I'm probably guilty of over using analogies in my daily life, I'm like a bulweavel in a.... see, there I go again! But the "spring" can be used to represent anything. One could string together a set of springs in a certain way to completely mimic the behavior of a building in an earthquake, or a table's jiggle'ability. A spring's springy'ness or compliance is measurable, hang weigth from it and it extends only so far, it has a resonant frequency, etc. these are all properties of the muscles and tendons in your "running chain" .
The beauty of the springiness of your legs is that it's variable. The engagement of muscles change tension, resonance, compliance of this spring. It's a wonderful thing!
Have you ever tried to run on a trampoline? There's only one slow speed you can go other wise the springiness of the trampoline will push back so hard it can drive your knee into a tooth! This is what's happening when you add 'springs' in series with your running chain. Why would I insert a spring? It's what you're doing when adding a springy cushion under your foot, or running on a rubber track, or spring loaded treadmill. Cascading these compliances adds to your work load. You are now in charge of absorbing your weight, absorbing energy returned at a different rate due to these dissimilar cascaded resonances.
Remove the cushions and you will see! It's less work, more economical, and more conducive to long term injury-free running. (...more to come).