There is no clear place to begin talking about the running stride, as the success of each phase is ultimately a product of how well the phase before it was performed. As will be explained later, I believe the recovery phase to be the most important of the running phases. Therefore, I will begin this discussion with the phase immediately following recovery, and build up to that crucial phase of form. That way, any errors in the recovery phase, being a product of things happening before it, will be able to be detected more easily.
I have separated the running stride into the Five "P's":
The first four are actual phases of the running stride, while the percussion is more of a tool of self-check.
The preparation phase begins after the foot has swung down from its recovery phase position close to the upper thigh, and come into the position it will hold until impact with the ground. This will be described as the time when maximum knee lift has occurred (this depends on the speed of the run, where faster running means more leg lift). The foot should be in a dorsi-flexed position, with the mid to forefoot falling directly below the knee. As the knee is a support mechanism in running (detailed later), it stands to reason that it should be directly above the contact with the ground at the time of impact. The dorsi-flexed foot will minimize absorption of running energy by the calf muscle. If the foot is plantar flexed, then as gravity pushes the body downward, the calf will be forced to lengthen in order to provide a push off role (see push off phase for more details). This eccentric contraction of the calf is extremely costly, as this type of contraction is the most straining on the body. Landing on the mid to forefoot will also minimize "braking" and trauma on other joints. By landing on the heel, impact forces are transferred up the legs, and can even reach the back.
After the leg has got into the position described above, it begins the downward swing to the ground. Muscularly this is caused by the extension of the hip muscles (glutes, upper hamstrings). Because of this extension, which will continue throughout the running motion, your foot will actually be moving backward upon impact. Therefore, you want the foot to land slightly in front of the centre of mass (COM), so that by the time it becomes "useful" it will be directly under the COM, if not slightly behind. (The moment the foot touches the ground, it has merely made contact, and has not yet become a supporting mechanism. Since your body is travelling forward this entire time, the COM will move ahead of the foot strike by the time it becomes a supporter.) If the hip extensors are called into action while the foot is in front of the COM, then they are becoming active in simultaneously pulling and helping support the body's weight, which is a great strain on the muscles and can eventually lead to great hamstring difficulties, including overuse injuries and premature tiring. Should the foot fall in front of the COM, a "braking" effect will occur. Tired quads can be a product of over striding, as it causes the quads to support the body's vertical and horizontal components simultaneously. Conversely, if the foot should fall behind the COM by too much, an inefficient falling motion will occur. The knee must be slightly bent upon impact. This will allow the mid to forefoot to position itself directly under the knee and the supporting system of the body. A straight leg will not only negate much of the lower legs power potential (quads), but it will also cause a greater strain on the hamstring and calf muscles when they are called into action to unbend the joint; moving any joint through a range of motion is significantly easier than the initial unbending of the joint itself.
For the most efficient stride, all of the energy of motion must be direction in the motion of travel, which, in the case of running, is forward. Any alternative motions are merely wasted energy. The COM should remain at a constant height so as to eliminate the use of energy in any vertical component of forces. In analyzing the forces in the running stride, a vertical component is present due to the need to counter the forces of gravity. However, to be most efficient, the forces supplied by the body will be just enough to counter the gravity, and not superfluous to that; in other words, no net change in COM height.
The forward motion is caused primarily by hip extension. In order to maximize each stride, the range of motion of the hip must be adequate enough to allow for maximal hip extension. Obviously, the farther one can push with each step, the longer the stride will be (frequency and stride length are the primary components in overall running speed). If you merely extended your hip without changing the angle of your knee or ankle, you would lower your COM. So, while your hip extends, your knee must extend simultaneously, also. The ankle comes into play at the end of the stride, which will be examined in the next section
The push off phase is actually a continuation of the propulsion phase, but deserves special attention, as it can help determine whether you run forward faster, or run with more of a bounce in your stride. Seemingly more than any other phase, this final push off will be the cause of wasted energy. The two major components of the final push off are near-maximal knee extension and a plantar-flexion of the ankle joint.
As previously described, the knee is primarily a height maintenance mechanism in running; as the hip extends, so must the knee. When the hip is at full extension, the knee has yet to completely extend. Hence, as there is no more extension of the hip, there is no need to further extend the knee. Doing so will only cause a greater vertical component to the running stride, and give the sensation of leaping or bounding with each stride, rather than running. As previously discussed, completely straightening the knee joint will require an undue stress on the hamstrings and calves in order to bend it for the recovery phase.
Additionally, it will take more time to get the lower leg into the recovery phase, which will create more upper body twisting. Excessively tired quads can be a product of having too much of a vertical component in the running stride. The final aspect of the running stride is the toe-off. After the hip has been fully extended, the ankle joint is the last chance to add horizontal movement and with it, length to the stride. With virtually no added time cost to this toe-off, there is clear benefit to the motion. I say virtually no added time because a small time component is present.
For the toe-off to be a horizontal component, the leg must be as far back as possible. The timing of the toe-off also coincides with the beginning of the recovery phase [pull through] of the leg to minimize the extra time of contact on the ground. To gain the greatest force from this toe-off, the principles of plyometrics must be heeded to; a loaded muscle will provide a greater response than an unloaded one.
When the foot first strikes the ground, the added weight of the body on the calf muscle becomes the loading. If landing with the ankle in a plantar-flexed position, the loading will be too much and too slow, and the golgi tendon organ (responsible for muscle relaxation) will win out, cancelling any potential load-fire coupling benefits.
Additionally, any extra strain on the calf from the landing will tire the calf, naturally decreasing its potential to give back energy through the toe-off. Strong quads are then also important for a proper toe-off, as they will support much of the load of the body, leaving the calves available for propulsion rather than support.
When training the body, it is said that increases in performance come during the recovery phase, rather than during the actual training session. The same principle can be applied to the running stride; the increases in stride efficiency will come from the recovery phase of the stride, or how fast you can get the leg through in order to begin the next preparation-propulsion-push off cycle.
The pattern of movement for the pull through phase can be classified simply by the pneumonic "heel up, toe up, knee up." This, again, emphasizes the need for a toe-off motion in completing the propulsion phase of the stride. The "heel up" begins with the toe-off creating the heel to rise, and continues with the need to get the heel to the upper thigh as quickly as possible. This will shorten the lever that needs to be brought forward, creating a faster pull through phase.
The toe up and knee up basically occur at the same time (keep in mind that all three of these events happen nearly simultaneously, as the goal is to have them occur as quickly as possible). As the heel is being brought to the upper hamstring, the knee is already being driven forward. As the foot swings through, it is then dorsi-flexed (toe up), and placed in the position it will remain in until contact with the ground. This flexing of the anterior shin muscles also helps begin the flexing of the knee.
Bringing the knee up is almost a misnomer, as it gives the illusion that the goal is to create a vertical component of movement. However, the primary thought behind "knee up" is in allowing the lower leg a slight amount of extra time to fall down into position for the landing. In essence, this is merely a slight pause in the motion of the upper leg while the lower leg uncoils.
The reasoning behind bringing the leg as close to the body involves more than just creating a shorter lever for quicker movement. By bringing the lower leg up against the upper leg, the hip flexors (a traditionally weaker muscle) do not need to exert as much force during the pull through phase. Instead, the hamstrings help support the weight of the lower leg during this phase. Raising the leg higher will also make the legs less of a rotational force. Because of this, the upper body does not need to counteract as much rotary movement, allowing for a more forward-focused movement. A strong core will assist even more with this process, as it will provide additional inhibition of rotary movement through its stabilization properties.
The final "P" of the running stride is percussion. This is merely a means of self-check in the absence of technical coaching and/or video equipment. Looking in a mirror does not provide great feedback, as a head-on mirror will reflect too small an image and not allow adequate time to get to a cruising speed (when a patterned stride occurs). A mirror on the side requires a turn of the head, which is not a natural part of the running stride, and will therefore provide inaccurate assessments of form. Energy cannot be created nor destroyed, it merely changes forms during its existence. One of these forms is movement, and another is sound. Optimally, while running, the goal is to put complete energy into movement. This, then leads to the assumption that the most efficient stride will also be the quietest (assuming all other things are equal). The sound produced by your feet hitting the ground is a transfer of energy your body is producing to the noise you hear, and is a result of the vertical component of force you place into the ground (and it conversely gives back to you). In a gravitational environment, some vertical component will always be necessary, so some sound will occur. The goal is to simply minimize it.
What to look for in a runner's upper body:
From one thing evolves another, and such is the whole of the running motion. As running is a cyclical pattern, an error can compound itself many times over. Ultimately, the most basic test of form is the sound the foot makes with the ground. Any noise is a transfer of energy in a downward motion, rather than the forward motion of running.