  This paper illustrates how multilevel functional models can detect and
characterize biomechanical changes along different sport training sessions. Our
analysis focuses on the relevant cases to identify differences in knee
biomechanics in recreational runners during low and high-intensity exercise
sessions with the same energy expenditure by recording $20$ steps. To do so, we
review the existing literature of multilevel models, and then, we propose a new
hypothesis test to look at the changes between different levels of the
multilevel model as low and high-intensity training sessions. We also evaluate
the reliability of measures recorded in three-dimension knee angles from the
functional intra-class correlation coefficient (ICC) obtained from the
decomposition performed with the multilevel funcional model taking into account
$20$ measures recorded in each test. The results show that there are no
statistically significant differences between the two modes of exercise.
However, we have to be careful with the conclusions since, as we have shown,
human gait-patterns are very individual and heterogeneous between groups of
athletes, and other alternatives to the p-value may be more appropriate to
detect statistical differences in biomechanical changes in this context.
