6. The muscle: hamstrings
Sure all those squats, dead lifts and lunges indirectly hit the hamstrings—but not enough. "Most guys are quad dominant and ignore their hamstrings," says Andrews, who notes that—naturally—hamstrings are only about 60 percent as strong as the quads. So what’s the big deal? Any imbalance of opposing muscle groups, like big quads and weak hamstrings, can cause unequal pull on the joint. And in this particular case, that sets the stage for knee injuries. (Just ask Andrews—who recently tore an ACL himself while jumping over a puddle during a run.)
Strengthen it: Prone hamstring curl machines and standing leg curl machines are both effective, or try this leg curl move using an exercise ball. Lie on the floor with your heels on top of the ball, toes up toward the ceiling, and legs slightly bent. Lift the hips by pushing down on the ball with your heels, then roll the ball towards you by pulling your heels towards your seat, kneecaps pointed towards the ceiling. Keeping the hips off the ground, roll the ball back out to the starting position and repeat. Do 8-10 reps 2-3 times a week.
7. The muscle: forearm extensors
Since these muscles are responsible for gripping heavy things, like dumbbells and barbells, weak ones my hamper your ability to train larger muscles and weaken your entire workout—not to mention your tennis backhand. "We get the forearm flexors with all pulling moves or curls or even tricep presses but nothing other than reverse biceps curls hits those muscles," says Irv Rubenstein, PhD, exercise physiologist and founder of S.T.E.P.S., a fitness facility in Nashville, TN.
Strengthen it: While watching TV, you can simply squeeze and release a tennis ball, doing 3 sets of 10 reps 2-3 nights a week. Or try this: tie a light weight to the end of a rope, then tie the rope around a piece of broom handle or a wooden dowel. Stand with arms extended in front of you and roll the weight up and then down by rotating the broom towards you and away from you. Do 2-3 reps.