Sprint running is extremely technical. It involves the use and coordination of your entire body. Olympic sprinters finely tune the biomechanics of their sprint running form for years to reduce their times by fractions of a second. There are four basic phases of sprint running: support, early flight, mid-flight, and late flight. These four motions occur in every stride you take while running. Also in each phase your muscles contract either eccentrically or concentrically. In eccentric motion, tension increases on the muscle as it lengthens. In concentric motion, tension increases on the muscle as it shortens. For example, your biceps muscle is undergoing eccentric muscle flexion -- bending motion -- during the downward phase of a biceps curl, and concentric muscle flexion during the upward phase.
The Support Phase
The support phase is also known as the push-off. This phase is responsible for your forward propulsion. In this phase there are several movements occurring. The first is eccentric hip flexion. This motion is responsible for slowing your backward thigh rotation. The next is concentric knee extension -- straightening motion. This motion propels your center of gravity forward. The last motion to occur is concentric plantar flexion. The plantar flexion is what gives you propulsion forward.
The Early Flight Phase
During the early flight phase, your hips and your knees do the majority of the work. They are preparing for the next step in your stride. Two movements occur during this phase; eccentric hip flexion and eccentric knee extension. During eccentric hip flexion you are decelerating the backward rotation of the thigh. The eccentric knee extension motion involves the quadriceps muscles. They are acting to slow the backward rotation of the leg and foot.
The Mid-flight Phase
The mid-flight phase takes only milliseconds and is the most basic of the phases. One major motion is occurring during this phase and that is concentric hip flexion. During concentric hip flexion, you are accelerating your thigh forward. The concentric hip flexion phase is where you prepare for your next propulsive step to occur.
The Late Flight Phase
The last milliseconds before your foot contacts the ground is when the late flight phase occurs. This phase involves two major motions: concentric hip extension and eccentric knee flexion. The concentric hip extension movement is responsible for rotating your thigh backwards. This balances your body over your feet, allowing you to take your next step. The eccentric knee flexion accelerates the entire leg backwards. This motion limits your knee extension and helps to minimize braking when your foot reaches the ground.