>I guess in the second case the plantar flexors are acting as a knee
>flexor if the foot is fixed.
- No: the knee doesn't flex. It stays at the same angle (well, it extends slightly)
The forward rotation of the shank might then be a part of maintaining equilibrium as the knee flexes?
- the shank rotates backwards (you are seeing the loop in the video - look at the slow part of the motion).
But what keeps the foot on the ground in the second case and why does it rise in the first?
Dr. Chris Kirtley MD PhD
Dept. of Biomedical Engineering
Catholic University of America
620 Michigan Ave NE, Washington, DC 20064
Tel. 202-319-6247, fax 202-319-4287
Not much response to the latest teach-in so far.
I don't know whether this means that the solution is absurdly obvious or bewilderingly difficult!
Perhaps I could offer some possiblilities:
1. The force of the contraction (ankle moment) may be less in the case
when the foot does not rise
off the floor.
2. The centre of pressure is more anterior when the foot is flat, allowing
a larger moment to be
generated before the foot rises.
3. Some other muscle group is acting.
I can't think of any other plausible solutions, but none of these strike me as credible.
In #1, I would expect that the shank would rotate backwards until the
point at which the foot
rises. I guess the tibialis anterior could act to prevent this.
#2 could be possible, but I don't think so. I'm pretty sure I maintained
the same starting
#3 also doesn't seem reasonably, because an inverse dynamics analysis
need only consider the foot
and shank segments.
Well, I guess the time has come to gather some data on the problem.
It worries me, though, that the
answer isn't immediately obvious!
mailto:[n/a] if you disagree!
The Catholic University of America
This problem would be a good way to get students thinking
about coupled dynamics. I always cite the article by Zajac & Gordon
(Exerc Sport Sci Rev, 1989) who used equations of motion to show
how the function of a muscle can depend on body posture. Their example
also involved the plantarflexors, but not the same movement.
I suspect that the upper body is positioned more backwards in the
second movement, and that this caused the difference in movement.
But the subject could also be cheating by using the knee extensors
more in case 2. You would have to do the inverse dynamic analysis to
verify that all joint moments are indeed the same, and initial posture
alone caused the difference in movement. Since it is such a delicate
balance, it is possible that small differences in joint moments can
cause large difference in movement. A method such as Zajac's would
be able to predict these things.
For the last question, I would say the joint power goes into the backward
acceleration (kinetic energy increase) of the body. The joint moment is
probably be much smaller in the second movement than in the
first, with considerable antagonistic activity, otherwise you
would lose balance.
Power flow is something I like to stay away from (though I have been
guilty of using it). Power flow is not invariant when transforming the
gait data to another inertial reference frame (think treadmill gait),
and so it may not be a variable that gives useful information about the
movement. The variable that you call "passive foot power" is very
large during treadmill walking. What does that mean? It is related to
the fact that the treadmill motor is always pushing and pulling to
maintain constant belt speed against the horizontal ground reaction
forces. And we "see" this power flow through the patient's joints too.
Is that interesting? I don't think so. After all, the ground that we
walk on is moving through interplanetary space with some speed also.
A.J. (Ton) van den Bogert, PhD
Department of Biomedical Engineering
Cleveland Clinic Foundation
9500 Euclid Avenue (ND-20)
Cleveland, OH 44195, USA
Phone/Fax: (216) 444-5566/9198
> 1. The force of the contraction (ankle moment) may be less in the
case when the foot
does not rise off the floor.
I think this is the main cause. The ankle moment generates
on the trunk and the segments that are upward, and slightly
backward. Gravity counteracts this with downward forces (segment
weights). When ankle moment is low, less than needed to
counteract gravity, the resultant force is directed backward. When
it rises quickly to a high value, the backward acceleration lasts
only a short time and gives only a small backward movement. An
experiment might verify this.
> 2. The centre of pressure is more anterior when the foot is flat,
allowing a larger
moment to be generated before the foot rises.
No, the position of the CoP is directly related to the ankle moment.
Ground reaction force is about equal to body weight in both cases.
Low moment -> CoP close to ankle.
> 3. Some other muscle group is acting.
> In #1, I would expect that the shank would rotate backwards until
the point at which the
foot rises. I guess the tibialis anterior could act to prevent this.
> #2 could be possible, but I don't think so. I'm pretty sure I maintained the same
> #3 also doesn't seem reasonably, because an inverse dynamics analysis need only consider
the foot and shank segments.
The argument is true, but not relevant here. In fact you need a
forward dynamics analysis, and this includes all segments above
the ankle. Shank thigh and trunk are all moved by the ankle
> Well, I guess the time has come to gather some data on the problem.
It worries me,
though, that the answer isn't immediately obvious!
This "worry" is justified. The mechanics of this kind of simple
biomechanical problems must all be in Newton's equations, but the
problem is to get them out of it.
Institute of Human Movement Science
University of Groningen
PO Box 196
9700 AD Groningen
Tel: (31) 50 363 2645
The Teach-In Problem is interesting but sends different messages to
me than is mentioned in the replies. My thinking is that increased
strength or use is what changes the subject from plantargrade to an equinus position when the muscles are functioning normally or typically.
On the second video, the quadriceps appears more active than in the
first video and the calf muscles appear to be more relaxed. When
I try to
hyperextend my knee, I need to relax the calf and increase the force of the quadriceps muscle. Houtz and Walsh discussed just this long ago and it
still seems logical to me. [Houtz, SJ and Walsh FP. Electromyographic Analysis of the function of the muscles acting on the ankle during weight
bearing with special reference to the triceps surae. J. Bone and Joint Surg., 41-A:1469-1481, Dec 1958.]
They also found that the gastrocnemius played a major role in
adjusting the relationship of the femur on the tibia. When a body
was leaning forward
the lateral gastrocnemius showed few action potentials. In the erect posture the gastrocnemius acted on the knees to maintain a slight degree of
flexion, medial head more active than lateral. They found that the triceps surae is active in raising the heel off the ground and forming part of the
longitudinal arch and that hamstring weakness does not result in gneu recurvatum.
Houtz and Walsh also mentioned that loss of the gastrocnemius muscles
in those with polio often causes hyperextension of the knee. I find
when neuromuscular electrical stimulation, NMES/FES, is used to the calf muscles in children with CP that knee hyperextension is decreased or
blocked if the medial gastrocnemius muscle is stimulated during stance, from initial contact to heel rise. Stability of course is also improved. Others
have found that the calf muscles are the major postural muscles of the body. [Simon SR, Mann RA, Hagy JL and Larsen LJ. Role of posterior calf
muscles in normal gait, J. of Bone and Joint Surg., 60-A: 465-471, June 1978, and also Sutherland D, Cooper L, and Daniel D, The Role of Ankle Planar
Flexors in Normal Walking, J. of Bone and Joint Surg. 62-A:354-363, April 1980.]
Judy Carmick PT