The squat is a great exercise for many different sports. It serves as both a general exercise and a specialized exercise, depending upon the sport or joint movement involved in a particular sport. In running, the typical back or front squat can be a great general exercise but when modified, it can be a specialized exercise and have a direct effect on your performance.
In a typical squat you lower the body to approximately a thigh level position or as close to this position as possible while still maintaining the natural curvature in the spine. When done in this manner you strengthen the quadriceps muscles of the anterior thigh and the hamstrings and gluteus maximus of the posterior thigh and buttocks respectively.
At the same time there is some strengthening of the erector spinae muscles of the lower back because of the isometric contraction of these muscles to maintain spinal position. But the isometric contraction is not as effective in developing strength of the muscles as the concentric contraction in which the muscles shorten during a contraction. For this you must do the back raise exercise on a Glute Ham Back machine.
In running, the hamstrings and gluteus maximus muscles are involved in the pawback action which takes place near the end of the flight phase to get the leg moving backward so that touchdown is more under the body. These muscles do not however, undergo a concentric contraction while the leg is in contact with the ground. They do the opposite as they undergo an eccentric contraction and stretch during touchdown to amortize the landing forces.
The hamstring and gluteus maximus muscles do not participate in the pushoff. Many coaches believe that these muscles, as part of the posterior chain, are critical in producing a strong pushoff. However, they are not involved in the pushoff since they are undergoing an eccentric contraction which in turn helps to keep the body level (minimizing up-and-down movements) during the run.
With this information in mind, we must now look at the anterior quadriceps muscles of the thigh. They too undergo an eccentric contraction during touchdown and play the major role in preventing the body from sinking too low during the support phase. The stronger these muscles are, the more upright they can hold the body, and as a result, minimize up-and-down movements. This makes the run more economical which is critical to all runners but especially to a long-distance runner. Maintaining the upright body position is the most important function of these muscles.
To make the squat more specific to the run, it is not necessary to go to the thigh level position. Doing a one half to one quarter squat is quite sufficient. The position reached in the one quarter squat more closely duplicates the body position seen during the run. As a result, this squat variant now becomes a specialized exercise for both sprinters and long-distance runners.
However, do not be surprised to find many coaches advocating a wide stance deep squat (thigh level) and in some cases turning the knees out to the sides as often seen in bodybuilding and powerlifting. These variants are not suitable for runners if you are looking to enhance your running form or performance. In running, you want your knees moving forward, not out to the sides, and you want the legs functioning under the body and not out to the sides.
Because of this you should do most of your squats with the feet directly under the hips rather than shoulder width or wider. In this position you will feel a distinct difference not only in the squat depth attainable, but also the feel of the muscular contractions. Doing a squat in this manner is especially beneficial in the specialized period of training to prepare you for upcoming competition.
For more information on squat execution and its role in running, see Biomechanics and Kinesiology of Exercise and Explosive Running.