The strongest muscles affecting hand motions aren't actually in your hand; they are in your forearm. Tendons attached to the muscles of the forearm reach into your hand to move your fingers and thumb. So when you train your hand for bowling, perform resistance exercises for the forearms. It's not as simple as squeezing handgrips. You'll have to get into the gym. But it'll pay off with added endurance, power, accuracy and torque.
Grab two dumbbells, hold them at your sides like suitcases and go for a walk. Straighten your spine, standing tall, and tighten your core and shoulders while taking short, quick steps. Take three walks of 30 yards each. This grip-strength exercise also works the hips and core for more power in your delivery. Work your way up to body weight. If you weigh 200 pounds, shoot for two, 100-pound dumbbells.
Wrist rolling requires a special workout tool called a wrist roller, featuring a small bar with a rope attached to the middle. At the end of the rope is a weight, usually 10 pounds, but it may be adjustable. Grab the bar using both hands with an overhand grip. Raise it straight out in front of you. Roll each wrist, one by one, in an upward motion until you pull the weight to the bar; then reverse, lowering the weight in the same rolling fashion. Perform three sets of five reps.
Pronation and Supination
This exercise gives you wicked spin for curved shots. Grab the ends of two light dumbbells, one in each hand. Sit on a bench with your knees in front of you. Rest your forearms on your thighs with your hands in front of your knees, palms facing each other and dumbbells pointing straight up. Turn your palms up and then roll them over to the facedown position, keeping your forearms on your thighs. Repeat 15 times, performing three sets.
Wrist Flexion and Extension
Grab two dumbbells and kneel next to a bench. Rest your forearms on the bench with your wrists hanging off the edge 2 to 3 inches, palms facing up. Curl your wrists into your body. Repeat 12 times and then turn your hands over, hanging your wrists off the bench, palm down. Raise the backs of your hands toward your body 12 times. Do three sets of 12 for each direction.
Take two, 10-pound plates and pinch them together between your fingers and thumb. Hold the plates at your side like a suitcase and maintain grip for as long as you can. As you get stronger, you may work up to two, 25-pound plates, or you can pinch more than two, 10-pound plates together, stacking three to six plates on top of each other. Record your time and try to beat it every time you train.
Fill a five-gallon bucket with rice, leaving 3 or 4 inches of space at the top of the bucket. Put your hand into the rice and grab. Drive a fist deep into the rice, grab a handful and remove your hand. Pinch the rice between your thumb and fingers. Drive your fist into the rice and rotate counterclockwise, clockwise, sideways back and forth, and then up and down. Grab handfuls of rice while driving your hand to the bottom of the bucket. Do each motion repeatedly for 20 seconds at a time.