## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Fitted Data**} & \multicolumn{4}{c}{**Dominant Leg**} & \multicolumn{4}{c}{**Non-Dominant Leg**} & \multirow{2}{*}{**\%Diff**} \\ \cline{2-2} \cline{4-9}  & **M (SD)** & **CV** & **SEM** & **ICC** & **M (SD)** & **CV** & **SEM** & **ICC** \\ \hline RFD\({}_{0-50}\) (\(N\)-\(s^{-1}\)) & & & & & & & & \\   Session 1 & 5465 (2000) & 21.2\% & 1161 & 0.802 & 4801 (1756) & 13.6\% & 651 & 0.914 & \(-\)13.8\% \\ Session 2 & 5589 (1840) & 13.6\% & 761 & 0.901 & 5308 (1637) & 18.7\% & 991 & 0.816 & \(-\)5.3\% \\ RFD\({}_{0-150}\) (\(N\)-\(s^{-1}\)) & & & & & & & & \\   Session 1 & 3315 (850) & 12.2\% & 406 & 0.956 & 3119 (766) & 11.5\% & 358.2 & 0.934 & \(-\)6.3\% \\ Session 2 & 3411 (808) & 6.8\% & 232 & 0.966 & 3042 (790) & 8.1\% & 244.9 & 0.959 & \(-\)12.1\% \\ RFD\({}_{0-20}\) (\(N\)-\(s^{-1}\)) & & & & & & & & \\   Session 1 & 2318 (463) & 8.1\% & 188 & 0.959 & 2120 (457) & 8.5\% & 181 & 0.950 & \(-\)9.3\% \\ Session 2 & 2365 (484) & 5.3\% & 126 & 0.958 & 2142 (448) & 5.9\% & 126 & 0.968 & \(-\)10.4\% \\ RFD\({}_{0-35}\) (\(N\)-\(s^{-1}\)) & & & & & & & & \\   Session 1 & 7561 (3447) & 28.8\% & 2179 & 0.912 & 6363 (2892) & 18.9\% & 1202 & 0.874 & \(-\)21.3\% \\ Session 2 & 7721 (3340) & 21.3\% & 1644 & 0.952 & 7459 (2763) & 24.9\% & 1856 & 0.944 & \(-\)1.4\% \\ \hline \hline \end{tabular} SEM: standard error of measurement, CV: SEM expressed as a coefficient of variation, ICC: intraclass correlation coefficient. Diff %: percentage of difference between dominant and non-dominant leg.  
\end{table} 
Table 2: _Cont._should examine whether the use of external attentional focus may have an acute impact on the reliability of rapid force production measures during the isometric knee extension tests, specially at early phases of the contraction (0-50 ms). A main practical resource herein provided for a better understanding of the isometric knee extension test reliability is the reporting of intra-session and test-retest differences in absolute terms (i.e., SEM). This information assists in the interpretation of results from a practical viewpoint. A large SEM relative to the between-participant variance contributes to poor reliability. In other words, if we would like to compare the differences after a training program, changes greater that the SEM would be likely to be a result of the intervention rather than a measurement error [29]. Accordingly, taking the Session 2 fitted readings and the dominant leg reference (Table 2), the current field-based method would permit us to identify, at least, changes in MVC over 35.9 N or changes in RFD\({}_{0-250}\) and Impulse\({}_{0-250}\) from 126 N\(\cdot\)s\({}^{-1}\) and 7.4 N\(\cdot\)s respectively. Hence, despite we cannot deny the existence of a high intra-individual variability during an isometric leg extension test [21], the use of evidence-based protocols, rapid-response warm-up and visual feedback make it possible to obtain a reliable RFD/Impulse measurement with field-based, portable and low-cost equipment. This is particularly relevant for coaches and sport clubs dealing with athletes with similar strength status than our sample (MVC from \(\sim\)500 to \(\sim\)850 N; 7- to 10-fold their body mass). The implications of these findings are that using a portable force sensor under proper measurement guidelines allows to collect reliable RFD, Impulse and MVC data (altogether from the same trial) to evaluate the rapid isometric contraction of the knee extensors in young athletes. Such knowledge, along with the interpretation of the measurement errors, will aid both in the assessment of performance at a given time-point (e.g., pre-season, tapering, diagnostic analysis) and the identification of true changes due to training-induce adaptations (e.g., training program, injury rehabilitation).