This might be a primary disconnect, then - since if you're not understanding how the ground provides a counter force to gravity - without you having to do anything, flex any muscles, etc. Internal strength at it's baseline takes advantage of that when you push on the ground, the ground pushes back - how purely you've trained your body (to cite your example of the legs above - there's much more than that going on) to capitalize on this will affect the effect you have when you move/hit/throw with internal strength.
The ground does not provide a counter force to gravity in the sense of the ground manufacturing energy and adding it to the ground/person system. The stability of bulk matter (the reason the ground supports you, even under reasonable loading) is explained by the Pauli exclusion principle (short range) in combination with Coulomb's law (long range). Reference http://en.wikipedia.org/wiki/Pauli_e...lity_of_matter
Even if you wish to contend that Pauli and Coulomb are forces, the net gain/loss of energy in the system as a whole is zero, the system is in a state of equilibrium. In short, the ground does not provide you with any usable potential energy which you may convert to kinetic energy and use to perform work.
ps - If someone can show me the physics of how "Internal strength at it's baseline takes advantage of that when you push on the ground, the ground pushes back " I will happily stand corrected.