Based on what I have read and experienced, here's a synopsis of my understanding of increasing stride length.

There are two ways to lengthen stride. Reach out with the leading leg/foot or a longer, stronger support leg "push."

The first way is what I think results in what we usually think of as classic overstriding. It causes foot placement to be too far in front of the body's center of gravity and results in a hard heel landing. It extreme cases, it produces a foot "slap" or "thud." We have all heard people near us in a race whose feet sound like someone is clapping their hands. It has three negative consequences. One is a braking or deceleration action, which the runner must overcome in order to maintain or increase pace. The second is higher risk of injury due to the harder landing. The third, I think, is that all of the resultant stride length increase occurs in the flight or float phase of the stride since the body must travel longer before the leading foot strikes the ground, thus delaying the beginning of the support phase which is where all the power is generated to run faster.

This form of overstriding is caused by excessive knee flexion of the forward leg, hence excessive lower leg extension, which permits the foot to swing further in front of the body's center of gravity. I think it's common among beginning runners because it's a "muscle memory" carry over from walking, where the foot does land well in front of the body's COG. Among more experienced runners, it's often due to insufficient hamstring strength, since these are the muscles that are used to slow lower leg swing and prevent excessive knee flexion. After stopping the forward swing, the hamstrings also have to pull the leg backwards to position it for proper foot placement. If the hamstrings permit, excessive knee flexion occurs, along with insufficient recovery of the leg for proper foot placement.

This form of stride length increase has absolutely no positive benefit and is to be avoided like the plague. Fortunately, it's relatively easily controlled through strengthening the hamstrings and stride manipulation. This is the form of overstriding problem that is, and should be, most often corrected by shortening stride length and increasing stride rate.

The other way to increase stride length, a longer and stronger support phase, has mostly positive consequences. It's the primary biomechanical way we run faster, stride rate increase being the other (secondary) way. The forward thrust generated during the support phase of the stride determines how far we travel with each stride and, ultimately, how fast we run. The stronger the thrust, the greater the potential to propel forward farther and faster. However, the thrust must be channeled in the right direction. Thrust generated by the support leg has horizontal and vertical components. The horizontal component determines how far and fast we are propelled during the float phase. The vertical component determines our vertical displacement during the stride. The greater the horizontal component of thrust, relative to the vertical, the less "pop up" or bounce we will experience relative to distance traveled and the more efficient our stride will be, i.e., the more energy generated will go into moving us forward and the less wasted in moving our body up and down. So, the challenge is to increase the amount of thrust we can efficiently generate, while maximizing the horizontal component and minimizing the vertical component.

Increasing thrust requires increasing ankle and hip flexibility, as well as strengthening of the muscles of the leg and hip. In discussing ankle flexibility, Martin and Coe say "It is a well known fact that a muscle can generate greater shortening (power) if it has been prestretched before tension generation begins. The longer the heel remains near to or in contact with the ground while the knee moves forward, the greater the prestretch on the calf muscles. This will increase both stride length and power." To keep the heel on or near the ground longer, the angle of the support leg has to increase, with respect to vertical, while keeping the foot flat on the ground. How long we can keep the heel on or near the ground is mostly determined by the flexibility of the ankle and hip. Thus, one of the reasons runners should stretch is that it makes you faster, as well as helping to prevent injuries. Of course, increasing leg and hip strength occurs over time through a training program that includes strength runs and weight training.

Increasing thrust increases float time, since you are propelled forward at a faster pace. But, the mechanics involved in generating the increased thrust keep the support leg on the ground for an even longer period of time. Thus, a greater percentage of time during a stride is spent in contact with the ground and a lesser percentage in flight resulting in a net gain in efficiency.

I also think that ankle flexibility is the most important factor in maximizing the forward component of thrust and minimizing vertical displacement. Martin and Coe's "Better Training for Distance Runners" contains an illustration of early and late takeoff on page 27. It shows early takeoff occurring with most of the forefoot still in contact with the ground and the rest of the foot at about a 60-70 degree angle from the ground. OTOH, it shows just the toes in contact with the ground and the foot already beyond 90 degrees for a late takeoff. With the early takeoff, the foot hasn't had a chance to finish rolling forward and presents a more flatfooted "launch pad", which causes more of the thrust to be directed in the vertical direction. Thus, early takeoff will result in a shorter stride length and more vertical displacement, as Martin and Coe say. Said another way, a shortened stride will result in early takeoff and greater "popup."

I think it is possible to overstride from the support phase. It occurs when we try to force a longer support phase before our body is trained to handle it. There are two problems with this. It moves us off of the point of optimum running economy and it increases the risk of injury. However, understriding in the support phase is much more common, and is often caused by a forced increase in stride rate. I still think it's best to let your body find the stride length with which it's most comfortable at any given pace, as long as you control the first form of overstriding.....reaching with the leading leg.

So, my answer to your question is that, I agree with you that a longer stride will increase the flight phase, but not necessarily vertical displacement, which it can even reduce. More importantly, the flight phase is not all that is increased. Far from it! I think most of the increase potential is during the support phase, but it can't be forced. It has to be developed through training. Leg length is the least of the factors involved. Muscle and joint flexibility, along with leg and hip strength, are much more important. That's why I think that shortening stride beyond the point of optimum running economy is a step backward for a runner looking to advance. That's what I meant in an earlier post when I said that it trains us to be "short striders", which is counterproductive in the long run. (No pun intended. :))

Jim2