He raises doubts about the efficiency of human power and questioned its economic viability.

CNN: Are we heading for a human-powered future?

Just as DARPA and other humanoid robot developers have drawn on biology for design inspiration, the agency hopes that robotic actuation can approach the efficiency of human and animal actuation where muscles, tendons, and bones cooperate in highly efficient ways.

FORBES: DARPA's New Challenge: Increase Robotic Power Efficiency By 2,000 Percent

That, and improving efficiency really made human powered-flight move from sustained hops and skips to the international crossings of the Albatross.

ENGADGET: The Highs and Lows of Human-Powered Flight Alt