I heard Jack say this to me about 30 years ago when I worked in the Nike lab with him. His research backs up the claim. The lighter the shoe, the less energy the runner used until the hardness or roughness of the surface forces the runner to cushion his footfalls with his own muscles.
Not only do you use less energy barefoot but you stay cooler when your feet are not wrapped in insulation, which is what a running shoe is mostly made of. But ideal conditions for barefoot racing hardly ever present themselves.
I know I would run my fastest at anything not requiring a sprint start barefoot on dry, green grass. But on wet soaked grass I would need spikes or on rough asphalt or concrete at fast race pace (not my old man pace of today) I would need shoes or my feet would become hamburger. I have training barefoot on nice grass fields and prefer it but have never found a cross-country race on all dry, green grass without rocks, sticks, or acorns.
Ice, snow, slush, would require shoes. I bet the fastest times would be set on track if they were made of the artificial turf that you see sometimes on the infield. I wish someone would build one for barefoot racing.
Anyone? Anyone?
Tom