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unknown user
11-17-1992, 04:12 PM
On Nov 16, John Martinson said:

> [introduction deleted]
> Many studies have been done on the vertical or lifting force of a standing
> person at various angles at the knee between the femur and the tibia. But
> in going from a flexed to a fully upright position imagine that the back
> of the knee is contact with a dynamometer so that the (nearly) horizontal
> movement of the knee exerts a force "backwards" or horizontally.

In a vertical lifting task, it is certainly possible to exert a
horizontal force at the knee, as you describe. However, the
results of such an experiment would not tell you anything
interesting, and probably would not be reproducible. The reason
is, that this is a task with only one degree of freedom (vertical
movement, or leg extension). If you have *two* restraining
forces (vertical and horizontal), you create a statically
indeterminate system. This means that, with the same muscle
forces, you could get an infinite number of different
distributions between the horizontal and vertical restraining
force. That distribution would depend highly on the rigidity of
the connections with the dynamometers. With only one dynamometer,
there is no such problem and a unique result will be obtained.

A similar indeterminacy occurs if you would have a rigid
four-legged table with perfect symmetry. You would expect the
weight to be equally distributed, but actual measurements could
give *any* result, depending highly on small differences in leg
length or ground stiffness. In a table with three legs, there is
no such discrepancy between the degrees of freedom and the number
of forces and measurements will produce good results.

> I also suspect that precise measurements very close to full extension
> might help resolve the seemingly interminable debates about the origins,
> advantages, disadvantages of hominid bipedalism among physical
anthropologists.
> In general, the occasional bipedalism of the other primates does not involve
> the full extension of big toe, ankle and knee of humans (consider the
ballerina
> up "on point"). But what kinds of mechanical advantages or energy
efficiencies
> might we humans enjoy because of this? Your suggestions are invited.

This is an intriguing problem. Most quadrupeds do not have, as
you say, their joints fully extended at any time. My speculation
would be, that humans have full extension because it is more
efficient, by requiring less muscle force to maintain the
posture. However, flexed joints also have some advantages. It
makes the limb more 'spring'-like, providing a good shock
absorption. It also allows a quick reaction when threatened by
predators, for example. Humans have to flex their knees before
jumping. Most animals can do that quicker, because the knee is
already flexed.

Note however that there is no universal correlation between
quadrupedality and the amount of joint flexion. An elephant has
its limbs fully extended during standing and during the stance
phase of locomotion. Supporting the weight with flexed joints
would simply require too much muscle force. That is purely a
matter of scaling: body weight increases with the 3rd power of
size, and muscle strength only with the square of size.

These are just some ideas. Hopefully useful.

-- Ton van den Bogert
Human Performance Laboratory
University of Calgary, Canada