When watching a 100m dash, I've never thought about the work and power that was used by the runner until now. If the runner exerted 500N of force over the course of the 12 second run, then you can determine the runners work and power. For solving for work, W = Fd. So, (500N)(100m) = 50,000N*m of work done. Then, to solve the power that was used, P = W/t. So, (50,000N*m)/(12s) = about 4,167 Watts of power used.