“Stress” is a way of characterizing how much of a pitcher’s workload has been compressed into high pitch count outings. The higher the stress ratings over the course of a pitcher’s career, the higher likelihood of arm injuries.
Stress = Pitcher Abuse Points(PAP)/# of Pitches
PAP = (# of Pitches -100)^3
According to Keith Woolner, starting pitchers who regularly work deeper into games decline in performance, as a group, after throwing a high number of pitches. In Baseball Prospectus 2001, he compared three weeks before and after pitchers’ high-pitch outings and found that pitchers threw fewer innings per start, struck out fewer batters, and allowed more hits and walks in the subsequent starts.