## 4 Discussion
### Reliability and Sensitivity  
Neither the validity nor the reliability of spatiotemporal metrics obtained from skate-mounted IMUs has been extensively studied to date. However, two recent and one older study have shown the application of IMUs to be an accurate and a valid method for identifying spatiotemporal variables in skating, when compared to 3D video analysis [7] and foot pressure sensors [13; 30] as reference methods. On the other hand, both the validity [11; 26; 27; 28] and reliability [22; 23; 24; 25] of spatiotemporal data derived from IMUs during running have been widely investigated and, therefore, a comparison with those studies is possible. Previous studies have reported good to excellent relative reliability of spatiotemporal data, obtained during running, using shoe-mounted IMUs. Specifically, [PERSON] et al. [22] and [PERSON] et al. [23] reported a high level of reproducibility in running stride kinematics (ICC = 0.70 to 0.99) at different speeds (i.e., 12, 15, 18, and 21 km/h) and on different surfaces (i.e., asphalt, macadam, grass). The findings from the current study corroborate the findings from these studies, evidencing acceptable test-retest reproducibility for all spatiotemporal variables when averaged over an 80 m skating distance (ICC = 0.73-0.97), and when obtained during the acceleration (ICC = 0.76-0.90) and steady-state phases (ICC = 0.57-0.98). Only the step frequency showed \"low\" relative reliability (ICC = 0.26) during the steady-state phase. This means that the participants' ranking order could be accurately determined, based on all spatiotemporal outcomes in all skating phases, except for the step frequency derived during the steady-state phase [18]. However, the step frequency did not differ significantly between the trials, and its absolute reliability estimated by the CV% (i.e., within-subject variability) was less than 5%, showing good test-retest reliability. Likewise, the absolute reliability of the spatiotemporal parameters obtained in this study was good (CV \(<\) 5%), which is in line with that previously reported in the running studies. Specifically, [PERSON] et al. [22] reported CVs of 6.5-9.9%, 4.6-5.2%, and 3.9-4.4% for the contact time, aerial time, and step frequency at different running speeds. Comparably, [PERSON] et al. [23] reported CVs of 1.2-7.3%, 1.2-1.9%, and 0.8-1.0% for the contact time, stride length, and step rate at different running speeds and on different surfaces. The good absolute reliability (i.e., a small amount of within-subject variability) observed in the \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**The Acceleration Phase**} & \multicolumn{4}{c}{**The Steady-State Phase**} \\ \hline
**Max Velocity (m/s)** & \(B\) & **SE \(B\)** & \(\beta\) & **R\({}^{2}\)** & \(\Delta\)**R\({}^{2}\)** & \(B\) & **SE \(B\)** & \(\beta\) & **R\({}^{2}\)** & \(\Delta\)**R\({}^{2}\)** \\ \hline Model & & & & 0.62 & 0.54 & & & & 0.72 & 0.67 \\ Constant & \(-\)10.97 & 6.29 & & & \(-\)2.19 & 2.25 & & & & \\ Double support time (s) & 0.02 & 0.04 & 0.65 & & \(-\)0.01 & 0.01 & \(-\)0.14 & & \\ Glide length (m) & \(-\)13.06 & 10.55 & \(-\)5.79 & & 2.19 & 0.58 & 1.55 \({}^{*}\) & & \\ Double support length (m) & 11.67 & 11.07 & 1.98 & & \(-\)1.45 & 1.06 & \(-\)0.49 & & \\ Step length (m) & 17.45 & 11.18 & 5.94 & & \(-\)0.32 & 0.54 & \(-\)0.17 & & \\ Step frequency (steps/s) & 3.55 & 1.03 & 1.70 \({}^{*}\) & & 1.86 & 0.41 & 0.89 \({}^{*}\) & & \\ \hline \hline \end{tabular} Legend: Model = “stepwise” method; \(B\) = unstandardized regression coefficients; SE \(B\) = standard error of the coefficient; \(\beta\) = standardized coefficient; R\({}^{2}\) = coefficient of determination; AR\({}^{2}\) = adjusted R\({}^{2}\); \({}^{*}\) \(p\) \(<\) 0.001.  
\end{table} 
Table 5: Prediction of maximal skating velocity using the spatiotemporal variables derived from the acceleration and steady-state phases.