# Heart Rate Dynamics and Quantifying Physical Fatigue in Canadian Football
###### Abstract  
The cardiac response to physical exertion is linked to factors such as age, work intensity, and fitness levels. The primary objective of this study was to characterize within-session changes in cardiac response to running in Canadian football athletes, which may indicate physical fatigue. Performance profiles were collected from GPS and heart rate (HR) sensors worn by 30 male Canadian varsity football players (20-26 years old) over 28 games and practices. Running efforts with 60-180 s of rest were detected, and the maximum HR (HRmax) and peak HR recovery (HRRpk) during rest were extracted. Additionally, a new metric of cardiovascular cost (CVC) was developed to reflect the efficiency of the HR response to physical workload. HRmax was higher in games (\(p<0.001\)) and in linemen (\(p<0.001\)), and it increased over time (\(p<0.001\)). HRRpk was higher in skilled players (\(p<0.001\)) and changed over time (\(p<0.001\)) depending on the rest period. CVC was higher in linemen (\(p<0.001\)) and increased over time (\(p<0.001\)). This study demonstrated the utility of HR response metrics to quantify ongoing fatigue experienced by Canadian football athletes and proposed a novel fatigue metric capable of monitoring an athlete's fatigue state in real time. workload; sport performance; recovery; high-intensity running; cardiac demands +
Footnote †: journal: Medical  
judgments on the physical readiness of an athlete prior to a session [6; 18], such markers have not been used to evaluate the level of physical fatigue within a session. Thus, identifying a marker sensitive to the accumulation of fatigue within a session could be relevant information for tactical decisions and injury prevention. HR dynamics and physical work was investigated in other team sports such as rugby and soccer by examining the time spent above a certain HR threshold set relative to HRmax [19; 20; 21]. The use of relative thresholds accounts for individual differences in performances, while HRmax or percent-based thresholds are still arbitrary and assume that the distribution of the athlete's HR is consistent across individuals and performances. Recent work in evaluating speed thresholds in Canadian football athletes has shown that such assumptions do not align with the multimodal nature of running performances and that data-driven approaches like a Gaussian mixture model are better suited to determine threshold levels [22]. As an alternative to thresholding approaches, HRR was used in field hockey players to evaluate the short-range response (10 to 60 s) of HR to physical exertion during games, which involves discrete blocks of activity and rest like football [23]. The differences in HRR across playing positions was related to the amount of high-intensity running performed by the athletes and thus could be used to assess the physical fitness of players relative to positional demands. Typically, HRR was computed as the percentage of maximal heart rate (%HRmax) post-exercise, and this metric showed high correlations with aerobic fitness, especially in the first 10 to 30 s of rest [10]. However, reducing the short-range HR response to a single summary metric such as %HRmax does not completely describe the relationship between HR and physical bouts of exertion, such as the timing of the HR response or the rate of recovery. The primary aim of this paper was to characterize the within-session cardiac load experienced by skilled players and linemen in Canadian football beyond limited summary metrics such as HRmax by evaluating the short-range HR response to intermittent physical exertions. It was hypothesized that HRmax following runs of similar intensity would increase with time and HRR metrics during rest would decrease with time. However, computing these HR response metrics is dependent on the detection of distinct bouts of activity and rest, which are unevenly distributed over the course of a session. To mitigate that effect, this study proposed a metric allowing for the live and continuous monitoring of the athlete's physical fatigue levels.