Technique for changing terrain
I don’t believe that one effectively consciously adapts running technique to suit changes in terrain; on the contrary, I believe that it is the terrain changes that alter running technique. Of course, I also believe that children do not actually learn how to run; I believe that the mechanisms involved in running are innate, as they are in all mammals. And that these mechanisms adapt to changing conditions without instruction when good dynamic posture is present.
The whip and leg recovery
Let’s look at leg recovery in uphill versus downhill running. I think that we can understand something of the physics of leg recovery by imagining a whip (or, if you like, a fly-fishing rod). If you hold a whip in your hand, with its length trailing behind you, and your arm at your side, you can sense that the movement that will bring the whip forwards will begin with your body. A slight rotation of your trunk away from the whip will move your shoulder forward, and begin to drag the length of the whip along. As your arm begins to rotate forward from the shoulder, the movement accelerates, and then abruptly decelerates before changing direction. At this point, when the arm has finished its forward travel, the business end of the whip is still accelerating. Add a little flick of the wrist, and it accelerates even more, causing it to move quickly forward. At this point, if you stop your arm movement, the whip will simply lengthen out before you. If, however, you begin to swing your arm backwards, the whip will follow your movement, but its tip will be the last part to change direction. And, because a pivoting object will travel fastest at it outermost point, the tip of the whip will be moving dangerously fast. So, although to think of the recovery of a leg in running as analogous to the movement of a whip is a huge oversimplification, it does help to understand how muscular actions closest to the trunk are primarily responsible for leg recovery.
Acceleration/deceleration
The body is accelerated forward by the extending leg. As the trunk approaches its peak speed, a foot is still on the ground. As that foot leaves the ground, the body begins to decelerate, but the foot has just begun its acceleration and, assisted by the iliopsoas muscles, it begins to move forward and upward at a speed eventually faster than that of the decelerating trunk. The thigh, pivoted forward by the iliopsoas muscles, draws the lower leg along. The hamstring maintains muscle tone to support the weight of the lower leg, such that the flexion of the hip-joint creates flexion at the knee. By the time the recovering leg is under the trunk, the body’s weight has descended onto the other leg, which then begins to extend. As the powerful contraction of the gluteus muscle begins to extend the supporting leg, the gluteus medius muscles at the side of the pelvis contract, tilting the pelvis slightly toward the supporting side and lifting the side of the recovering leg. The strong contraction of the glutes also creates a rotation of the pelvis towards the recovering leg. This combined lift and rotation gives added impetus to the forward movement of the recovering leg and whips it forward. As with the whip, the last part of the leg to move forward is the foot. In fact, the foot continues to travel forward even after the thigh has begun to pivot rearward, further straightening the recovered leg. On level ground, this means that the foot will already be traveling rearward as it reaches the ground. And, also in level conditions, the foot reaches the ground before the body begins to decelerate and fall.
Uphill vs. downhill
Uphill, things change slightly. The more powerful and complete extension required to lift the body as it moves forward results in a fast and high leg recovery. Downhill, the leg will not need to drive as strongly, and will rarely fully extend, as no lift is required. Thus, the recovering leg will not be drawn up after the body, and will tend to recover lower, which means the foot will reach down more as it reaches forward. At steep inclines, the foot will not be able to reach the ground before the body falls, as the body isn’t lifted by a stride off of a decline. Thus, we need to consider that there is some impact in downhill running which is not present in level or uphill running. This impact can be reduced by fully extending the leg in recovery, so that the knee and ankle will be nearly straight at the point of full recovery. Flexed foot heel-striking is especially ineffective in downhill running, but landing on the toes with an almost straight leg allows the forces of descent to be distributed over a long distance (like the longer travel in descent mountain bike shocks). Still, it is important that the foot be travelling back towards the body before it reaches the ground, because this will mean that the joints in the legs will already be folding when the body lands, and will thus be in a better condition to absorb the shock of landing. A foot that is not moving in relation to the trunk, or that is still moving forward at landing, will tend to create a stiff landing. This is like the advice given to parachutists to do a forward roll when the land. Of course you don’t really need to do a forward roll, but doing one, or at least thinking about it, will make you hinge at the hips and knees, and will enable you to use the glutes as shock absorbers, rather than stiffening the muscles and taking the impact through the bones.
Lifting the feet – NOT
Finally, I think we can put to rest the idea that one can improve or accelerate leg recovery by consciously lifting the foot towards the buttocks, especially in downhill running, where it is clear that there can be no advantage to forcing the foot and leg to travel further upward than they would as the result of an appropriate leg extension. Remember, the longer and more complete the leg extension, the more distance the runner travels per stride, and the faster he can run. The acceleration of the body continues through the stride, and, as in a vertical jump, only reaches peak speed as the leg reaches full extension. As one cannot jump high without full extension, one cannot reach peak running speed without full extension. Even small jumps are easier and more efficient when done to full extension from a less flexed beginning point.
Try this. Bend your knees slightly, incline your trunk forward, and extend one leg behind you. When you lift the extended leg so that your foot leaves the ground, note your hamstring – it will be active. Note your calf muscles – they will not. From this state, when running, as the extended leg begins to recover, the tone in the hamstring will cause the knee to flex in coordination with the flexion of the hip. The joints are linked through the action of the biarticular muscle of the posterior thigh. The ankle, however, need not flex, and the foot can be easily recovered without ankle flexion. Thus, a heel strike cannot occur unless some action (unnecessary) it added to the anterior muscles of the lower leg. Heel strikers are flexing their ankles when they recover their legs, or, they are simply wearing shoes with heel lift. Neither is very helpful, except, perhaps to correct other defects in posture.
Extending, not falling
Further, full extension of the leg gives impetus to and time for ample recovery, as, with a longer extension phase, the runner stays airborne for a longer time/distance. We see many barefoot runners taking very small steps to avoid the consequences of bad running technique. They are not extending enough at their ankles to get much lift into their strides, so they have little time to get their recovered foot on the ground. (Some runners have described barefoot running at a series of “falls”, but if there are only “falls” with no prior or subsequent “lift”, every step would bring the runner closer to the ground – toss this notion out along with that of lifting the foot.)