Reliability of W´ using time-trials under laboratory conditions
Critical power (CP) and the maximum work above CP (W´) serve as important parameters to characterize high intensity exercise (Jones et al. 2010). However, recent research reported poor reproducibility of W´ (Galbraith et al. 2011; Karsten et al. 2015; Triska et al. 2015) and it was suggested that differences in the environmental conditions (e.g. terrain, seating position) or exhaustive durations might have influenced W´ between tests.
Therefore, the aim of this study was to determine W´ under controlled laboratory conditions using time-trials (TT). We hypothesized non-significant differences and a high reliability for W´.
Ten well-trained cyclists (MAP: 329±41 W) participated in this study. Reliability was assessed across three tests comprising three exhaustive TT each. These TT were 12, 7, and 3 min in duration and interspersed by 30 min passive rest. Tests were separated by at least 72 h. TTs were performed on a Cyclus2 ergometer (RBM Elektronik GmbH, Leipzig, Germany) where the participants’ personal bikes were mounted on. To replicate real-world TT cycling, participants consequently utilised a self-pacing strategy were gearing was adjusted throughout efforts using the virtual gear changer mounted to the handlebars. W´ was estimated using a linear regression where power is plotted against the inverse of time (1.s-1):
P = W´.t-1 + CP
where P is the power output (W) and t is the time (s). The standard error (SE) for W´ was calculated in absolute and relative values for each test.
A repeated measures ANOVA assessed the differences between the trials and significant main effects were followed-up by Bonferroni post-hoc procedures. Reliability was analysed using the intra-class correlation coefficient (ICC) and the coefficient of variation (CoV) (Hopkins 2000). Statistical significance was set at P<.050.
Table 1 illustrates the results of the tests. Non-significant differences between repeated tests were revealed for W´ (F1.513,9.935 = 2.951; P = .115). However, significant differences were found for the absolute and relative SE (F2,18 = 10.865; P = .001; and F2,18 = 5.428; P = .014, respectively). Using Bonferroni post-hoc procedures, absolute SE of Test I was significantly higher compared to Test II and Test III (P = .008-.042).
Discussion and Conclusions
Using TT efforts in trained cyclists, a learning effect for W´ between Test I-Test II was identified. Furthermore, the ICC was low between Test I-Test II, but high for Test II-Test III. This is supported by CoV values for Test I-Test II which were notably above the recommended upper limit of 10% (Atkinson and Nevill 1998), however, improved to acceptable values in the following tests. The relative SE of the first test was slightly above the accepted upper limit of 10% (Ferguson et al. 2013), but well below that in the two following tests. Furthermore, absolute SE was significantly lower in Test II and Test III. To accurately determine W´ our findings suggest a familiarisation trial even when testing trained cyclists.
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