There are three primary arm injuries from throwing baseball: 1) Elbows (usually ulnar nerves, or torn tendons) from throwing curve balls and sliders. 2) Shoulders (usually rotator cuff) from throwing too hard or too many times and it just wears out. and 3) bicep usually from a small tear in the muscle fibers from throwing too hard without proper warm up. This soreness will last all season and will usually go away from a three month rest. For some crazy reason, throwing split finger fastballs seems to take its toll on biceps too. That's why there are only 5 splitters in the Bigs, and three are from Japan and 2 from Cuba. Don't throw a splitter. And in the offseason do alot of pushups, not curls. Curls hurt hitting and throwing and are only for looks.