If you're a runner and you want to increase your speed, build your endurance, and have the ability to tackle hills with a breeze, you need to have strong quads. Strengthening your thighs can also prevent injuries such as runner's knee. This common injury, which often happens to new runners, is tied to a combination of weak quads and tight hamstrings and calves. Wall squats (also called wall sits) are the perfect exercise to strengthen your upper legs.