The reproducibility of three different indicators of fatigue ... - IOS Press

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Isokinetics and Exercise Science 21 (2013) 227–236 DOI 10.3233/IES-130507 IOS Press

The reproducibility of three different indicators of fatigue from plantar-flexion isokinetic testing at two knee flexion angles is not sufficient to be termed ‘acceptable’ Kim Hébert-Losier∗ , Sarah J. Willis and Hans-Christer Holmberg Swedish Winter Sports Research Centre, Department of Health Sciences, Mid Sweden University, Östersund, Sweden

Received 18 August 2012 Accepted 26 April 2013

Abstract. OBJECTIVE: To characterize the reproducibility of three indicators of fatigue (FIs) specific to concentric plantar-flexion isokinetic testing at knee flexion (KF) angles of 0◦ (straight) and 45◦ (bent). METHODS: On two separate days one week apart, thirty-four males performed 50 consecutive maximal concentric isokinetic plantar-flexion contractions at 60◦ /s with 0◦ and 45◦ of KF. Differences in the pre- and post-maximal voluntary isometric contraction torques (FIstatic ), powers during the initial five and last five isokinetic contractions (FIdynamic ) and powers during the 50 isokinetic contractions (FIslope ) were used as FIs. Changes in means, intra-class correlation coefficients (ICCs) and coefficients of variations (CVs) were computed to quantify the reproducibility of the FIs. Comparisons were made between the two KF angles and three FIs using two-way repeated measures ANOVA. RESULTS: For both KF angles and three FIs, ICCs ranged from 0.52 to 0.71 and CVs from 10.0 to 29.3%. The CVs from the two isokinetic-based FIs were lower than those from the isometric FI and a trend towards larger ICCs at 0◦ was observed. CONCLUSIONS: The reproducibility of the three FIs was not sufficient to be termed ‘acceptable’. The FIslope and FIdynamic were more reproducible than FIstatic and are recommended – with the knee straight rather than bent – until more reliable indicators become available. Keywords: Ankle, endurance, IsoMed2000-dynamometer, lower extremity, musculoskeletal, test-retest reliability, triceps surae muscle

1. Introduction Adequate strength and endurance of the triceps surae muscle group, the prime mover for ankle plantarflexion [1], is not only required to perform activities of ∗ Corresponding author: Kim Hébert-Losier, Swedish Winter Sports Research Centre, Department of Health Sciences, Mid Sweden University, Kunskapens väg 8, Hus D, 83125 Östersund, Sweden. Tel.: +41 46 6316 5632; Fax: +41 46 6316 5740; E-mail: [email protected].

daily living, but also to optimize sports performance. Since this muscle-tendon unit responds to repetitive high loads and is important in human locomotion [2, 3], the ability of the plantar-flexors to resist to fatigue is routinely assessed in health care. In both clinical and experimental settings, the triceps surae muscle group is often assessed and trained at two or more angles of knee flexion (KF) to vary the relative contributions of the biarticular gastrocnemius and monoarticular soleus muscles to plantar-flexion [4]. In laboratory settings, isokinetic dynamometry ser-

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K. Hébert-Losier et al. / The reproducibility of three different indicators of fatigue from plantar-flexion isokinetic testing

ves as a useful tool for evaluating the neuromuscular function of the triceps surae muscle group and has provided valuable insight into its morphology and function [5], pathogenesis [6] and response to training [7]. However, clinical application of ankle isokinetics requires that outcomes from testing be reproducible in terms of its absolute and relative quantifiers. Only reproducible outcomes can be used to monitor small, but nonetheless functionally meaningful, changes; document the evolution of pathology or training status; and compare various testing methods or instruments [8,9]. In research, adequate reproducibility is necessary to assess changes in experimental trials and the effectiveness of interventions [8]. In general, there are only a handful of investigations concerning the reproducibility of isokinetic measurements of muscles operating on the ankle [10] and most of these focus on strength rather than endurance [10– 13]. Since higher numbers of contractions are needed to evaluate triceps surae muscle endurance, employing the reproducibility measures from these studies to guide the assessment of plantar-flexion endurance is not appropriate. The indicators of fatigue (FIs) most often computed from muscle testing protocols include pre-topost changes in maximal voluntary isometric contraction and reductions in peak power during serial contractions [14,15]. However, there are generally few reports on the reproducibility of such FIs [16] and only two concerning the triceps surae muscle group [17, 18], with both involving individuals with intermittent claudication secondary to peripheral artery disease and lacking healthy controls. This study was designed to characterize the reproducibility of three different FIs specific to the performance of 50 maximal concentric isokinetic plantarflexion contractions at 60◦ /s and at two KF angles (0◦ and 45◦ ) in healthy individuals. These FIs and KF positions were chosen because of their scientific and clinical value in the field of physical therapy, sports and exercise science and rehabilitative medicine.

2. Methods 2.1. Study design This study used a repeated-measures design that required each subject to attend two experimental sessions, including familiarization, at the muscle performance laboratory of a sports research facility. The two

test sessions were performed one week apart at the same time of day and led by the same experienced examiner. At both sessions, each individual was tested at 0◦ and 45◦ of KF in a block-randomized order, with half the subjects beginning at 0◦ and the others at 45◦ . The same sequence of testing was used on the two test occasions for a given individual. The research protocol was pre-approved by the Regional Ethical Review Board and adhered to the latest amendment of the Declaration of Helsinki. 2.2. Subjects After providing informed written consent, a sample of convenience consisting of 34 males (mean ± SD; age: 27.9 ± 5.6 years; body height: 181.3 ± 6.0 cm; body mass: 79.2 ± 10.0 kg; body mass index: 24.1 ± 2.8 kg·m−2 ) with no previous experience in using an isokinetic dynamometer for testing or training completed this study. The inclusion criteria were good selfreported general health, regular recreational physical activity, and no current or recent (less than 3 months) musculoskeletal injury, joint pathology or other medical condition that could limit isokinetic plantar-flexion performance. Highly competitive or elite athletes were excluded, as were highly sedentary individuals. All subjects were requested to refrain from strenuous physical activity and lower-body training for the 24-h period prior to experimentation and the ingestion of caffeine on the days of testing. Since dominancy can influence dynamometric measurements [19], only the dominant leg was tested. Each subject performed 4 skilled tasks using either the right or left foot: kick a ball, stamp out a simulated fire, pick up a marble and trace a shape. The side that was used most often to perform these tasks defined the dominant leg [20]. Based on this approach, the right foot was found to be dominant in 29 subjects and the left foot in the remaining 5. 2.3. Instrumentation All isokinetic testing procedures were performed using the IsoMed2000-dynamometer (D. & R. Ferstl GmbH, DE) with the foot adapter in place. Calibration of this device was certified and verified each day prior to collecting data that were sampled at a rate of 200 Hz R using the IsoMed2000 Software 2008.1.2RV1. 2.4. Experimental set-up With the subject in supine on the IsoMed2000dynamometer, the dominant foot was firmly attached to the adapter using 3 Velcro straps: two on its prox-


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Fig. 1. Positioning of the subject on the IsoMed2000-dynamometer for the maximal isokinetic plantar-flexion fatiguing test at two angles of knee flexion (KF). A. The dominant foot was secured to the adapter using 3 Velcro straps. B. Shoulder pads and a pelvic belt were used to minimize extraneous and upper-body motion. C. A support was placed above the thigh to secure the knee at 0◦ . D. A support was placed below the thigh to secure the knee at 45◦ .

imal aspect immediately distal to the ankle joint-line and one on the distal aspect of the foot (Fig. 1A). The position was adjusted to align the axes of rotation of the ankle joint and dynamometer, and two shoulder pads and a broad pelvic belt were used to minimize extraneous movements. The subject was also permitted to grasp the shoulder pads of the isokinetic device to enhance upper-body stabilization (Fig. 1B). Subsequently, the knee was fixed at an angle of either 0◦ (knee straight with the femoral and tibial bones in alignment, Fig. 1C) or 45◦ (knee bent, Fig. 1D). The examiner confirmed proper positioning using a goniometer (Model01135, Lafayette Instrument Europe, UK) and secured the knee angle with a thigh support. The knee and ankle were stabilized as to prevent superior displacement and the proper alignment of the axes of rotation of the ankle and dynamometer during maximal plantar-flexion efforts was verified prior to data collection. Once the subject was positioned on the IsoMed2000dynamometer with either 0◦ or 45◦ of KF, the range of motion of the ankle for the isokinetic trials was set following a series of systematic steps (Fig. 2). First,

the ankle was placed in a neutral position (i.e. with the 5th metatarsal and fibula bones perpendicular to one another, as verified with the goniometer (Fig. 2A). The foot adapter was thereafter moved passively to the end-range of dorsi-flexion (Fig. 2B). From this position, the dynamometer was moved 2 to 3◦ in the opposite direction towards plantar-flexion to avoid strain from forceful muscle contraction in an elongated position [21]. This ankle position defined the starting position of the concentric plantar-flexion contractions of the isokinetic trial (Fig. 2B). Thereafter, the foot adapter was moved actively to the end-range of plantar-flexion, stopping when the subject or examiner judged that the force of plantarflexion was no longer being transferred effectively to the foot adaptor. The dynamometer was then moved passively 2 to 3◦ in the opposite direction towards dorsi-flexion to ensure effective force transmission and define the ending position of the concentric plantarflexion contractions of the isokinetic trial (Fig. 2C). The settings were recorded for each individual on the first session and used on the second session a week later.

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Fig. 2. The procedure followed to set the range of ankle motion for the 50 concentric plantar-flexion contractions performed in the fatiguing isokinetic trial. The steps are presented in sequence from A to C. represents a joint angle. A. First, the ankle was placed in a neutral position. B. Next, the starting angle was set by moving the ankle first to a maximal dorsi-flexion position (the white bar outlined in black dashes) and then 2 to 3◦ towards plantar-flexion (the white bar outlined in solid black). C. Finally, the ending angle was set by moving the ankle first to maximal plantar-flexion (the white bar outlined in black dashes) and then 2 to 3◦ towards dorsi-flexion (the white bar outlined in solid black).

For the isokinetic trials, the IsoMed2000-dynamometer was programmed to monitor 50 consecutive concentric plantar-flexion contractions at 60◦ /s, with passive return of the ankle to the starting position at 90◦ /s. A total of 50 contractions was selected since the torque has been reported to remain relatively stable after 50 contractions in trials involving 100 such contractions [22]. Utilizing the angular velocity of 60◦ /s allowed comparison to prior isokinetic reports [5, 22,23]; whereas programming the passive return at 90◦ /s ensured a plantar-flexion cycle of approximately 1 Hz [24]. To optimize the isokinetic segment of the concentric plantar-flexion contractions, rapid accelerations and decelerations were utilized; and to minimize force oscillations, 50 N was required to initiate each contraction. 2.5. Experimental protocol After determining body height, body mass and the dominant foot, each subject was positioned on the IsoMed2000-dynamometer at either 0◦ or 45◦ of KF (Fig. 3), and the range of ankle motion for the isokinetic trial was set. The range of ankle motion was 49 ± 8◦ with the knee straight and 50 ± 10◦ with the knee bent. Following subject set-up on the dynamometer, the ankle was placed in the neutral position and the subject was asked to remain as relaxed as possible while the software of the isokinetic dynamometer compensated for gravity. Thereafter followed familiarization with performance of maximal voluntary isometric contractions (MVICs) involving 5 sub-MVICs. This was followed by three 7-sec MVICs, separated by 2 min of rest, to assess peak isometric torque. During the subsequent warm-up and more specific familiarization, each subject practiced initiating and

completing concentric isokinetic plantar-flexion contractions at 60◦ /s throughout the previously determined range of motion, relaxing as the dynamometer passively moved the ankle back to the dorsi-flexion starting position at 90◦ /s. The isokinetic familiarization involved performing 5 isolated sub-maximal, 3 isolated maximal, 5 successive sub-maximal and 3 successive maximal concentric plantar-flexion contractions (Fig. 3). Each subject then rested for 10 min to alleviate fatigue. Prior to the actual isokinetic fatiguing test, each subject performed a single isolated 7-sec MVIC identical to the three that were performed previously to ensure that the peak torque was ± 5% of the three others (indicating no residual fatigue). Immediately prior to the isokinetic trial, each subject was reminded of the importance of pushing as hard as possible throughout the entire range of motion during each of the 50 consecutive plantar-flexion contractions, relaxing afterwards to allow passive return of the ankle to the starting position. The verbal introduction of “ready, 3–2–1–PUSH” was the cue provided to initiate the 50 maximal isokinetic contractions. Throughout the isokinetic trial, the examiner provided strong and consistent verbal encouragement and the IsoMed2000 monitor displayed continuous visual feedback to promote maximal effort. The halfway point of 25 completed contractions was indicated to the subject and the final 10 contractions were counted down. After completion, each subject rated the perceived exertion in their triceps surae muscles on the 6– 20 point Borg [25] scale. One final 7-sec MVIC was then performed to assess the post-trial peak isometric torque. Each subject then rested for 1 hr before repeating the procedure with the knee in the alternate position.


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contraction that followed the protocol to compute an FIstatic using the equation: [(Pre − Post)/Pre) × 100%]. Similarly, the peak torque achieved during each concentric isokinetic contraction was converted to units of power [torque × velocity (W)] to provide a measure specific to this mode of testing. An FIdynamic value was derived from the highest peak power from the first five contractions and the lowest from the last five using the equation: [(First − Last)/First) × 100%]. Finally, the line provided by the linear regression of the peak powers for all 50 isokinetic contractions was extrapolated to the y-axis. The slope of this line was normalized to its intercept (%/rep) and used to define an FIslope value. For both FIstatic and FIdynamic , higher FI values represented greater fatigability whereas lower values indicated greater resistance to fatigue. In the case of the FIslope , a more negative slope indicated a more rapid decrease in power and greater fatigability. 2.7. Data analysis

Fig. 3. Flow diagram illustrating the experimental procedure for a subject. Abbreviations: KF, knee flexion; MVIC, maximal voluntary isometric contraction; ROM, range of motion. * Concentric plantar-flexion contractions at 60◦ /s with passive dorsi-flexion return at 90◦ /s through the set ankle ROM.

2.6. Data processing Angular position (◦ ), velocity (◦ /s) and torque (Nm) data were transferred wirelessly from the R IsoMed2000 Software to a computer in the ASCII format; and then processed with a 4th order zero-lag Butterworth filter and cut-off frequency of 10 Hz. The peak isometric torque (Nm) from the MVICs completed before the fatiguing protocol in each of the two KF angles was compared to the corresponding

Mean ± SD values were computed for all variables to describe the data. Heteroscedasticity was formally inspected by plotting the absolute difference between the values obtained on the two test days against the individual means and calculating the correlation coefficients by linear regression. Heteroscedascity correlations were close to zero for all three FIs at both KF angles, ranging from −0.050 to 0.209 with associated P -values of 0.110 to 0.885. Accordingly, raw FI values were used to derive the reproducibility measures. The intra-class correlation coefficient (ICC) for each variable was computed to determine the relative reproducibility of measurements at one-week. A modified statistical spreadsheet was used to compute the ICC as: [ICC = 1 − (TEM/SD)2 ], where TEM is the typical error of measurement calculated√as: [TEM = SD(between-day difference in values)/ 2] and SD is the mean between-subject standard deviation in the two trials resulting by weighing the variances on the basis of their degrees of freedom [26]. To quantify the absolute reproducibility, the change in the mean between sessions (Δmean ) and the TEM were computed. The TEM was expressed as a coefficient of variation (CV) as: [CV = (TEM/mean) × 100%], where the mean takes into consideration all 34 subjects and both sessions [8,27]. Upper and lower 90% confidence limits were calculated to provide a measure specific to the quantifiers of reproducibility. A two-way repeated measures ANOVA was used to identify the main effect of KF (0◦ and 45◦ ) and FI

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Table 1 Three indicators of fatigue (FIs) specific to a maximal isokinetic plantar-flexion test performed at 0◦ and 45◦ of knee flexion on two separate days one week apart. The mean ± standard deviation (SD) and range (minimum, maximum) of values are reported Knee flexion Day 1 0◦ 25.8 ± 9.7 (knee straight) (−6.8, 41.8) 45◦ 15.5 ± 7.2 (knee bent) (3.1, 30.9)

FIstatic (%) Day 2 26.8 ± 10.5 (9.1, 48.3) 17.9 ± 6.4 (6.0, 32.6)

Combined 26.3 ± 10.1 (6.8, 48.3) 16.7 ± 6.8 (3.1, 32.6)

Day 1 60.1 ± 10.9 (31.6, 77.4) 58.0 ± 9.3 (38.4, 77.6)

FIdynamic (%) Day 2 57.5 ± 9.3 (39.4, 80.6) 57.1 ± 10.7 (34.1, 83.2)

Combined 58.8 ± 10.4 (31.6, 80.6) 57.6 ± 10.0 (34.1, 83.2)

Day 1 −1.2 ± 0.3 (−0.7, −0.5) −1.2 ± 0.2 (−1.7, −0.7)

FIslope (%/rep) Day 2 −1.1 ± 0.3 (−1.6, −0.5) −1.2 ± 0.3 (−1.5, −0.3)

Combined −1.2 ± 0.3 (−1.6, −0.5) −1.2 ± 0.3 (−1.7, −0.3)

Table 2 The reproducibility (Δmean , ICC and CV) of three indicators of fatigue (FIs) specific to a maximal isokinetic plantar-flexion test performed at 0◦ and 45◦ of knee flexion on two separate days one week apart. The 90% confidence limits [lower, upper] of values are reported Knee flexion

FIstatic (%) FIdynamic (%) FIslope (%/rep) ICC Δmean CVa ICC Δmean CVa ICC Δmean CVa 0◦ 0.65 1.0 23.6 0.71 −2.5 10.0 0.67 0.08 15.3 (knee straight) [0.44, 0.81] [−1.5, 3.6] [19.6, 29.8] [0.51, 0.85] [−5.0, −0.1] [8.3, 12.6] [0.47, 0.83] [0.00, 0.15] [12.8, 19.4] 45◦ 0.52 2.5 29.3 0.55 −0.9 12.0 0.62 −0.01 13.1 (knee bent) [0.25, 0.75] [0.2, 4.7] [24.1, 37.9] [0.32, 0.75] [−3.7, 2.0] [10.0, 15.1] [0.39, 0.79] [−0.08, 0.06] [19.4, 17.6] Δmean : difference in values recorded on the first and second test day. ICC: intra-class correlation coefficient; CV, coefficient of variation. a Expressed as a percentage of the mean FI value.

(static, dynamic and slope) on the quantifiers of reproducibility (ICC and CV), with post-hoc pair-wise comparisons using the Scheffe correction method. To ensure a similar level of exertion between days, paired −t tests were employed. All statistical analyses were performed using Stata/IC v.11.2 (StataCorp LP, TX) and Microsoft Excel 2010 (Microsoft Corp., Redmont, WA). The level of significance was set at α 0.050, with 0.050 < α 0.100 indicating a trend.

3. Results The subjects perceived similar levels of (calf muscle) exertion on the first and second test days at both 0◦ (p = 0.2047) and 45◦ (p = 0.4508) of KF. The mean ratings for both days combined was 18.8 ± 1.2 and 18.2 ± 1.6, respectively. As reported in Table 1, the mean FIdynamic was approximately 2 to 3 times greater than the mean FIstatic at both 0◦ and 45◦ , with comparable FIslope values at these two KF angles. As documented in Table 2, the ICCs ranged from 0.52 to 0.71, being lowest and highest for FIstatic at 45◦ and FIdynamic at 0◦ . The CVs ranged from 10.0% to 29.3%, being lowest and highest for FIdynamic at 0◦ and FIstatic at 45◦ . The analysis of variance revealed no main effect from KF on CVs (p = 0.4226) and ICCs (p = 0.0746), although a trend towards greater ICCs at 0◦ than 45◦ of KF was apparent (0.68 ± 0.03 versus 0.56 ± 0.05%). On the other hand, the type of FI exerted no main ef-

fect on ICCs (p = 0.4533), but a significant influence on CVs (p = 0.0470). Pair-wise comparisons revealed significantly greater CVs for FIstatic than either FIdynamic (15.5 ± 3.0%, p = 0.021) or FIslope (11.8 ± 3.0%, p = 0.042), with no marked difference between the latter two indicators (3.6 ± 1.2%, p = 0.463).

4. Discussion This study reveals that the reproducibility of three different indicators of fatigue (FIs) specific to a maximal concentric isokinetic plantar-flexion fatiguing protocol performed at two different angles of knee flexion (KF) is not sufficient to be termed ‘acceptable’ as reflected in the differences in the means, intra-class correlation coefficients and coefficients of variation. Although there are no universally accepted criteria for classifying the reproducibility as adequate [28], relative reproducibility is commonly considered to be poor, fair and good when the corresponding ICC value is below 0.4, from 0.4 to 0.75, and equal to or greater than 0.75 [29–31]. Furthermore, many researchers arbitrarily regard a 10% CV value as the upper limit defining adequate reproducibility [32–34]. Accordingly, the three FIs computed here demonstrate fair relative reproducibility and less than adequate absolute reproducibility at one week for both KF angles. None of the absolute and relative quantifiers attained good or adequate reproducibility thresholds commonly cited in research and clinical contexts.


Although better reproducibility of the FIs investigated here would have been desirable, a multicentre trial has reported similar outcomes with respect to several FIs specific to isokinetic fatigue of the knee extensors and flexors [35]. Even though this multicentre trial involved a different protocol (40 reciprocal maximal concentric contractions performed at 120◦/s), both studies indicate that alterations in FIs from repeated maximal isokinetic contractions must be substantial before these can be interpreted as indicative of any actual change in the fatigability and/or endurance of select muscle groups. When considering our overall findings, i.e. ICC values together with their relevant quantifiers of absolute reproducibility [27,28], the following points are noteworthy. First, it becomes apparent from the three measures of reproducibility that FIdynamic with the knee at 0◦ was the most reproducible indicator of fatigue; whereas FIstatic with the knee at 45◦ , the least. Secondly, the absolute reproducibility as indicated by the CV was significantly better for FIdynamic and FIslope (extrapolated from the peak powers of the isokinetic fatiguing protocol) than for FIstatic (obtained from the peak torques of the pre-and-post maximal voluntary isometric contractions. Thirdly, testing with the knee straight at 0◦ tended to provide indicators of fatigue that were relatively more reproducible at one week than with the knee bent at 45◦ . Collectively, these observations suggest that using an outcome measure specific to the mode of testing increases the overall reproducibility and that individual change in plantar-flexion fatigability using isokinetic testing may be more readily detected with the knee positioned at 0◦ than 45◦ (see below). In this study, quantifying the plantar-flexion fatigue on the basis of change in peak isokinetic power (i.e. FIslope and FIdynamic ) was more specific to the experimental task than quantification based on changes in torque (i.e. FIstatic ). Conformingly, FIdynamic was 2 to 3 times larger than FIstatic , which is similar to the findings by Roy et al. [36] in connection to testing of the shoulder complex. However, it should be stated that the post MVIC we used to compute FIstatic was initiated ∼20 sec following the fatiguing task, because the IsoMed2000-dynamometer does not allow a shorter timeframe. This delay can allow muscles to partly recover and contribute to decreasing FIstatic values [37]. Furthermore, our CV values indicate that the absolute reproducibility of the isometric FI was lower than for the two isokinetic-based FIs, which were similar

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in this respect. These observations suggest that fatigue expressed as the change in peak power indicated by the normalized slope of a line based on all the contractions is as reproducible as when indicated using the percentage difference between subsets of contractions at the start and end of an isokinetic fatiguing protocol. Conversely, Maffiuletti and co-workers [38] have proposed that the reproducibility of indicators of fatigue expressed as the slope of a linearly regressed line is better than as a percentage change in isokinetic output. However, this claim was neither supported by inferential statistics nor CV comparisons. Indeed, paired t-tests on their reported ICCs rather indicate no difference between the slope and percentage loss (p = 0.830), with a similar interpretation drawn from comparing their respective CV values (15.2 ± 3.6% and 11.3 ± 3.1%, p = 0.1472). Hence, further inspection of their data indicate no pronounced differences between the ICC and CV values for the absolute and relative reproducibility of FIs computed using linearly regressed lines and percentage changes in isokinetic performance. The CV values associated with the absolute reproducibility of the two isokinetic-based plantar-flexion FIs obtained (10 to 15%) are similar to those associated with the reproducibility of measures from ankle plantar-flexion isokinetic endurance [18] and strength [10,13,39,40] tests, while slightly greater than those reported for knee endurance tests [41,42]. These lower CV values reported for the knee compared with the current ankle measurements are consistent with the standard observations of better reproducibility of isokinetic measures at this joint [12,43] and were expected outcomes. There was a trend towards better reproducibility of the plantar-flexion FIs when tests were performed at 0◦ versus 45◦ of KF, agreeing with previous indications that joint position can influence the reproducibility of measurements [44] and variability in plantar-flexion performance [45]. Repositioning of the knee at 45◦ is presumably more prone to error and, as discussed by Wenneberg [44], inaccurate repositioning of this joint at subsequent testing occasions may influence plantarflexion muscle length, lever arm and torque generation, particularly of the gastrocnemius muscle; and thereby exert a negative impact on the reproducibility. Moreover, the 45◦ knee bent position appeared less stable than the 0◦ straight knee position and, despite considerable efforts to restrain motion at the ankle, may have augmented the ankle joint rotation which inevitably occurs during maximal efforts [46–48]. More rotation

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at the ankle would, in turn, increase the variability of measurement and be detrimental to the overall reproducibility of the FIs. We acknowledge that the degree of reproducibility is relative to the frame of reference, susceptible to a learning effect [49,50] and influenced by the homogeneity of the study population [51] – all factors that can alter the interpretation of our study findings. For instance, Kline [52] and Vincent [53] have proposed that correlation indices of the reproducibility of clinical measures should not fall below 0.70, meaning that all of the FIs reported here should be avoided or, at least, interpreted with caution in clinical settings. At the same time, it is highly probable that the reproducibility of our FIs would improve with further training, familiarization or if the subjects were highly (rather than recreationally) trained, as demonstrated elsewhere [49,54]. Therefore, plantar-flexion FIs for elite athletes quantified using our protocol and three or more test occasions would undoubtedly show a much better reproducibility.

Conflicts of interest There are no relevant conflicts of interest to declare. No source of funding was necessary for the preparation of this paper.

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5. Conclusions Based on common thresholds, the relative and absolute reproducibility of three different indicators of fatigue specific to 50 maximal concentric isokinetic plantar-flexion contractions were less than desired at both 0◦ and 45◦ of knee flexion. The reproducibility of indicators was lower with isometric-based computations, with a tendency towards better reproducibility when the knee was fully extended. Our findings suggest that more reproducible indicators of fatigue from isokinetic testing of the plantar-flexors are required for monitoring individual change. However, until more reliable FIs are available, utilizing indicators specific to the mode of testing and positioning the knee at 0◦ rather than 45◦ may increase the overall reproducibility of measurements and likelihood of identifying changes in plantar-flexion fatigability or endurance from an isokinetic protocol.

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Acknowledgements The authors would like to thank Dan Backlund and Jonas Winberg for assistance in data collection, and all the subjects who volunteered to take part in this study.

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