View Full Version : muscle fibre length changes in response to exercise

Susannah Thorpe
10-25-1998, 02:29 AM
Many thanks to all who responded to my query on the
response of muscle fascicle length to concentric and
eccentric exercise. A number of people have expressed an
interest in the results so I have posted the responses

Susannah Thorpe.

I'm not sure whether there is any direct research on this issue, but my
prediction would be that any changes in muscle fiber length would depend on
the length range over which exercise took place, not whether the exercise
was concentric or eccentric (although this may have a small effect).
Research has shown that fixing a joint in an extended or flexed position
that lengthens some muscles and shortens others causes the muscle fibers to
lengthen/shorten to establish a new average sarcomere length such that
active force production is maximal at the maintained joint position (for a
review, see the book by R. Lieber - "Skeletal Muscle Structure and Function".

Warren Darling

Hi Sue,

During the World Congress of Biomechanics held in Sapporo
last August, Dr. David Morgan (Monash University,
Australia) showed that after strenuous eccentric exercise
the shape of muscle force/length relationship changes.
That could be a clue in favour of muscle architectural changes.
Best regards.


Alberto Minetti


Dear Susannah,

Useful references.

Lynn, R. & D. L. Morgan. 1994. Decline running produces more sarcomeres in
rat vastus intermedius muscle fibers than does incline running. Journal of
Applied Physiology 77, 1439-1444.

Koh, T. J. & W. Herzog. 1998. Eccentric training does not increase
sarcomere number in rabbit dorsiflexor muscles. Journal of Biomechanics
31, 499-501.

Best regards,

Mark Willems


Dear Susannah --

There are two studies to my knowledge that have been performed in this
area. The first, by Lynn and Morgan (1994) J Appl Physiol 77, 1439-1444,
showed a larger fiber length (as measured by the number of sarcomeres in
series) in downhill versus uphill trained (daily for 1 week) rats. This
appeared to be a result of sarcomere addition in downhill trained rats, and
the authors interpreted this to be a response to eccentric contractions.

Based on this study, Walter Herzog and I measured serial sarcomere number
in eccentrically trained dorsiflexor muscles of the rabbit (Koh and Herzog
(1998) J Biomechanics 31, 499-501). Training was performed (2 times/week,
12 weeks) on anesthetized animals, with muscle contractions controlled by
an electrical stimulator and a motor driven footplate. We found that
eccentric training had little or no effect on sarcomere number (or fiber

The differences between studies could be related to species differences, or
mode of exercise differences. Clearly, more work needs to be done in this

I would be happy to discuss this topic further.

Tim Koh.

Great question Susannah.

As you probably know, there is good evidence from animal studies that
muscles adapt sarcomere number and rest length in response to altered
patterns of use. Most of this is from immobilisation studies, but there are
also a number of studies that look at changes that occur without
immobilisation (see, for example, old study by Supinski in AM J Physiol on
changes in diaphragm brought on by contraction at short lengths with
emphysema). Perhaps of most interest is a recent series of studies by David
Morgan and colleagues (eg, he is second author on a very recent paper in
Journal of Applied Physiology) that have looked at length-tension changes
and sarcomere number number changes produced by eccentric exercise. Morgan's
hypothesis is that sarcomere addition will occur in response to eccentric
exercise to reduce damage caused by instability attributable to sarcomere
length inhomogeneities.

All these studies show that altered patterns of use can produce sarcomere
number changes, but most do not distinguish between whether the sarcomere
number changes are produced by the altered *length* at which the muscle is
made to contract, or whether the adaptation is because the muscle is made to
do lots of contracting concentrically or eccentrically. A lovely study in
the very latest Journal of Biomechanics by Koh and Walter
Herzog and colleague looked at adaptation of sarcomere
number in response to eccentric training at controlled
lengths and found, contrary to Morgan's hypothesis, no
(detectable) change in sarcomere number.

One caution when looking at this literature. You need to be clear by what
you mean when you refer to the "length" of a fascicle, because "rest" length
(or "slack" length) may change independently of optimal length (I have
briefly discussed this in a recent paper I wrote with Jack Crosbie in the
1997 European Journal of Applied Physiology)

Hope this is of some help.

Rob Herbert

I only have one good article on this although there is plenty of info on
sarcomere adaptation to immobilisation at lengths greater or lesser than
rest length. If you want them, let me know.

Decline running produces more sarcomeres in rat vastus intermedius muscle
fibers than does incline running.
R Lynn and D L Morgan. Journal of Applied Physiology 77(3): 1439-1444, 1994.

If you check out DL Morgans page at the Monash University web site you'll
see all the articles the research team has published. It is probably worth
a look.

Hope this helps. I'm interested in your results.

Tony Blazevich.

Dear Dr. Thorpe,
as far as I am aware it is the cross sectional area of
muscle (of all fibre types) that increase with exercise, i.e. it is
the number of fibres that increase and not their length. Also
increased is the number of motor units and their size (see Astrand.
Textbook of Work Physiology 1988.). Decrease in size would only occur
with detraining (atrophy).

However, there are changes in the length of series elastic components
(tendons, ligaments.) of the musculature. This can be demonstrated by
simple flexibility tests such as sit and reach tests. In fact laxity
of joints can occur as a result of certain exercises which overstretch
the connective tissue e.g. deep knee bends (and injury of course).

As I am not a Biomechanist though, I am not completely familiar with
all the literature of the last few years so I don't know if Astrand's
description still holds. During my Master's course in biomechanics
there was no mention of changes in fibre length.

Putting some keywords through databases such as Sportsdiscus and
Medline will probably give you the best results. One very useful
keyword is SSC (Stretch-Shortening Cycle) which is the acronym for
eccentric muscle action in the biomechanics/exercise physiology

I hope this is of some help and if you have, or do find, evidence of
changes in fibre length I would be grateful for the references

Ian Rogers M.Med.Sci.



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