"Bend ze knees!" he shouted, "pretend like you are making love, ja!" Those were the memorable instructions I heard years ago from the Austrian ski instructor behind me on the Alps. They hold true today for all sports.
Watch a great surfer, the back leg is bent so far that the thigh is parallel to the board. A soccer player with power bends his planted leg in order to generate enough force with the striking foot to fire the ball into the net. The best linemen in the NFL bend low despite their massive size. And, like my instructor pointed out, who can make love without bending their knees?
Bending the knees sufficiently is the hardest part, and this only gets harder as athletes age. Golfers progressively stand up more, losing that ideal position for addressing the ball. Tennis players fail to get low enough to generate rotation power from their core. Bowlers don't plant with enough knee bend and the ball hits the alley hard losing spin.
Knowing this, you might think that people train to bend their knees. Odd, but watch most people cross-train, if they do at all, they "go the gym" or "ride the bike" usually with the TV on; in the gym they might bench press or use the elliptical machines; they might even take a Yoga class, believing that is the key to improving their flexibility.
The reality is that most sports demand power in knee bending. To generate power for striking a ball, to hold a position while rotating the hips and the back, to get low enough to apply pressure to the back of a surf board or a ski, it takes concentric and eccentric power in the muscles, meaning the ability to generate force while bending and extending. You can only optimally develop that coordinated power by repeating that motion with progressively higher weights and volume of repetitions.
So if you are going to train to become a better athlete, the ideal method is the simple squat exercise. The squat, when done properly, perfectly trains all the muscles of the lower extremity and the core muscles, the gluteal (butt) muscles, the abdominal muscles and the paraspinal muscles, in a coordinated fashion. Done with body weight alone, it can be performed anywhere. When adding weights to a bar or making the squat more complicated, such as adding an overhead thrust of a bar with weights, more of the body is engaged and the goals of coordinated fitness are achieved earlier.
Proper squat form means the head is in a neutral position, the chest is upright, facing forward, the knees are bent over but not beyond the toes, the hips are lowered as far as they can go, the weight is on the heels, the abdomen muscles are sucked in, and the butt is pushed backwards. The cheapest, most available and best exercise for all sports exists right in your own body.
So "bend ze knees," at the very least you may notice it improves your love life!