Like I said, the energy has to come from somewhere. It's not necessarily the case that we expend more energy, but it's certainly plausible (especially when you start substituting in materials with a different elasticity or parts that need moving).
I'm not a mechanical engineer, but I'm decently familiar with thermodynamics and the idea that there aint no such thing as a free lunch. I'm more experienced with alternative energy systems, to really evaluate them, you always have to do a rigorous energy balance. So, what I'm really getting at here isn't that I am certain that we expend more energy in walking, but that someone needs to do the math for that balance. It's often the case (sadly) that that analysis reveals that the power generation just shifts the environmental burden upstream.