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Young-hoo Kwon, Ph.d.
09-18-1998, 02:19 AM
Dear colleagues:

According to the textbook I am using for my undergraduate Kinesiology
course, 'antagnist' is defined as "the role played by a muscle generating
torque opposing that generated by the agonists at a joint". Then it goes,
"When a muscle opposes a movement at a joint through development of
eccentric tension, it is acting as an antagonist." I witnessed similar
statements in some other textbooks I happened to possess. But I believe
there is a discrepancy between these two statements.

The definition implies that the antagonists opposes the agonists for fine
control of the movement and safety. When the agonists produces too much
tension or when the joint angle reaches the extreme of the joint range of
motion, the antagonists will produce tensions to control or slow down the
joint motion for fine control and safety since muscles produce tension only.
When the agonists contract concentrically, it is obvious that the
antagonists should contract eccentrically to slow down the joint motion. So
both statements above seem to agree. But what about the eccentric
contraction of muscles due to external load such as gravity?

Let's imagine some one is doing pushups. The triceps contract concentrically
during the up-phase, but eccentrically during the down-phase. According to
the second statement above, the triceps are the agonists during the
up-phase, but the antagonists during the down-phase. Which group of muscles
are the agonists in the down-phase then? Since the elbow flexion during the
down-phase is basically caused by the gravity, the elbow flexors can not be
the agonists. So the definition does not hold here.

Or can we view the triceps as the agonists in both phases? The main job of
the muscles which the man intended during the down-phase was to slow down
the elbow flexion against the torque produced by gravity. Triceps are
undertaking this job. When triceps produce too much tension causing too slow
an elbow flexion, the elbow flexors can compensate that as antagonists.
According to the same author, agonist is defined as "a role played by a
muscle acting to cause a movement". The triceps are causing 'slow-down' of
the elbow flexion during the down-phase of pushup. Whthout the activation of
the triceps, he will simply collapse. If we stick to the definition, the
second statement is incorrect.

I'd like to get some comments or feedback on this issue from the readership.
Anything helpful to clarify this issue will be welcome. I'll post the
summary later. Thanks!

Young-Hoo Kwon
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- Young-Hoo Kwon, Ph.D.
- Biomechanics Lab, PL 202
- Ball State University
-
- Phone: +1 (765) 285-5126
- Fax: +1 (765) 285-9066
- E-mail: ykwon@cs.bsu.edu
- Homepage: http://www.cs.bsu.edu/~ykwon/
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