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Ton Van Den Bogert
12-10-1999, 03:41 AM
"Devita, Paul" wrote:

> It seems that isokinetic and inverse dynamic estimates of joint torque
> capabilities are in disagreement. Isokinetic measures of maximal torques
> seem to underestimate predicted, inverse dynamic values by almost 50% (see

This is a very interesting question and I am posting a reply to Biomch-L
in the hope that this topic will stimulate a public discussion.

My thoughts:

(1) Methodology

In dynamic activities that include a ground impact phase, such as
drop jumps, there is a methodological problem in determining joint
moments. High frequencies in the kinematic data are usually filtered
out, but left intact in the ground reaction force signal. So the
different terms in the equation of motion, used to compute joint moment,
are not treated consistently. Typically, this leads to high moment peaks
during the impact phase which I consider to be an artifact. Muscle
torques should never show impact peaks since muscle is soft tissue
and force can only change gradually. A solution would be to filter
both the kinematics and the forces (van den Bogert AJ, de Koning JJ,
On optimal filtering for inverse dynamics analysis. Proc. 9th CSB
Congress, Burnaby, B.C., pp. 214-215, 1996).

However, the discrepancy mentioned by Paul de Vita also exists
in movements without impact (e.g. Bobbert, 1988, vertical jump),
so this is definitely not the only source of the problem.

(2) Stretch-shortening cycle

Although this certainly contributes in many of the activities
listed by Paul de Vita, I don't think that eccentric force-velocity
properties or stretch-induced force enhancement provide a sufficient
explanation. In a later study, Bobbert showed that a 300 Nm knee
moment can also be generated during a jump that starts from a
squatting position, a movement with *no* muscle lengthening at any
time (Bobbert et al., Med Sci Sports Exerc 11:1402-1412, 1996).
I think that the squat jump results also show that muscle shortening
during the isokinetic test is not the cause of the smaller torque.
The muscles are also shortening, quite fast, during the squat jump.

(3) Inhibition

I propose that an inhibition must exist which limits the muscle
activation during a typical strength testing protocol. This
could be a protective mechanism, or simply a consequence of our
central nervous system being "wired" to use muscles efficiently.
If extra activation does not result in a change in movement, why
even try? Movement is not influenced at all by muscle activity
during isokinetic or isometric tests. If the latter hypothesis is
true, one would expect to see larger maximal voluntary torques
being produced against inertial or viscous loads, when compared
to isometric or isokinetic tests. Is motor control naturally lazy?

There is some evidence that in addition to torque, the muscle
activation can also be larger during functional movements when
compared to MVC. Jacobs and van Ingen Schenau reported that the
EMG amplitude of the Vastus Lateralis was 200%(!) of MVC during
sprint running (J Biomech 25:953-965, 1992). Analysis of EMG
amplitude is also somewhat sensitive to methodological problems,
but those results may still indicate a true mechanism.

Twitch interpolation, if I remember correctly, can be used to
determine how close the voluntary activation is to the maximal
activation. I am not familiar at all with this field of
research and not sure if those results are consistent with a
30-50% inhibition. Perhaps someone can comment on this.

Also I am not familiar with the literature references on maximal
isokinetic knee torques. If some of those were obtained by
electrical stimulation, the inhibition theory would not apply.

Finally, I can't resist quoting the muscle physiologist Henk ter
Keurs, who once commented "it is good to be lazy and ambitious
at the same time, because it makes you efficient". Although he
intended this to apply to academic pursuits, it may just be true
for muscles as well.

Ton van den Bogert

--

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

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