The law of the pendulum states that the swing is
dependent on the length of the
string, not the weight at the bottom(the weight of the ball).
When you want a pendulum to swing more quickly, the string must
be shortened. When you want a pendulum to swing slower, the
string must be elongated. You would not change that weight at the bottom
to make adjustments, you would simply slide it up or down the string
http://www.chasingglennbeck.com/homeblog/2013/2/4/who-should-swing-the-pendulum.html
When running, the arms play a role in "shortening your string". As do the legs.
The arms and legs act as weight, and when they are bent, they are technically
moving the center of gravity upward. Therefore, bent arms and legs
enable the you to swing them faster without using more muscle.
http://www.7sruns.co.uk
Energy expenditure has to do with how much up and down movement a runner has.The
more they bob up and down, the more energy is loss. The more forward a runner leans,
the less bobbing there is, and gravity can do more work.
The runners lean is not done from the waist, but from the ankles.
http://www.kaikuusisto.com/43383/366251/portfolio/nike-finland-running-club