  In American football, a pass rush is an attempt by the defensive team to
disrupt the offense and prevent the quarterback (QB) from completing a pass.
Existing metrics for assessing pass rush performance are either discrete-time
quantities or based on subjective judgment. Using player tracking data, we
propose STRAIN, a novel metric for evaluating pass rushers in the National
Football League (NFL) at the continuous-time within-play level. Inspired by the
concept of strain rate in materials science, STRAIN is a simple and
interpretable means for measuring defensive pressure in football. It is a
directly-observed statistic as a function of two features: the distance between
the pass rusher and QB, and the rate at which this distance is being reduced.
Our metric possesses great predictability of pressure and stability over time.
We also fit a multilevel model for STRAIN to understand the defensive pressure
contribution of every pass rusher at the play-level. We apply our approach to
NFL data and present results for the first eight weeks of the 2021 regular
season. In particular, we provide comparisons of STRAIN for different defensive
positions and play outcomes, and rankings of the NFL's best pass rushers
according to our metric.
