Abstract: A motion analysis device uses an inertial sensor to calculate a measure of a user's athletic energy efficiency using only readings provided by the inertial sensor. The motion analysis device further provides suggestions to the user as to how to alter his posture or movements to improve his athletic energy efficiency. For example, in the case of a runner, the motion analysis device may instruct the runner to lean in one direction or another in order to alter his landing angle in such a way so as to improve his athletic energy efficiency. The device may further provide continuous feedback to the athlete to let him know when he is moving closer to, or away from, preset athletic energy efficiency goals.