P77,
You are doing what we call knuckling up or squeezing the ball. This is incorrect..the smaller thumb is actually better for you. Try this, place your hand in the ball, put the ball down on the ground and relax. Try releasing the ball (with relaxed thumb), can you come out? If you do, this means when you're bowling, your gripping the ball with your thumb, tip down making contact. When you do that, you're physically altering the diameter of your thumb. You're making it wider then it really is. Here's another test, leave the ball on the ground, can you place your thumb only in the ball? Does it slide in and out easily? There's a simple rule, if you can get your thumb in, then you can get it out. You have to learn one of 2 things, relax as much as possible because this will not cause your thumb to bend or if you want to apply some pressure, apply it at the first joint of the thumb so that it remains straight and not bent at the tip. The catch here is that in order for you to relax, you have to have a snug thumb(tape it), if the thumb is loose at all, you have no choice but to grip it when it's in motion...it's a catch 22. Hope this info helps...contact me directly if you need any further help.
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Rick Leong
Ten Pins Pro Shop
Track Pro Shop Staff
Vise Grip Staff