[quote author="danimal9" date="1167364320"]I may be over simplifying this but using a spring-mass model when you steepen the angle of attack, meaning the foot would land closer to underneath the COM, this lessens GCT, increases the vertical force component, thus with a stiffer spring, that elastic energy contribution is higher and the foot because of all this wouldn't be trailing the body in contact with the ground much anymore behind than it touched down in front for take-off.
You're actually over-complicating. The spring mass model is actually quite simple and by itself doesn't say anything about angles of attack, etc. Having said that, you're right on some things and a little off on others. Elastic energy contribution actually shouldn't be affected greatly by ground contact position…beneficial elastic energy contribution likely is. Somewhat of a semantical issue but an important one. Otherwise things are pretty much right.
The Seyfarth, et. al 2002 study "A Movement Criterion for Running", used the spring-mass model and indicated that angle of attack is dependent to leg stiffness for a stable running pattern of a given speed. Granted it was done by simulation of a model, but the model is assumed correct and the evidence and data so far seems to indicate that it will continue to be the case. However, one could work backwards in the system knowing a less steep angle would require a higher initial velocity at the COM to keep the GCT at a minimum the leg stiffness would have to be increased. My thoughts are this would lead to system instability, therefore a steeper angle of attack must occur to provide stability to the spring-mass system at greater speeds to maintain velocity.
Figure copied w/o permission from Seyfarth, et. al. "A Movement Criterion for Running", Journal of Biomechanics, Vol 35, 2002, p. 651