One of the most important pieces of running mechanics is the position of your foot when it lands on the ground. When your foot strikes the ground it will land either ball-of-the-foot first, mid-foot, or heel first. Many runners make the mistake of reaching out in front of their body by purposely taking a longer stride and landing heel first. This is called heel-striking and is not ideal.
Heel-striking is usually very inefficient and can be the cause of a long list of injuries. When you land on your heel first, your leg is close to straight and extended in front of your body. The combination of a near straight leg and a heel-strike transfers a lot of impact through your heel and up through your knee to your hip and spine. Heel-striking places greater stresses on your joints and can cause pain and injury to your hips, knee, ankle, foot and attitude.
A heel-first foot plant also means you are over-striding or just not executing paw-back. Paw-back is a term used to describe the motion of beginning to pull your foot back underneath you after extending the leg out in front of you at the end of the leg swing phase. Paw-back done properly feels like you are pawing at the ground so that at foot-strike, you are actually grabbing the ground and pulling yourself forward over your foot. When not executed properly, you will reach out in front of your body with your leg and “let” it drop and will likely land heel first which results in a slight braking action with each step. It is like trying to drive your car while pressing on both the gas and brake pedals at the same time. You are wasting energy and making your training run harder than it should be. If executing paw-back properly, you are more likely to land mid-foot.
Landing on the ball of the foot is ideal for sprinters, but generally not for distance runners. This results in a lot of up and down motion for distance runners and leads to a lot of stress on the calf muscles and Achilles tendon.
As a distance runner, your most efficient foot plant is one in which your foot lands directly under your knee which is slightly ahead of your center of gravity. The ideal landing position is very slightly toward the outside edge of your foot, just behind your little toe. Your foot would then naturally roll slightly inward while pushing off your big toe. The slight inward roll of your foot is called pronation and provides some cushioning during the running stride. Know that as you toe-off and your ankle flexes, the ankle unpronates some. A small amount of pronation followed by unpronation is normal and desirable.
Excessive pronation can also be the cause of injury and stride inefficiencies. Excessive pronation might be prevented through the use of motion-control shoes (much controversy and discussion exists on this shoe and running mechanics topic). That type of shoe has strong heel inserts that inhibit to a degree the inward rolling motion of pronation. While motion-control shoes will temporarily solve the problem, it is like putting a band aid on a cut that will never heal. It solves the immediate problem but it not a long-term cure. Excessive pronation can be caused by weak muscles in your lower leg, a flatter foot, a very straight foot (not slightly curved) or stride inefficiencies. Doing some barefoot walking (and possibly running) will help strengthen the ankle and foot-stabilizing muscles in your lower leg. Doing exercises and drills on an unstable surface such as a wobble board or a Bosu (half) ball can further help with this problem. If you pronate severely I would suggest consulting with a physical therapist to find out if there are alternatives to motion-control shoes in your specific case.