and extension, but due to the small mean value of APM during flexion the score of relative SEM is that huge. Because of this reason both SEM values are reported in Tables 2 and 3. The present results support general recommendations, that PM and CW serve as reliable parameters to evaluate isokinetic knee flexor and extensor characteristics, whereas APM should not be used and evaluated as an independent parameter [2,8,21,55]. 4.6. Limitations and further research The authors are aware of the fact that the present study comprised potential methodological limitations (e.g. minimal stabilization, no pre-activation, maximum values) which might have impaired the reproducibility. Nevertheless, the results obtained from inexperienced subjects revealed moderate to high reliability scores of unilateral concentric and eccentric knee flexion and extension with regard to PM and CW which allow a meaningful and reliable interpretation of the present data. Other important aspects have to be considered because their impact on isokinetic data and reliability appears to be obvious, but has not been investigated and statistically analysed yet. Some of them are the isokinetic dynamometer itself (e.g. Biodex vs. IsoMed), subject’s stabilization (e.g. straps and pads vs. hand grips), test design (e.g. reciprocal vs. discrete), joint configuration (e.g. flexed vs. extended hip joint) and pre-activation level (e.g. selected threshold value vs. none) as well as data treatment (e.g. normalisation for body weight vs. absolute values) and descriptive statistics (e.g. maximum vs. mean values). Further research is needed to determine the number of test sessions required to produce stable eccentric isokinetic data. Furthermore, the potential impact of other factors (e.g. axis alignment, pre-activation) on the reliability of isokinetic exercises must still be proven. Although angular velocity showed no influence, it should be verified whether the reliability of functional agonist-antagonist ratio of the knee flexors and extensors is velocity-independent, too. This is es-

sential for valid and meaningful assessment of muscular balance and subsequently purposeful training application and injury prevention [5–8].

5. Conclusion Test-retest reliability of unilateral isokinetic knee flexion and extension varied considerably (p 0.05) between muscles (ecc flex > ecc ext), contraction modes (con ext > ecc ext) and test parameters (PM = CW > APM), but did not depend on angular velocity (30 = 90 = 150◦ /s). Due to familiarisation effects the reliability of PM obtained from eccentric knee extensions significantly increased over the course of test sessions (T2–T3 > T1–T2). This statistical evidence clarified the impact of internal and external factors influencing test-retest reliability of isokinetic knee exercises. Eccentric extensions revealed a continuous improvement of PM and CW throughout all test sessions accompanied by a significant shift of APM towards higher knee flexion. Nevertheless moderate to high reliability indices of PM and CW ensured a meaningful interpretation of maximum muscular capacities of the knee flexors and extensors, whereas a valid analysis of APM remained questionable. For concentric isokinetic knee flexion and extension, a single habituation session revealed diagnostically conclusive data obtained from inexperienced subjects, whereas eccentric knee exercises appeared to require multiple familiarisation sessions to eliminate practise-induced improvement. These results enhance the development and execution of reliable isokinetic strength testing protocols for unilateral knee flexion and extension together with the interpretation of different test parameters.

Acknowledgement There is no conflict of interest. The authors would like to thank all subjects who volunteered to participate in this study and demonstrated great motivation and commitment.

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