Try tennis or another racket sport. Racket sports like tennis or squash are great for building arm muscles and for a total body workout. Join a recreational tennis league in your area or take tennis lessons from the tennis pro at your gym. If a family member enjoys playing squash or racquetball, ask them to give you lessons and practice your skills. You should notice marked improvements in your arm strength and better arm muscle definition the more you play racket sports.
HOW TO DO IT: Place your left foot on a stable low box or step (even a sturdy phone book will work). Hold your right arm forward. Quickly alternate which foot is on the box and which is on the ground from side to side. Stay on the balls of your feet throughout the movement. Perform the move at a slower, more controlled tempo at first, focusing on nice, clean exchanges of your hands and feet, and gradually increase your speed over time.
Get down on all fours with your knees placed directly below your hips and palms placed directly below your shoulders. Now, raise your right arm forward and stretch your left leg backward at the same time. Create a tension in your back by flexing your foot. Hold the position for a few seconds and then come back to the starting position. Repeat the same using your left arm and right leg. Repeat 15 to 20 times on both sides.