View Full Version : Measuring energy recovery from leg compression?

01-23-2008, 12:47 AM
Hello Everyone,

With all the recent postings regarding Mr. Pistorius I was wondering if we
could begin a discussion on developing a method for measuring the energy
recovery from leg compression during running.

Voluntary muscle contraction and the leg's energy transfer to the upper body
obscure the measurement. Maybe it would be possible to measure the energy
return of the leg alone without these complications.

One method might be similar to the "bounce" method for measuring the
coefficient of elasticity of a ball (1). Here the percentile energy return is 100
times the return height divided by the drop height.

Of course, it wouldn't be possible to drop an anesthetized upright subject
with activated leg muscles on a surface and measure their return height. But,
suppose an alert subject lies on their back in a frame (2). The frame
could have a pair of stops that would contact the top of the pelvis. The stops
would prevent the subject from sliding head-ward due to leg compression.
Then, the spring-like behavior of the leg could be observed -- independent of the
motion of the upper body.

Initially, the subject would lie on the frame and slide head-ward until
their pelvis contacted the stops. They would then bend their leg so their knee
was at the same angle that it would be midway between touchdown and
mid-stance. The base of the bent leg would rest on a "frictionless" carriage, so the
leg could compress and expand. A compressive load could be applied to the
bent leg with a system of weights and pulleys. The load might be 1.5 BW. A
"sanity check" could be run at this point to confirm that the leg stiffness was
approximately 10 to 20 KN/m.

The subject would tension his leg to oppose the 1.5-BW compressive force and
close his eyes. A lightweight pendulum could be released from a small angle
so that it impacts the foot carriage and compresses the leg. The pendulum's
impact would compress the leg and rebound to a fraction of its release
angle. If the angles were small, the energy recovery from the leg would be 100
times the return angle over the release angle.

Admittedly, there are some problems with this approach. It does not include
the angular moment of the leg segments. There is an issue regarding the
energy absorption of the foot carriage and using a pendulum for the compressive
loading. But maybe this idea makes a start at developing a method for
determining the energy recovery from leg compression? Any suggestions?

Ted Andresen
St. Petersburg, Florida

(1) _http://staff.hightechhigh.org/~ajgloag/MathIV/bouncingball.doc_
_http://members.aol.com/EasyExperiments/GaitCycle/TestingEnergyReturn.gif_ (http://members.aol.com/EasyExperiments/GaitCycle/TestingEnergyReturn.gif)

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