S-LIKE BODY SHAPE IN RUNNING POSE
What is the best S-like body shape you should have in running? This seemingly simple question became a topic for discussion because of no clarity on this matter. I do not see any problems in posing any questions as long as they allow us to understand things better and clearer. This question indicates the lack of comprehensive description of the running Pose, which makes it difficult for newcomers to understand and learn the Pose Method, so it's necessary to return to its explanation with more depth and clarity.
The original running Pose was based on the description of a mid-stance position of the runner on support, which is characterized by a vertical and balanced position of the body on one leg, with the body weight GCM (general center of mass) located on the ball of the support foot. The body is slightly bent in three major joints: hip, knee, and ankle. How much is "slightly" bent to get into the correct S-like body shape? This is a question for many people learning the Pose Method.
The answer is pretty much easy because it's related with both S-like body shape and bent joints, which is necessary to have the body springiness condition. That's right, the springiness of the body is the main reason why we are trying to have slightly bent joints and assume an S-like body shape. But with that some more benefits come together, such as an ability to reduce the load on joints during the support time, to get quicker and softer interaction with the ground and to pull the foot from support more efficiently.
The only question remains, how much is "slightly" bent? How are we supposed to choose this undefined position, if we are all different as to our height, weight, strength, etc? The answer comes from the ability of our body to have springiness by itself. So our "slightly" bent joints position means the best springiness, which we can achieve by hopping softly in place on two legs. You can play with joints bent while hopping to find out, which position provides better springiness for you and by doing this get an idea of slightly bent joints good for you. Surely, it will be different for different people, but it will be different for different height of hops, as well. The main thing is to keep the perception of springiness as your guidance and then transfer this into your running.
If you keep this guidance in mind, it will be no problem for you to maintain a proper angle of bending your joints through any length of the running distance. One more note about the perception of springiness, it shouldn't be associated with any muscles tension, but on the opposite, with a perception of lightness and effortlessness, a short time of support and easiness of changing support. If any of these perceptions are changing to the opposite, it is a sign that your joints are in the wrong bending position and it is necessary to adjust their angles of bending in order to return to the best springiness condition. And this logic remains the same from sprint to long distances, from one level of fitness to another, from novice to advance level of running.