Sports almost never require you to simultaneously work both arms or both feet equally hard, so unilateral exercise more directly applies to athletic training.
A limb working unilaterally often has more than half the strength of both limbs working together. That’s because when you work a pair of limbs together, a protective mechanism called the bilateral deficit shuts down some of your motor units (combinations of nerve cells and muscle fibers) to keep you from getting hurt during the heaviest lifts. When you work with one limb at a time, however, the deficit doesn’t kick in, so you can work the muscles in each limb more intensely.
So if you’ve maxed out the available weight on your leg curl when using both legs, try it with one leg at a time. (Start with your left leg if you’re right-handed, and vice versa.) Even if you haven’t maxed out, it’s a good idea to work unilateral exercises into your program from time to time.
A physiological phenomenon that’s somewhat better known than the bilateral deficit is the fact that your muscles are about 40 percent stronger when lowering a weight than when raising it. So, returning to the example of the leg curl, try lifting the weight with both legs but lowering it with one leg. That will give you a more potent muscle-building stimulus than would lowering with both legs.
It improves your balance
Unilateral exercise also changes the balance component of a movement. That’s why a one-legged squat or deadlift is much harder than you’d expect—you might not be able to do these with any weight other than your own body weight. So you improve your balance and coordination as well as your strength.