Muscle Contraction or Action? By Mel Siff
Here we are witnessing a discussion bwteen Mel Siff and another distinguished member on the Supertraining Forums
Member<< I might take issue with some of this and ask for greater clarification.
While it is true that for general purposes a muscle is “measured relative to
its resting, unactivated length”, the relationship between lengthening and
shortening is relative to the extremes of the specific action being examined
and would seem to have nothing to do with “resting, unactivated length”….
While it is true, that the “attempt” to contract against opposing force
provides tension to the muscle in all three muscle action/contractions, true
contraction “only” occurs when the filaments actually “slide”, providing
a “shortening”. So even though we might be able to loosely term the act of
tensioning, “contracting”, we would be acknowledging that the “attempt to
contract” is “understood”. >>
Mel Siff:
***I was clarifying the definitions and analysis of muscle action according
to what is accepted in standard high level texts and was not trying to create
a new body of knowledge, because literature that I came across did not
reflect an adequate degree of accuracy on the topic. You have offered
comments which apply to muscles in different states of excitation, including
altered cognitive states, which intentionally have been excluded from the
accepted definitions because they are acknowledged not to be true resting
states. It would also help if you provided some references to support any
opinions, so that we don’t simply end up having a jolly good rumble in the
playground of speculation.
Member:
<<It is also true that due to the elastic nature of the “muscle complex” that
it is possible to have a small amount of “shortening” during an isometric or
eccentric action, this action is limited to the elastic properties and “is
not” representitive of the total action in these cases.>>
Mel Siff:
***This is not exactly correct, since the sliding filament model of muscle
and photomicrographs of muscle action show clearly that the actin-myosin
units shorten and thereby tension the other non-contractile elements in the
muscle complex. Also, there are elastic and viscoelastic components in the
muscle complex, so that the issue is not one of simple elastic extension.
Member:
<<I think we more accurately would say during a “forced lengthening” eccentric
action, the muscle “attempts” to contract. >>
Mel Siff:
***No, whether the action is “forced” by a heavy load or allowed to happen
voluntarily when a person lowers a light load, the muscle will contract. It
does not “attempt” to contract – it contracts. Muscle contraction is not a
matter of ‘half-hearted’, semi-committed action, but the result of all or
nothing excitation of muscle fibres. If there are any “attempts” to initiate
a motor action, then this process happens at a neural level, not at a
muscular level.
Member:
<<I am pressed to see that a true contraction (shortening) can happen under
these conditions. Even though, as I have stated, a muscle has a degree of
elasticity that may allow a small amount of actual contracting (within the
limits of structural integrity), I think somehow we are confusing the
“attempt to contract” with the actual “act of contracting”. The resistive
action in the muscle to a “forced lengthening” is not called an eccentric
contraction. It is called an eccentric action (or attempt to contract
against an active/superior force)>>
Member:
*** How can elasticity allow for contraction? Elasticity is always
associated with lengthening in extensible tissues such as the muscles and
elastic bands. This elasticity and possibly some ’sliding’ within the muscle
complex contribute to the electromechanical delay associated with the
activation of a muscle from its relaxed to its contracting state.
You will notice that I referred to eccentric “action” throughout my post, so
that this comment has no bearing on what I wrote. I take great care not to
confuse action with contraction, as do any biomechanists working in the same
field. As noted above, the concept of “attempting” to contract is not a
local muscular process or even one of spinal motor reflex action.
In one of my recent posts I even mentioned that the resting length in some
cases (as in individuals suffering measurement of muscle length is a relative
one. I simply emphasized that there happens to be a well-accepted definition
of resting (unexcited) muscle length in every individual.
Member:
<<Biomechanically, the two actions (concentric-eccentric) are distinct and
different. You might simply say that one acts as a brake and the other acts
as a motor.>>
Mel Siff:
***Interestingly, though research has shown that isometric action is
controlled by different brain mechanisms from dynamic action, no such
difference has been found between concentric and eccentric muscle action (we
discuss this point in Ch 1 of “Supertraining”). During all forms of JOINT
action, the underlying process of muscle CONTRACTION is the same, though
there are differences in the utilisation of deformable passive tissues and
the various reflexes.
I can anticipate your possibly detouring into some lengthy semantic arguments
about what we are discussing, and that will simply induce me to summarise
even more information on this topic from some very competent authorities.
So, to fill the gaps in your interpretation of muscle structure and function,
first please read the summaries of current muscle research such as that in
“Supertraining” (1999, pages 38-39) and go to the references cited. Other
relevant texts are:
Fung Y Biomechanics: Mechanical Properties of Living Tissue 1981
Frankel V & Nordin M Basic Biomechanics of the Skeletal System 1980
Scientific American has also featured more recent work on this topic, plus a
Medline search will also yield a huge amount of useful information.
Mel Siff
Denver, USA
Mel Siff Dot Com
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