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    "textoCompleto" => "<p class="elsevierStylePara"><span class="elsevierStyleBold">Introduction</span></p><p class="elsevierStylePara"> The maximal oxygen uptake &#40;VO<span class="elsevierStyleInf">2max</span>&#41; is a basic measure of physical fitness for athletes&#44; mainly in the cases where the performance is influenced by the aerobic power&#46; The direct measurement is considered the gold standard to assess it&#44; but this is rather complex and expensive&#46; In consequence&#44; a variety of indirect tests have been developed to estimate VO<span class="elsevierStyleInf">2max</span>&#44; such as &#197;strand-Rhyming 6-minute cycle ergometer test&#44;<span class="elsevierStyleSup">1 </span>Balke 15-minute run&#44;<span class="elsevierStyleSup">2 </span>Cooper 12-minute run&#44;<span class="elsevierStyleSup">3 </span>Bruce treadmill test&#44;<span class="elsevierStyleSup">4 </span>the multistage 20-metre shuttle run tests of L&#233;ger and Lambert<span class="elsevierStyleSup">5 </span>and L&#233;ger et al&#46;&#44;<span class="elsevierStyleSup">6 </span>1-mile track jog<span class="elsevierStyleSup">7 </span>and Yo-Yo endurance test&#46;<span class="elsevierStyleSup">8 </span>The Yo-Yo endurance test &#40;YET&#41; is a continuous multistage field test that is widely implemented to estimate VO<span class="elsevierStyleInf">2max</span> as an alternative to the direct laboratory measurement&#44; because of its specificity&#44; its practical and easy implementation and the simple testing environment required&#46; It is one of the three Yo-Yo tests&#46;<span class="elsevierStyleSup">8 </span>The other ones are the Yo-Yo intermittent endurance test &#40;YIET&#41; and the Yo-Yo intermittent recovery test &#40;YIRT&#41;&#44; the last one also providing an estimation of VO<span class="elsevierStyleInf">2max</span>&#46;<span class="elsevierStyleSup">9</span></p><p class="elsevierStylePara"> The Bland-Altman approach<span class="elsevierStyleSup">10-12 </span>has been extensively applied to compare methods of measurement in diverse research areas&#46; It allows to assess the degree of agreement between two measurement techniques&#44; to determine if they can be used interchangeably&#46; However&#44; method comparison studies are sometimes analysed inappropriately&#44; by comparing the mean responses&#44; by using correlation coefficients or by testing the slope of the linear regression between the methods&#46;<span class="elsevierStyleSup">10</span></p><p class="elsevierStylePara"> Different studies have compared the direct VO<span class="elsevierStyleInf">2max</span> measurement obtained through a treadmill exercise test with the outcomes of the Yo-Yo tests in athletes&#46;<span class="elsevierStyleSup">13-26 </span>In most cases the population under study was football players&#46; In these works&#44; the direct assessment of VO<span class="elsevierStyleInf">2max</span> was compared with the performance &#40;distance covered&#41; in the Yo-Yo tests&#44; or with the indirect estimation of VO<span class="elsevierStyleInf">2max</span> obtained from YET or YIRT&#46; Linear correlations were examined or mean responses were contrasted&#46; Some of the studies that compared the VO<span class="elsevierStyleInf">2max</span> estimation given by the Yo-Yo test &#40;YET or YIRT&#41; with the direct measurement also reported Bland-Altman limits of agreement&#46;<span class="elsevierStyleSup">15&#44;21&#44;25 </span>However&#44; confidence intervals for these limits were not included&#46; Furthermore&#44; it would be ideal to define the acceptable limits of agreement <span class="elsevierStyleItalic">a priori</span>&#46; Although it may be difficult for physiologic variables&#44; an attempt should be made&#59; the opinion of experts may be used&#46; Without <span class="elsevierStyleItalic">a priori </span>setting of limits of agreement&#44; widely discrepant limits may be selected&#46;<span class="elsevierStyleSup">27</span></p><p class="elsevierStylePara"> The uncertainty due to sampling error should be considered not only when estimating the difference between methods &#40;bias&#41;&#44; but also when estimating the limits of agreement&#46;<span class="elsevierStyleSup">12 </span>This issue is of crucial importance when the sample size is small&#46; The Bland-Altman approach provides statistical methodology to quantify the precision of the estimated limits of agreement&#46; Nevertheless&#44; no method comparison studies were found between the direct measurement of VO<span class="elsevierStyleInf">2max</span> and the indirect estimation by the Yo-Yo test &#40;YET or YIRT&#41; that include confidence intervals for the limits of agreement&#46; This study aimed to examine the extent of agreement in the assessment of VO<span class="elsevierStyleInf">2max</span> in athletes between the Yo-Yo endurance test and the direct measurement performed in an incremental treadmill running test&#44; and to quantify the precision of the estimated limits of agreement&#46;</p><p class="elsevierStylePara"><span class="elsevierStyleBold">Material and methods</span></p><p class="elsevierStylePara"><span class="elsevierStyleBold">Participants</span></p><p class="elsevierStylePara"> Data were obtained from a group of 11 competitive male field hockey players&#46; All subjects or their guardians gave consent to participate in the study after being informed of the aims and procedures&#46; The study was conducted based on the ethical principles of the Declaration of Helsinki of the World Medical Association&#46; Each participant underwent a physical examination before the testing sessions&#46; Table 1 displays a statistical summary of the kinanthropometric characteristics of the athletes&#46; Age was calculated in decimal years&#44; by subtracting the date of birth from the date of assessment&#46; Percentages of fat mass and muscle mass were estimated using the four-compartment model based on the strategy of De Rose and Guimaraes&#46;<span class="elsevierStyleSup">28 </span>This model was adapted using the simple regression equation for male athletes developed by Withers et al&#46;&#44; which was cited by Norton&#44;<span class="elsevierStyleSup">29 </span>to estimate body density&#44; and Siri formula<span class="elsevierStyleSup">30 </span>to calculate the percentage of fat mass&#46;</p><p class="elsevierStylePara"><img alt="Table 1&#46; Kinanthropometric characteristics of the athletes&#46;" src="276v52n193-90460847fig1.jpg"></img></p><p class="elsevierStylePara"><span class="elsevierStyleBold">Study design</span></p><p class="elsevierStylePara"> The present research is a study of simple cross-over design&#44; in order to evaluate agreement in assessing VO<span class="elsevierStyleInf">2max</span> in ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1 </span>between the Yo-Yo endurance test and the direct measurement&#59; a special focus is on the interval estimation of the limits of agreement&#46; The direct measurement of VO<span class="elsevierStyleInf">2max</span> &#40;DM&#41; was conducted in the Exercise Physiology Laboratory of the National Center of High Performance Athletics &#40;CeNARD&#44; Buenos Aires&#44; Argentina&#41;&#44; using an incremental running test on a motorized treadmill &#40;Technogym Excite Run 700i&#59; Technogym SpA&#44; Gambettola&#44; Italy&#41;&#46; Breath-by-breath data collection was obtained by means of a computerized open-circuit metabolic system &#40;Medgraphics Cardiopulmonary Exercise System CPX&#47;D&#44; Breeze Ex v3&#46;06 software&#59; Medical Graphics Corporation&#44; St&#46; Paul&#44; MN&#44; USA&#41;&#46; Increments in treadmill speed of 1 km h<span class="elsevierStyleSup">&#8722;1</span> every minute were stipulated until volitional exhaustion &#40;initial speed &#61; 9 km h<span class="elsevierStyleSup">&#8722;1</span>&#41;&#46; Heart rate was controlled with a heart rate monitor &#40;Polar 610i&#59; Polar Electro Oy&#44; Kempele&#44; Finland&#41;&#46; VO<span class="elsevierStyleInf">2</span>plateau &#40;change in VO<span class="elsevierStyleInf">2</span> difference less than 2&#46;1 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1 </span>with a further increase in workload&#41; was the primary criterion for the attainment of VO<span class="elsevierStyleInf">2max</span>&#59; secondary criteria were&#58; respiratory exchange ratio greater than 1&#46;1 and heart rate within 10 beats min<span class="elsevierStyleSup">&#8722;1 </span>of age-predicted maximum heart rate&#46;<span class="elsevierStyleSup">31&#44;32 </span>The Yo-Yo endurance test level 1 was performed indoor at CeNARD&#44; in a sport hall with wooden sprung floor&#46; It consisted of repeated 20-m shuttle runs at a progressively increased speed &#40;initial speed &#61; 8 km h<span class="elsevierStyleSup">&#8722;1</span>&#41;&#44; which was controlled by audio beeps from a portable audio player&#46; The test was considered ended when the participant was unable to maintain the currently indicated speed&#46; The final speed level and the number of performed 20-metre distances at this speed level were recorded&#46; The estimation of VO<span class="elsevierStyleInf">2max</span> was then obtained using the corresponding nomogram&#46; Both YET and DM took place at the same time of day &#40;&#177;2 hours&#41;&#44; to minimize the effects of diurnal biological variation on the results&#44; and were carried out at least 72 hours apart from each other to avoid residual fatigue&#46; The order of the tests was randomly assigned for each subject&#46; Prior to the evaluations&#44; the participants had a 20-minute related warm-up&#46;</p><p class="elsevierStylePara"><span class="elsevierStyleBold">Statistical analysis</span></p><p class="elsevierStylePara"> The kinanthropometric characteristics of the athletes and the heart rate and respiratory exchange ratio values reached when achieving VO<span class="elsevierStyleInf">2max</span> were reported as mean &#177; standard deviation &#40;SD&#41;&#46; Summary statistics were produced to describe the VO<span class="elsevierStyleInf">2max</span> values obtained by means of YET and DM&#46; Bland-Altman analysis was employed for assessing agreement between the two methods of measurement&#46;<span class="elsevierStyleSup">10-12</span> In accordance to Bland&#44;<span class="elsevierStyleSup">33 </span>the acceptable 95&#37; limits of agreement were established on the basis of experience&#44; and were set <span class="elsevierStyleItalic">a priori </span>at &#177;5 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#46; The 95&#37; confidence intervals for the limits of agreement were also computed&#46; In view of the small sample size&#44; they were estimated using the exact expression for the estimator of the variance of the limits of agreement instead of the approximate one<span class="elsevierStyleSup">12</span>&#58;</p><p class="elsevierStylePara"><img src="276v52n193-90460847fig2.jpg"></img></p><p class="elsevierStylePara"> where <img alt="F&#243;rmula" src="277v52n193-90460721figsi4.gif"></img> is the mean difference between methods and <span class="elsevierStyleItalic">s<span class="elsevierStyleInf">d </span></span>is the standard deviation of the differences&#46; Pearson&#8217;s and Spearman&#8217;s correlation tests were implemented to evaluate possible associations of the difference between YET and DM with the magnitude of the measurement&#46; Statistical significance was set at the 0&#46;05 probability level&#46; All analyses were performed using the R software environment&#44; version 3&#46;2&#46;0 &#40;R Core Team&#44; Vienna&#44; Austria&#41;&#46;<span class="elsevierStyleSup">34</span></p><p class="elsevierStylePara"><span class="elsevierStyleBold">Results</span></p><p class="elsevierStylePara"> The VO<span class="elsevierStyleInf">2max</span> values obtained for each individual using the two methods of measurement were quite similar&#46; The VO<span class="elsevierStyleInf">2max</span> values estimated by YET were&#44; on average&#44; only 0&#46;6&#37; lower than those determined by DM&#46; Table 2 presents a descriptive summary of the outcomes of YET and DM and of the differences between them&#46; The heart rate and respiratory exchange ratio values reached when achieving VO<span class="elsevierStyleInf">2max</span> are summarized in Table 3&#46;</p><p class="elsevierStylePara"><img alt="Table 2&#46; VO2max assessment&#58; comparative summary of both methods of measurement&#46;" src="276v52n193-90460847fig3.jpg"></img></p><p class="elsevierStylePara"><img alt="Table 3&#46; Heart rate and respiratory exchange ratio values reached when achieving VO2max&#46;" src="276v52n193-90460847fig4.jpg"></img></p><p class="elsevierStylePara"> A non-statistically significant bias was found between YET and DM &#40;&#8722;0&#46;31 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#59; 95&#37; confidence interval&#58; &#8722;1&#46;69 to 1&#46;07 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#41;&#46; The estimates of the lower and upper 95&#37; limits of agreement were &#8722;4&#46;34 and 3&#46;72 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#46; And the 95&#37; confidence intervals for these limits were&#44; respectively&#44; from &#8722;6&#46;78 to &#8722;1&#46;90 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#44; and from 1&#46;29 to 6&#46;16 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#46; As previously mentioned&#44; given the small sample size&#44; the confidence intervals for the limits of agreement were constructed using the exact expression for the estimator of the variance of these limits &#40;Eq&#46; &#40;1&#41;&#41;&#46; The region determined by the estimated 95&#37; limits of agreement was narrower than the region bounded by the acceptable limits defined <span class="elsevierStyleItalic">a priori</span>&#46; However&#44; the total region covered when considering the 95&#37; confidence intervals for the 95&#37; limits of agreement was wider than the <span class="elsevierStyleItalic">a priori </span>acceptable limits&#46; The typical Bland-Altman plot with estimated bias and 95&#37; limits of agreement between methods is shown in Fig&#46; 1&#44; while all the aforementioned results are presented graphically in Fig&#46; 2&#46;</p><p class="elsevierStylePara"><img alt="Figure 1&#46; VO2max&#58; Difference between methods against the average measurement with bias and 95&#37; limits of agreement&#46;" src="276v52n193-90460847fig5.jpg"></img></p><p class="elsevierStylePara"><span class="elsevierStyleBold">Figure 1&#46; </span>VO<span class="elsevierStyleInf">2max</span>&#58; Difference between methods against the average measurement with bias and 95&#37; limits of agreement&#46;</p><p class="elsevierStylePara"><img alt="Figure 2&#46; VO2max&#58; Mean and 95&#37; limits of agreement of the difference between methods with 95&#37; confidence intervals&#46;" src="276v52n193-90460847fig6.jpg"></img></p><p class="elsevierStylePara"><span class="elsevierStyleBold">Figure 2&#46; </span>VO<span class="elsevierStyleInf">2max</span>&#58; Mean and 95&#37; limits of agreement of the difference between methods with 95&#37; confidence intervals&#46;</p><p class="elsevierStylePara"> It was observed in the data that neither the difference nor the absolute value of the difference appeared to be correlated to the magnitude of the measurement&#46; In that order&#58; Pearson&#8217;s <span class="elsevierStyleItalic">r </span>&#61; 0&#46;02 &#40;95&#37; confidence interval&#58; &#8722;0&#46;59 to 0&#46;61&#41;&#59; Spearman&#8217;s rho &#61; &#8722;0&#46;06 &#40;95&#37; confidence interval&#58; &#8722;0&#46;64 to 0&#46;56&#41;&#46;</p><p class="elsevierStylePara"><span class="elsevierStyleBold">Discussion</span></p><p class="elsevierStylePara"> The indirect measurement of VO<span class="elsevierStyleInf">2max</span> is frequently performed in sport evaluation&#46; Maximal and submaximal effort protocols have been proposed to estimate the maximal aerobic power of an individual&#46;<span class="elsevierStyleSup">1-9 </span>Good agreement with the direct measurement should be the main objective of these tests&#46; The Yo-Yo endurance test is a continuous multistage field test broadly used to indirectly assess VO<span class="elsevierStyleInf">2max</span>&#46;<span class="elsevierStyleSup">15 </span>Its mechanical characteristics make it suitable for athletes participating in sports that involve stop&#44; start and change of direction movement patterns&#46;</p><p class="elsevierStylePara"> Diverse studies have been conducted to compare the outputs provided by the Yo-Yo tests with the VO<span class="elsevierStyleInf">2max</span> obtained by direct measurement in an incremental running test on treadmill&#46; The comparisons involved contrasting the VO<span class="elsevierStyleInf">2max</span> values or testing the correlation between directly measured VO<span class="elsevierStyleInf">2max</span> values and distances covered in the Yo-Yo tests&#46; Some of the studies that compared the VO<span class="elsevierStyleInf">2max</span> results of both methods also included the evaluation of agreement&#46; For instance&#44; Metaxas et al&#46;<span class="elsevierStyleSup">22 </span>found in male football players &#40;<span class="elsevierStyleItalic">n </span>&#61; 35&#41; statistically significant differences &#40;<span class="elsevierStyleItalic">p </span>&#60; 0&#46;05&#41; when comparing the VO<span class="elsevierStyleInf">2max</span> estimations given by YET level 1 with the VO<span class="elsevierStyleInf">2max</span> values obtained by DM in either a continuous or an intermittent running protocol on a treadmill &#40;YET level 1 values lower than DM values by 11&#46;4&#37; and 13&#46;4&#37;&#44; respectively&#41;&#46; Castagna et al&#46;<span class="elsevierStyleSup">15 </span>contrasted the VO<span class="elsevierStyleInf">2max</span> results of YET level 2 with the ones of DM in male football players &#40;<span class="elsevierStyleItalic">n </span>&#61; 24&#41;&#44; evidencing a non-statistically significant bias &#40;<span class="elsevierStyleItalic">p </span>&#61; 0&#46;10&#41; of 1&#46;17 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#44; and 95&#37; limits of agreement of &#8722;5&#46;44 and 7&#46;79 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#46; Nazarali et al&#46;<span class="elsevierStyleSup">23 </span>reported a higher mean for the VO<span class="elsevierStyleInf">2max</span> values provided by YIRT level 2 &#40;50&#46;8 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#41; in comparison to the mean of the values determined by DM &#40;43&#46;6 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#41;&#44; in a study conducted with female football players &#40;<span class="elsevierStyleItalic">n </span>&#61; 20&#41;&#46; In contrast&#44; Mart&#237;nez-Lagunas and Hartmann<span class="elsevierStyleSup">21 </span>found in female football players &#40;<span class="elsevierStyleItalic">n </span>&#61; 18&#41; that VO<span class="elsevierStyleInf">2max</span> was significantly underestimated &#40;<span class="elsevierStyleItalic">p </span>&#60; 0&#46;001&#41; by YIRT level 1 &#40;45&#46;2 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#41; compared to DM &#40;55&#46;0 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#41;&#44; expressing the 95&#37; limits of agreement on a percentage scale &#40;&#8722;31&#46;8&#37; to &#8722;3&#46;8&#37;&#41;&#46; And S&#225;nchez-Oliva et al&#46;<span class="elsevierStyleSup">25 </span>examined the assessments of the two methods in male football players &#40;<span class="elsevierStyleItalic">n </span>&#61; 15&#41; and also found a statistically significant underestimation of VO<span class="elsevierStyleInf">2max</span> using YIRT level 1 &#40;51&#46;00 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#41; in comparison to DM &#40;60&#46;85 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#41;&#44; with 95&#37; limits of agreement of 3&#46;37 and 16&#46;33 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#46;</p><p class="elsevierStylePara"> On the other hand&#44; no research articles were found reporting method comparison studies by using Bland-Altman analysis between the direct measurement of VO<span class="elsevierStyleInf">2max</span> and the indirect estimation by the Yo-Yo test &#40;YET or YIRT&#41; that consider the computation of confidence intervals for the limits of agreement&#46; This is a matter of particular importance in the case of small sample size&#44; which is a frequent situation in studies involving maximal effort testing&#46; The variance of the mean difference and that of the limits of agreement are inversely proportional to the sample size&#46; Therefore&#44; the corresponding confidence intervals are wide when the sample size is small&#44; reflecting the great variation of the differences between methods&#46;</p><p class="elsevierStylePara"> This research presents a thorough method comparison study between the direct assessment of VO<span class="elsevierStyleInf">2max</span> in laboratory conditions and the indirect estimation by means of the Yo-Yo endurance test&#44; which is a continuous multistage field test especially useful for athletes of endurance disciplines&#44;<span class="elsevierStyleSup">8 </span>but also suitable for sports teams<span class="elsevierStyleSup">35</span>&#44; such as field hockey&#44; football and rugby players&#46; Estimates of bias and 95&#37; limits of agreement between methods are reported&#46; Furthermore&#44; accurate 95&#37; confidence intervals for the limits of agreement are also reported for the current sample size&#46; A small bias was observed between the two techniques&#44; which resulted to be not statistically significant &#40;<span class="elsevierStyleItalic">p </span>&#61; 0&#46;63&#41;&#46; The estimated 95&#37; limits of agreement set at &#177;4&#46;03 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1 </span>from the mean difference between them&#46; In the model proposed&#44; for the mean and standard deviation of the differences between methods to be meaningful estimates it must be assumed that they are constant throughout the range of measurement&#46;<span class="elsevierStyleSup">12&#44;36&#44;37 </span>It is worth emphasizing that the mean difference and the standard deviation of the differences appeared not to depend on the magnitude of the measurement&#44; which complies with the assumptions underlying the simple 95&#37; limits of agreement method applied&#46; The region determined by the estimated limits of agreement was within the acceptable limits defined <span class="elsevierStyleItalic">a priori</span>&#46; However&#44; these estimates have a large variance due to the small sample size&#44; so this last result has to be considered with caution&#46; Taking into account the 95&#37; confidence intervals for the 95&#37; limits of agreement&#44; the total region extended beyond &#177;6 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#44; exceeding the acceptable 95&#37; limits of agreement established <span class="elsevierStyleItalic">a priori</span>&#46;</p><p class="elsevierStylePara"> The confidence intervals for the mean difference and for the limits of agreement provide a measure of precision of the point estimates&#44; based on sampling error&#46; The variances of the estimators of these quantities are crucially affected by the sample size&#46; The smaller is the number of observations for the assessment of the difference between the methods&#44; the wider are the confidence intervals&#44; and the higher is the level of uncertainty&#46; Moreover&#44; although the Bland-Atman approach provides the methodology for constructing limits of agreement&#44; it does not state whether they are acceptable or not acceptable&#46; The acceptable limits should be defined <span class="elsevierStyleItalic">a priori</span>&#44; based on specific needs and goals&#46;</p><p class="elsevierStylePara"><span class="elsevierStyleBold">Limitations</span></p><p class="elsevierStylePara"> The small sample size is a limitation of this study&#46; The 95&#37; confidence intervals for the 95&#37; limits of agreement given the current sample size are &#177;1&#46;19 units of the standard deviation of the differences between measurements of the two methods&#46; Therefore&#44; it would be advisable to reproduce the present analysis with larger sample sizes to increase the precision of the estimates produced&#46; Furthermore&#44; it would be relevant to simultaneously carry out a comparison of the repeatability of each method by collecting replicated data&#44; because the repeatabilities of two methods of measurement restrict the amount of agreement that is possible&#46;<span class="elsevierStyleSup">12</span></p><p class="elsevierStylePara"><span class="elsevierStyleBold">Conclusions</span></p><p class="elsevierStylePara"> The point estimates of the 95&#37; limits of agreement were within &#177;5 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#44; satisfying the <span class="elsevierStyleItalic">a priori </span>requirement based on experience&#46; Thus&#44; a reasonable good agreement was found between the Yo-Yo endurance test and the direct laboratory measurement for the assessment of VO<span class="elsevierStyleInf">2max</span> in athletes&#46; According to this fact&#44; the two methods can be used interchangeably&#46; Nevertheless&#44; the need for caution in interpreting this result solely is further emphasized by the finding that the region covered when considering the 95&#37; confidence intervals for the 95&#37; limits of agreement was wider than the acceptable region of agreement defined <span class="elsevierStyleItalic">a priori</span>&#46; Hence&#44; although the estimated 95&#37; limits of agreement do not discredit the Yo-Yo endurance test as an alternative method to measure VO<span class="elsevierStyleInf">2max</span> in athletes&#44; the present study is limited in precision because of the small sample size and does not allow definitive conclusions&#46;</p><p class="elsevierStylePara"><span class="elsevierStyleBold">Conflict of interests</span></p><p class="elsevierStylePara"> Authors declare that they do not have any conflict of interests&#46;</p><p class="elsevierStylePara"><span class="elsevierStyleBold">Acknowledgments</span></p><p class="elsevierStylePara"> The authors wish to express special thanks to Mariela B&#46; Arangio&#44; Enrique D&#46; Balardini&#44; Claudio A&#46; Gillone&#44; Cristina Perez and Enrique O&#46; Prada for their technical assistance&#46;</p><hr></hr><p class="elsevierStylePara"><span class="elsevierStyleSup">&#9734;</span><span class="elsevierStyleItalic">Note</span>&#46; It is reported that a preliminary summary of this work was published given its presentation at &#8216;&#8216;57th Annual Meeting and inaugural World Congress on Exercise is Medicine of the American College of Sports Medicine&#8217;&#8217;&#46; The mentioned event took place in &#8216;&#8216;Baltimore Convention Center&#8217;&#8217; in Baltimore&#44; Maryland&#44; between 1 and 5 June 2010&#46; The summary was published in &#8216;&#8216;Medicine and Science in Sports and Exercise&#44; Volume 42&#58; 5 Supplement&#8217;&#8217;&#46;</p><p class="elsevierStylePara"> Received 12 April 2016&#59;<br></br> accepted 11 July 2016<br></br> Available online 15 September 2016</p><p class="elsevierStylePara"><span class="elsevierStyleSup">&#8727; </span>Corresponding author at&#58;<br></br> Exercise Physiology Laboratory&#44; National Center of High Performance Athletics &#40;CeNARD&#41; Miguel B&#46; S&#225;nchez 1050&#44; Buenos Aires &#40;Postal Code&#58; 1429&#41; Argentina&#46;<br></br><span class="elsevierStyleItalic">E-mail address&#58; </span><a href="mailto&#58;alongo&#64;deportes&#46;gob&#46;ar" class="elsevierStyleCrossRefs">alongo&#64;deportes&#46;gob&#46;ar</a> &#40;A&#46;F&#46; Longo&#41;&#46;</p>"
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        "resumen" => "<p class="elsevierStylePara"> Introducci&#243;n&#58; Si bien diferentes estudios han reportado l&#237;mites de concordancia en la valoraci&#243;n del VO<span class="elsevierStyleInf">2max</span> entre el Yo-Yo test y la medici&#243;n directa&#44; la precisi&#243;n de estos l&#237;mites en general no ha sido considerada&#46; El objetivo de este estudio fue examinar el grado de concordancia en la valoraci&#243;n del VO<span class="elsevierStyleInf">2max</span> en atletas entre el Yo-Yo endurance test &#40;YET&#41; y la medici&#243;n directa &#40;DM&#41;&#44; y cuantificar la precisi&#243;n de los l&#237;mites de concordancia estimados&#46;</p> <p class="elsevierStylePara"> Material y m&#233;todos&#58; Los datos fueron obtenidos de un grupo de 11 jugadores varones de hockey sobre c&#233;sped &#40;edad &#61; 22&#44;2 &#177; 3&#44;6 a&#241;os&#44; BMI &#61; 22&#44;1 &#177; 2&#44;4 kg m<span class="elsevierStyleSup">&#8722;2</span>&#41;&#46; La DM fue realizada usando un test de carrera incremental en cinta rodante&#46; El YET nivel 1 fue usado para la estimaci&#243;n indirecta del VO<span class="elsevierStyleInf">2max</span>&#46; El an&#225;lisis de Bland-Altman fue empleado para valorar la concordancia entre los 2 m&#233;todos&#46; Los l&#237;mites aceptables de concordancia del 95&#37; fueron fijados a priori en &#177;5 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#46;</p> <p class="elsevierStylePara"> Resultados&#58; Un sesgo estad&#237;sticamente no significativo fue observado entre YET y DM &#40;50&#44;78 vs&#46; 51&#44;09 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#44; p &#62; 0&#44;05&#41;&#46; Las estimaciones de los l&#237;mites de concordancia del 95&#37; fueron &#8722;4&#44;34 y 3&#44;72 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#46; Y los intervalos de confianza del 95&#37; para estos l&#237;mites fueron&#44; respectivamente&#44; desde &#8722;6&#44;78 hasta &#8722;1&#44;90 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#44; y desde 1&#44;29 hasta 6&#44;16 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#46; La diferencia entre m&#233;todos pareci&#243; no estar correlacionada con la magnitud de medici&#243;n&#46;</p> <p class="elsevierStylePara"> Conclusiones&#58; Fue encontrada una razonable buena concordancia entre YET y DM&#46; Sin embargo&#44; la gran varianza de los l&#237;mites de concordancia debido al peque&#241;o tama&#241;o muestral hace necesario considerar este resultado con precauci&#243;n&#46;</p>"
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        "resumen" => "<p class="elsevierStylePara"> Introduction&#58; Although different studies have reported limits of agreement in assessing VO<span class="elsevierStyleInf">2max</span> between the Yo-Yo test and the direct measurement&#44; the precision of these limits in general has not been considered&#46; The aim of this study was to examine the extent of agreement in the assessment of VO<span class="elsevierStyleInf">2max</span> in athletes between the Yo-Yo endurance test &#40;YET&#41; and the direct measurement &#40;DM&#41;&#44; and to quantify the precision of the estimated limits of agreement&#46;</p> <p class="elsevierStylePara"> Material and methods&#58; Data were obtained from a group of 11 male field hockey players &#40;Age &#61; 22&#46;2 &#177; 3&#46;6 yrs&#44; BMI &#61; 22&#46;1 &#177; 2&#46;4 kg m<span class="elsevierStyleSup">&#8722;2</span>&#41;&#46; DM was performed using an incremental tread-mill running test&#46; YET level 1 was used for indirect estimation of VO<span class="elsevierStyleInf">2max</span>&#46; Bland-Altman analysis was employed for assessing agreement between the two methods&#46; The acceptable 95&#37; limits of agreement were set a priori at &#177;5 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#46;</p> <p class="elsevierStylePara"> Results&#58; A non-statistically significant bias was observed between YET and DM &#40;50&#46;78 vs&#46; 51&#46;09 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#44; P &#62; &#46;05&#41;&#46; The estimates of the 95&#37; limits of agreement were &#8722;4&#46;34 and 3&#46;72 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#46; And the 95&#37; confidence intervals for these limits were from &#8722;6&#46;78 to &#8722;1&#46;90 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#44; and from 1&#46;29 to 6&#46;16 ml kg<span class="elsevierStyleSup">&#8722;1 </span>min<span class="elsevierStyleSup">&#8722;1</span>&#44; respectively&#46; The difference between methods did not appear to be correlated to the magnitude of measurement&#46;</p> <p class="elsevierStylePara"> Conclusions&#58; A reasonably good agreement was found between YET and DM&#46; However&#44; the large variance of the limits of agreement due to the small sample size means these results should be treated with caution&#46;</p>"
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Journal Information
Vol. 52. Issue 193.
Pages 17-22 (January - March 2017)
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VO 2max assessment in athletes: A thorough method comparison study between Yo-Yo test and direct measurement
Valoración del VO 2max en atletas: Un riguroso estudio de comparación de métodos entre el Yo-Yo test y la medición directa
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Aldo F. Longoa, Gustavo D. Aquilinoa, Marcelo L. Cardeya, Néstor A. Lentinia
a Exercise Physiology Laboratory, National Center of High Performance Athletics (CeNARD), Buenos Aires, Argentina
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Abstract
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Tables (3)
Table 1. Kinanthropometric characteristics of the athletes.
Table 2. VO2max assessment: comparative summary of both methods of measurement.
Table 3. Heart rate and respiratory exchange ratio values reached when achieving VO2max.
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Figures (3)
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Introducción: Si bien diferentes estudios han reportado límites de concordancia en la valoración del VO2max entre el Yo-Yo test y la medición directa, la precisión de estos límites en general no ha sido considerada. El objetivo de este estudio fue examinar el grado de concordancia en la valoración del VO2max en atletas entre el Yo-Yo endurance test (YET) y la medición directa (DM), y cuantificar la precisión de los límites de concordancia estimados.

Material y métodos: Los datos fueron obtenidos de un grupo de 11 jugadores varones de hockey sobre césped (edad = 22,2 ± 3,6 años, BMI = 22,1 ± 2,4 kg m−2). La DM fue realizada usando un test de carrera incremental en cinta rodante. El YET nivel 1 fue usado para la estimación indirecta del VO2max. El análisis de Bland-Altman fue empleado para valorar la concordancia entre los 2 métodos. Los límites aceptables de concordancia del 95% fueron fijados a priori en ±5 ml kg−1 min−1.

Resultados: Un sesgo estadísticamente no significativo fue observado entre YET y DM (50,78 vs. 51,09 ml kg−1 min−1, p > 0,05). Las estimaciones de los límites de concordancia del 95% fueron −4,34 y 3,72 ml kg−1 min−1. Y los intervalos de confianza del 95% para estos límites fueron, respectivamente, desde −6,78 hasta −1,90 ml kg−1 min−1, y desde 1,29 hasta 6,16 ml kg−1 min−1. La diferencia entre métodos pareció no estar correlacionada con la magnitud de medición.

Conclusiones: Fue encontrada una razonable buena concordancia entre YET y DM. Sin embargo, la gran varianza de los límites de concordancia debido al pequeño tamaño muestral hace necesario considerar este resultado con precaución.

Palabras clave:
Consumo máximo de oxígeno; Medición directa y Yo-Yo endurance test; Precisión de los límites de concordancia

Introduction: Although different studies have reported limits of agreement in assessing VO2max between the Yo-Yo test and the direct measurement, the precision of these limits in general has not been considered. The aim of this study was to examine the extent of agreement in the assessment of VO2max in athletes between the Yo-Yo endurance test (YET) and the direct measurement (DM), and to quantify the precision of the estimated limits of agreement.

Material and methods: Data were obtained from a group of 11 male field hockey players (Age = 22.2 ± 3.6 yrs, BMI = 22.1 ± 2.4 kg m−2). DM was performed using an incremental tread-mill running test. YET level 1 was used for indirect estimation of VO2max. Bland-Altman analysis was employed for assessing agreement between the two methods. The acceptable 95% limits of agreement were set a priori at ±5 ml kg−1 min−1.

Results: A non-statistically significant bias was observed between YET and DM (50.78 vs. 51.09 ml kg−1 min−1, P > .05). The estimates of the 95% limits of agreement were −4.34 and 3.72 ml kg−1 min−1. And the 95% confidence intervals for these limits were from −6.78 to −1.90 ml kg−1 min−1, and from 1.29 to 6.16 ml kg−1 min−1, respectively. The difference between methods did not appear to be correlated to the magnitude of measurement.

Conclusions: A reasonably good agreement was found between YET and DM. However, the large variance of the limits of agreement due to the small sample size means these results should be treated with caution.

Keywords:
Maximal oxygen uptake; Direct measurement and Yo-Yo endurance test; Precision of the limits of agreement
Full Text

Introduction

The maximal oxygen uptake (VO2max) is a basic measure of physical fitness for athletes, mainly in the cases where the performance is influenced by the aerobic power. The direct measurement is considered the gold standard to assess it, but this is rather complex and expensive. In consequence, a variety of indirect tests have been developed to estimate VO2max, such as Åstrand-Rhyming 6-minute cycle ergometer test,1 Balke 15-minute run,2 Cooper 12-minute run,3 Bruce treadmill test,4 the multistage 20-metre shuttle run tests of Léger and Lambert5 and Léger et al.,6 1-mile track jog7 and Yo-Yo endurance test.8 The Yo-Yo endurance test (YET) is a continuous multistage field test that is widely implemented to estimate VO2max as an alternative to the direct laboratory measurement, because of its specificity, its practical and easy implementation and the simple testing environment required. It is one of the three Yo-Yo tests.8 The other ones are the Yo-Yo intermittent endurance test (YIET) and the Yo-Yo intermittent recovery test (YIRT), the last one also providing an estimation of VO2max.9

The Bland-Altman approach10-12 has been extensively applied to compare methods of measurement in diverse research areas. It allows to assess the degree of agreement between two measurement techniques, to determine if they can be used interchangeably. However, method comparison studies are sometimes analysed inappropriately, by comparing the mean responses, by using correlation coefficients or by testing the slope of the linear regression between the methods.10

Different studies have compared the direct VO2max measurement obtained through a treadmill exercise test with the outcomes of the Yo-Yo tests in athletes.13-26 In most cases the population under study was football players. In these works, the direct assessment of VO2max was compared with the performance (distance covered) in the Yo-Yo tests, or with the indirect estimation of VO2max obtained from YET or YIRT. Linear correlations were examined or mean responses were contrasted. Some of the studies that compared the VO2max estimation given by the Yo-Yo test (YET or YIRT) with the direct measurement also reported Bland-Altman limits of agreement.15,21,25 However, confidence intervals for these limits were not included. Furthermore, it would be ideal to define the acceptable limits of agreement a priori. Although it may be difficult for physiologic variables, an attempt should be made; the opinion of experts may be used. Without a priori setting of limits of agreement, widely discrepant limits may be selected.27

The uncertainty due to sampling error should be considered not only when estimating the difference between methods (bias), but also when estimating the limits of agreement.12 This issue is of crucial importance when the sample size is small. The Bland-Altman approach provides statistical methodology to quantify the precision of the estimated limits of agreement. Nevertheless, no method comparison studies were found between the direct measurement of VO2max and the indirect estimation by the Yo-Yo test (YET or YIRT) that include confidence intervals for the limits of agreement. This study aimed to examine the extent of agreement in the assessment of VO2max in athletes between the Yo-Yo endurance test and the direct measurement performed in an incremental treadmill running test, and to quantify the precision of the estimated limits of agreement.

Material and methods

Participants

Data were obtained from a group of 11 competitive male field hockey players. All subjects or their guardians gave consent to participate in the study after being informed of the aims and procedures. The study was conducted based on the ethical principles of the Declaration of Helsinki of the World Medical Association. Each participant underwent a physical examination before the testing sessions. Table 1 displays a statistical summary of the kinanthropometric characteristics of the athletes. Age was calculated in decimal years, by subtracting the date of birth from the date of assessment. Percentages of fat mass and muscle mass were estimated using the four-compartment model based on the strategy of De Rose and Guimaraes.28 This model was adapted using the simple regression equation for male athletes developed by Withers et al., which was cited by Norton,29 to estimate body density, and Siri formula30 to calculate the percentage of fat mass.

Study design

The present research is a study of simple cross-over design, in order to evaluate agreement in assessing VO2max in ml kg−1 min−1 between the Yo-Yo endurance test and the direct measurement; a special focus is on the interval estimation of the limits of agreement. The direct measurement of VO2max (DM) was conducted in the Exercise Physiology Laboratory of the National Center of High Performance Athletics (CeNARD, Buenos Aires, Argentina), using an incremental running test on a motorized treadmill (Technogym Excite Run 700i; Technogym SpA, Gambettola, Italy). Breath-by-breath data collection was obtained by means of a computerized open-circuit metabolic system (Medgraphics Cardiopulmonary Exercise System CPX/D, Breeze Ex v3.06 software; Medical Graphics Corporation, St. Paul, MN, USA). Increments in treadmill speed of 1 km h−1 every minute were stipulated until volitional exhaustion (initial speed = 9 km h−1). Heart rate was controlled with a heart rate monitor (Polar 610i; Polar Electro Oy, Kempele, Finland). VO2plateau (change in VO2 difference less than 2.1 ml kg−1 min−1 with a further increase in workload) was the primary criterion for the attainment of VO2max; secondary criteria were: respiratory exchange ratio greater than 1.1 and heart rate within 10 beats min−1 of age-predicted maximum heart rate.31,32 The Yo-Yo endurance test level 1 was performed indoor at CeNARD, in a sport hall with wooden sprung floor. It consisted of repeated 20-m shuttle runs at a progressively increased speed (initial speed = 8 km h−1), which was controlled by audio beeps from a portable audio player. The test was considered ended when the participant was unable to maintain the currently indicated speed. The final speed level and the number of performed 20-metre distances at this speed level were recorded. The estimation of VO2max was then obtained using the corresponding nomogram. Both YET and DM took place at the same time of day (±2 hours), to minimize the effects of diurnal biological variation on the results, and were carried out at least 72 hours apart from each other to avoid residual fatigue. The order of the tests was randomly assigned for each subject. Prior to the evaluations, the participants had a 20-minute related warm-up.

Statistical analysis

The kinanthropometric characteristics of the athletes and the heart rate and respiratory exchange ratio values reached when achieving VO2max were reported as mean ± standard deviation (SD). Summary statistics were produced to describe the VO2max values obtained by means of YET and DM. Bland-Altman analysis was employed for assessing agreement between the two methods of measurement.10-12 In accordance to Bland,33 the acceptable 95% limits of agreement were established on the basis of experience, and were set a priori at ±5 ml kg−1 min−1. The 95% confidence intervals for the limits of agreement were also computed. In view of the small sample size, they were estimated using the exact expression for the estimator of the variance of the limits of agreement instead of the approximate one12:

where

is the mean difference between methods and sd is the standard deviation of the differences. Pearson’s and Spearman’s correlation tests were implemented to evaluate possible associations of the difference between YET and DM with the magnitude of the measurement. Statistical significance was set at the 0.05 probability level. All analyses were performed using the R software environment, version 3.2.0 (R Core Team, Vienna, Austria).34

Results

The VO2max values obtained for each individual using the two methods of measurement were quite similar. The VO2max values estimated by YET were, on average, only 0.6% lower than those determined by DM. Table 2 presents a descriptive summary of the outcomes of YET and DM and of the differences between them. The heart rate and respiratory exchange ratio values reached when achieving VO2max are summarized in Table 3.

A non-statistically significant bias was found between YET and DM (−0.31 ml kg−1 min−1; 95% confidence interval: −1.69 to 1.07 ml kg−1 min−1). The estimates of the lower and upper 95% limits of agreement were −4.34 and 3.72 ml kg−1 min−1. And the 95% confidence intervals for these limits were, respectively, from −6.78 to −1.90 ml kg−1 min−1, and from 1.29 to 6.16 ml kg−1 min−1. As previously mentioned, given the small sample size, the confidence intervals for the limits of agreement were constructed using the exact expression for the estimator of the variance of these limits (Eq. (1)). The region determined by the estimated 95% limits of agreement was narrower than the region bounded by the acceptable limits defined a priori. However, the total region covered when considering the 95% confidence intervals for the 95% limits of agreement was wider than the a priori acceptable limits. The typical Bland-Altman plot with estimated bias and 95% limits of agreement between methods is shown in Fig. 1, while all the aforementioned results are presented graphically in Fig. 2.

Figure 1. VO2max: Difference between methods against the average measurement with bias and 95% limits of agreement.

Figure 2. VO2max: Mean and 95% limits of agreement of the difference between methods with 95% confidence intervals.

It was observed in the data that neither the difference nor the absolute value of the difference appeared to be correlated to the magnitude of the measurement. In that order: Pearson’s r = 0.02 (95% confidence interval: −0.59 to 0.61); Spearman’s rho = −0.06 (95% confidence interval: −0.64 to 0.56).

Discussion

The indirect measurement of VO2max is frequently performed in sport evaluation. Maximal and submaximal effort protocols have been proposed to estimate the maximal aerobic power of an individual.1-9 Good agreement with the direct measurement should be the main objective of these tests. The Yo-Yo endurance test is a continuous multistage field test broadly used to indirectly assess VO2max.15 Its mechanical characteristics make it suitable for athletes participating in sports that involve stop, start and change of direction movement patterns.

Diverse studies have been conducted to compare the outputs provided by the Yo-Yo tests with the VO2max obtained by direct measurement in an incremental running test on treadmill. The comparisons involved contrasting the VO2max values or testing the correlation between directly measured VO2max values and distances covered in the Yo-Yo tests. Some of the studies that compared the VO2max results of both methods also included the evaluation of agreement. For instance, Metaxas et al.22 found in male football players (n = 35) statistically significant differences (p < 0.05) when comparing the VO2max estimations given by YET level 1 with the VO2max values obtained by DM in either a continuous or an intermittent running protocol on a treadmill (YET level 1 values lower than DM values by 11.4% and 13.4%, respectively). Castagna et al.15 contrasted the VO2max results of YET level 2 with the ones of DM in male football players (n = 24), evidencing a non-statistically significant bias (p = 0.10) of 1.17 ml kg−1 min−1, and 95% limits of agreement of −5.44 and 7.79 ml kg−1 min−1. Nazarali et al.23 reported a higher mean for the VO2max values provided by YIRT level 2 (50.8 ml kg−1 min−1) in comparison to the mean of the values determined by DM (43.6 ml kg−1 min−1), in a study conducted with female football players (n = 20). In contrast, Martínez-Lagunas and Hartmann21 found in female football players (n = 18) that VO2max was significantly underestimated (p < 0.001) by YIRT level 1 (45.2 ml kg−1 min−1) compared to DM (55.0 ml kg−1 min−1), expressing the 95% limits of agreement on a percentage scale (−31.8% to −3.8%). And Sánchez-Oliva et al.25 examined the assessments of the two methods in male football players (n = 15) and also found a statistically significant underestimation of VO2max using YIRT level 1 (51.00 ml kg−1 min−1) in comparison to DM (60.85 ml kg−1 min−1), with 95% limits of agreement of 3.37 and 16.33 ml kg−1 min−1.

On the other hand, no research articles were found reporting method comparison studies by using Bland-Altman analysis between the direct measurement of VO2max and the indirect estimation by the Yo-Yo test (YET or YIRT) that consider the computation of confidence intervals for the limits of agreement. This is a matter of particular importance in the case of small sample size, which is a frequent situation in studies involving maximal effort testing. The variance of the mean difference and that of the limits of agreement are inversely proportional to the sample size. Therefore, the corresponding confidence intervals are wide when the sample size is small, reflecting the great variation of the differences between methods.

This research presents a thorough method comparison study between the direct assessment of VO2max in laboratory conditions and the indirect estimation by means of the Yo-Yo endurance test, which is a continuous multistage field test especially useful for athletes of endurance disciplines,8 but also suitable for sports teams35, such as field hockey, football and rugby players. Estimates of bias and 95% limits of agreement between methods are reported. Furthermore, accurate 95% confidence intervals for the limits of agreement are also reported for the current sample size. A small bias was observed between the two techniques, which resulted to be not statistically significant (p = 0.63). The estimated 95% limits of agreement set at ±4.03 ml kg−1 min−1 from the mean difference between them. In the model proposed, for the mean and standard deviation of the differences between methods to be meaningful estimates it must be assumed that they are constant throughout the range of measurement.12,36,37 It is worth emphasizing that the mean difference and the standard deviation of the differences appeared not to depend on the magnitude of the measurement, which complies with the assumptions underlying the simple 95% limits of agreement method applied. The region determined by the estimated limits of agreement was within the acceptable limits defined a priori. However, these estimates have a large variance due to the small sample size, so this last result has to be considered with caution. Taking into account the 95% confidence intervals for the 95% limits of agreement, the total region extended beyond ±6 ml kg−1 min−1, exceeding the acceptable 95% limits of agreement established a priori.

The confidence intervals for the mean difference and for the limits of agreement provide a measure of precision of the point estimates, based on sampling error. The variances of the estimators of these quantities are crucially affected by the sample size. The smaller is the number of observations for the assessment of the difference between the methods, the wider are the confidence intervals, and the higher is the level of uncertainty. Moreover, although the Bland-Atman approach provides the methodology for constructing limits of agreement, it does not state whether they are acceptable or not acceptable. The acceptable limits should be defined a priori, based on specific needs and goals.

Limitations

The small sample size is a limitation of this study. The 95% confidence intervals for the 95% limits of agreement given the current sample size are ±1.19 units of the standard deviation of the differences between measurements of the two methods. Therefore, it would be advisable to reproduce the present analysis with larger sample sizes to increase the precision of the estimates produced. Furthermore, it would be relevant to simultaneously carry out a comparison of the repeatability of each method by collecting replicated data, because the repeatabilities of two methods of measurement restrict the amount of agreement that is possible.12

Conclusions

The point estimates of the 95% limits of agreement were within ±5 ml kg−1 min−1, satisfying the a priori requirement based on experience. Thus, a reasonable good agreement was found between the Yo-Yo endurance test and the direct laboratory measurement for the assessment of VO2max in athletes. According to this fact, the two methods can be used interchangeably. Nevertheless, the need for caution in interpreting this result solely is further emphasized by the finding that the region covered when considering the 95% confidence intervals for the 95% limits of agreement was wider than the acceptable region of agreement defined a priori. Hence, although the estimated 95% limits of agreement do not discredit the Yo-Yo endurance test as an alternative method to measure VO2max in athletes, the present study is limited in precision because of the small sample size and does not allow definitive conclusions.

Conflict of interests

Authors declare that they do not have any conflict of interests.

Acknowledgments

The authors wish to express special thanks to Mariela B. Arangio, Enrique D. Balardini, Claudio A. Gillone, Cristina Perez and Enrique O. Prada for their technical assistance.


Note. It is reported that a preliminary summary of this work was published given its presentation at ‘‘57th Annual Meeting and inaugural World Congress on Exercise is Medicine of the American College of Sports Medicine’’. The mentioned event took place in ‘‘Baltimore Convention Center’’ in Baltimore, Maryland, between 1 and 5 June 2010. The summary was published in ‘‘Medicine and Science in Sports and Exercise, Volume 42: 5 Supplement’’.

Received 12 April 2016;

accepted 11 July 2016

Available online 15 September 2016

Corresponding author at:

Exercise Physiology Laboratory, National Center of High Performance Athletics (CeNARD) Miguel B. Sánchez 1050, Buenos Aires (Postal Code: 1429) Argentina.

E-mail address: alongo@deportes.gob.ar (A.F. Longo).

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