# Heart Rate Dynamics and Quantifying Physical Fatigue in Canadian Football
## 2 Materials and Methods  
Data were recorded from 30 male football athletes (Table 1) recruited from two Canadian varsity football teams who underwent random doping assessments, were given no specific instruction given regarding caffeine intake, and followed the hydration and nutrition guidelines set by the team nutritionist. Data were collected during normal game and practice conditions, for a total of 311 collected performances (156 games, 155 practices). Games followed a weekly schedule with a duration of 190 min \(\pm\) 24 min, and practices were recorded once a week with a duration of 123 min \(\pm\) 20 min. Vest-worn Catapult sensors (Catapult Sports, Melbourne, Australia) collected GPS and heart rate data at 10 Hz, as illustrated in Figure 1. Athletes were categorized based on tactical position as either skilled players (receivers, defensive backs, running backs, linebackers) or linemen (offensive linemen, defensive linemen). Ethics clearance for the study was received through the Aging-Neuroimaging Research Ethics Committee of the CIUSSS Centre-Sud-de-I'lle-de-Montreal Office of Research Ethics Committee (ethics no. MP-53-2020-191). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Athlete Group** & **Age (Years)** & **Height (m)** & **Weight (kg)** & **BMI (\(\frac{\text{kg}}{\text{m}}\))** \\ \hline Skilled players & 23.1 \(\pm\) 1.6 & 1.79 \(\pm\) 0.06 & 85.6 \(\pm\) 9.5 & 26.5 \(\pm\) 2.6 \\ Linemen & 22.9 \(\pm\) 1.8 & 1.88 \(\pm\) 0.04 & 120.4 \(\pm\) 9.6 & 34.1 \(\pm\) 3.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical characteristics of athletes according to athlete group. Efforts of running activity were detected by first using a 2 km/h threshold to filter out standing and low-speed walking segments of the velocity data. The remaining activity was identified as a running effort if a velocity of at least 12 km/h was reached during the effort and if the effort duration was at least 2 s in duration, as the minimum duration of a football play is about 1.9 s [24]. In order to ensure adequate recovery when analyzing the HR response, efforts which were followed by less than 60 s of rest before the next run were excluded (Figure 2a). Additionally, to evaluate the HR response across longer periods of activity and rest, running series were detected by grouping individual runs such that there was at least 180 s of rest following the series of running efforts (Figure 2b). The HR response was processed using a second-order, lowpass Butterworth filter with a cutoff frequency (FC) corresponding to the recovery interval being analyzed (i.e., FC = \(\frac{1}{60}\) for 60 s recovery and FC = \(\frac{1}{180}\) for 180 s recovery). The overall running velocity of each series was taken as the average of the maximum velocity for each effort in the series. Running intensity speed thresholds for each performance were determined using a 5-zone Gaussian mixture model [22], and then each effort or series was categorized into medium-intensity running (MIR: one 3/4) and high-intensity running (HIR: zone 5). The HRmax in the recovery period was extracted for each running event. Additionally, the rate of change in heart rate was taken from a centered difference approximation of the first derivative of the filtered HR signal. The peak rate of HR recovery (HRRpk) was  
Figure 1: Depiction of Catapult vest and sensor location on athlete. Figure 2: Running event detection and windowing for a skilled player. The thresholded running velocity signal is in black (left y axis) with the heart rate signal in red (right y axis). Running and recovery periods are shown for (**a**) running efforts and (**b**) running series. The maximum velocity for each individual running effort is denoted by a black dot, while the average of all maximum velocities of runs within an effort or series is denoted by the black horizontal dashed line.