|Tim Lincecum aka The Freak|
It isn't structural. This is apparent from watching girls/women play high school or college softball -- they can throw well and hard. As Jame Fallows pointed out in an article (Throwing Like A Girl) which he wrote fifteen years ago,"if you ask an orthopedist, an anatomist, or (especially) the coach of a women's softball team there is no structural reason why men and women should throw in different ways."
Fallows describes the "kinetic chain" involved in producing a "proper" baseball throw, whereby energy passes progressively from the larger and heavier parts of the body to the smaller, lighter parts:
A good throw uses six links of chain. The first two links involve the lower body, from feet to waist. The first motion of a throw (after the body has been rotated away from the target) is to rotate the legs and hips back in the direction of the throw, building up momentum as large muscles move body mass. Then those links stop—a pitcher stops turning his hips once they face the plate—and the momentum is transferred to the next link. This is the torso, from waist to shoulders, and since its mass is less than that of the legs, momentum makes it rotate faster than the hips and legs did. The torso stops when it is facing the plate, and the momentum is transferred to the next link—the upper arm. As the upper arm comes past the head, it stops moving forward, and the momentum goes into the final links—the forearm and wrist, which snap forward at tremendous speed.In a more recent follow-up piece (Throwing Like A Tim Lincecum), Fallows points to this Red Bull-sponsored super slo-mo clip of the Giants' Tim Lincecum throwing the ball as a perfect illustration of how the kinetic chain works:
Fallows contends that throwing is not some innate skill but is an "intricate series of actions coordinated among muscle groups" that has to be learned. And, for a combination of evolutionary and cultural reasons, boys often begin playing with balls practically from infancy, while most girls don't. As Fallows asserts, boys generally figure out how to throw earlier with "hundreds of idle hours spent throwing balls, sticks, rocks, and so on in the playground or the back yard." In addition, as Fallows points out, boys on the playground tend to be more competitive, "trying to throw a rock or ball or whatever farther than their friends," while girls tend to do this less often.
In our league, we teach a series of steps to get the girls to throw a ball, including turning perpendicular to the target ("skateboard"), putting both arms straight out with the glove hand pointing to the target ("scarecrow"), and holding the ball with elbow up high (like a "waitress"). By this method, the girls eventually learn to throw properly, although nobody throws like Lincecum.