Maybe someone with more knowledge can help me out on this one: Why is it no problem at all for a starter to throw 100 pitches every 5th day, but a reliever cannot throw 20 pitches every day, adding up to 100 pitches every five days? I get that something will be lost to warming up 5 times instead of 1, but the starter throws warm-up pitches every inning but the reliever only once or twice per appearance, which should mitigate some of that. But these days it seems like it is viewed as abusive to have a pitcher throw even 3 20-pitch outings in 4-5 days. Is it just empirical observation that RPs' effectiveness declines over the year with that level of usage? If so, would that be the case for all RPs or would some have more endurance than others? Is there some physiological basis for the difference in 5-day pitch counts? Help me out!