I don't know why I put so much time into pitching, I'm just fascinated by the subject, and I read as much as I can about it. Whomever mentioned piggy backing, that's only for the A ball team as pitchers emerge from short season ball and don't have enough IP to go a full season. Piggy backing is strictly a mechanism to limit IP in a year. At A+ they've occasionally experimented with relievers starting and the starting pitchers finishing the game, but that's not piggy backing, That's a mechanism to get pitchers acclimated to pitching through the 9th inning. Why 24-25 years? Because as I said the average adult male is fully matured around that time... obviously some will mature more quickly or take longer to mature. The chance of injury is at it's lowest once the body reaches maturity then gradually begins to rise again with age becoming exponential by the early to mid 30s. As far as the actual pitch counts, I'm not sure that 100 is THE number for young pitchers, I don't think there is such a thing. I think it would vary by pitcher, even pitchers of the same age. It's very difficult subject to tackle, one which I would like the medical community to work on as opposed to statisticians number crunching, just get us some good ballpark numbers based on age so we can err on the side of caution. It might be 90... I don't think anyone really knows. This idea is based entirely on injury prevention, getting young pitchers to the majors healthy, then keeping them in the rotation. PAP (Pitcher Abuse Points) set the threshold at 100 for everyone, which is what is generally accepted, but I personally believe it is bunk. 130 pitches for older veteran pitchers is based more on the decline in production that follows a start of that many pitches and has been covered numerous times on numerous sites, If a pitcher is being regularly pushed over 130 I'm skeptical how long he will be able to stay healthy but pushing 130 here and there before an off day where the starters all get an extra day of rest before their next start doesn't bother me. I read the study on Baker previously, and I think it's flawed in the same way that all pitching studies are flawed, there's no context. The fatigue point is critical, and it's going to differ by start due to many factors... weather, opposing line up, how good the pitchers stuff is that day, etc. The point about fatigue is that every pitch past the point of fatigue carries exponential injury risk. Just to be clear, the injury is not a straight line graph, it's a very steep curve, it's exponential. For example if a pitcher reaches fatigue at 85 pitches, like Parra not being stretched out against St Louis the other day, by pitch 100 his performance is likely plummeting and his injury risk is sky rocketing with each successive pitch. It's not really about pitch velocity either, most pitchers can keep digging a little deeper to maintain their velocity, which is part of the problem. They put additional stress on their arms trying to maintain velocity the more tired they get. It's not the 3.5 pitches per start, it's the distribution of the those 3.5 pitches that make all the difference. I really like what the Brewers have begun doing. They have 3 levels of pitch counts in the minors... pitches in an inning, pitches in a game, and pitches in a season (total IP pitched). At first glance pitches in a inning may season a bit weird, but if you think about it, is pitching 7 innings that each last 15 pitches more fatiguing, or pitching a single inning of 40 pitches? When a pitcher is only going 15 pitches at a time there are periods of rest and recovery, pitching 40 straight pitches is incredibly taxing on the body. Generally speaking a young pitcher's workload should only increase 20 to 30 IP per season. These are approximations but what we typically see with high school pitchers is they are around 100+ coming out of A ball, 130ish out of A+, and 150ish out of AA and AA. (the season is shorter so they don't have the opportunity to work up to 180 IP). In some cases a guy is going to have to be pushed a bit if an injury sets him back, his progression is going to get messed up (Mark Rogers). So again, this isn't set in stone, but it is the guideline the organization has adopted recently.