View Full Version : SUMMARY - physiological vs. NMES contractions

Arnel Aguinaldo
12-06-1999, 11:36 AM
Since there was such an overwhelming response to my original posting, I had
to summarize all the points made by my respondents instead of posting their
original replies. Below you will find the original posting followed by a
respondent summary. Thank you to those who gave their insights on this
subject matter.

Arnel Aguinaldo, MA
Biomechanical Engineer
San Diego, CA
email: ala@znet.com


I'm currently conducting research on the use of neuromuscular electrical
stimulation (NMES) for therapeutic applications, and I would like to get
feedback on the specific effects of NMES on muscle activity.

Although the effects of NMES have been investigated numerous times in past
studies, there seems to be conflicting views on the overall effectiveness
of NMES as compared to voluntary exercise. During a physiological
contraction, motor units are recruited according to the size principle.
Slow (S), Fast-Fatigue-Resistant (FR), and Fast-Fatigue (FF) units are
recruited respectively as voluntary force increases. During a NMES induced
contraction, however, it is unclear which units are actually being
recruited. Further, the same motor units respond each time NMES is applied.
There does not seem to be a clear explanation to why these responses happen
during NMES so I pose the following questions to the group:

1) Do NMES induced contractions follow the size principle?
2) If not, which motor units are primarily recruited when NMES is applied
and why?
3) If the same units are activated with each stimulation, would muscle
fatigue be more prevalent than it would with physiological contractions?

These questions are important to answer because it would explain how NMES
could be optimized to control the tension levels associated with its
application. A muscle will respond according to the stress imposed upon it.
Thus if NMES can produce higher tensions, then it would result in more
effective strengthening. In a study conducted by Lieber et al (1992), it
was found that muscle strengthening with NMES is just "as effective as
voluntary exercise, provided the tension levels are equivalent." It is
obvious, therefore, that NMES needs to be applied at conditions appropriate
to reach tension levels comparable to voluntary exercise. However, a better
understanding of the physiological responses to NMES is needed in order to
optimize these conditions.

Any feedback on this subject will be greatly appreciated. Thank you.


1) There was a general agreement that motor unit recruitment during NMES
does not follow the size principle observed during volitional contractions.
Some respondents have suggested that due to the low depolarization
threshold of large FF and FR MUs, NMES recruitment works on the reverse
size principle.

However, there is also evidence that indicates that this reversal is not
always the case. In a study conducted by Binder-Macleod et al. (Med Sci
Sports Exerc, 27:4, 556-565, 1995), the rectus-femoris was stimulated at
three intensities (20, 50, & 80 %MVC) and at three frequencies (20, 40, &
60 Hz). They found that although fatigue increased with increasing
frequency, the differences in the amount of fatigue produced at each
intensity did not vary between frequencies. If the reverse size principle
was truly followed during NMES, then fatigue would decrease with increasing
intensity because FF & FR MUs are recruited at lower intensities while the
slower fatigable MUs are recruited with higher ones. Their conclusion was
that recruitment during NMES was random and less orderly. If you would like
more information on these specific results, I would suggest you contact
David Russ (druss@udel.edu) from the University of Delaware, who can
elaborate more on this study.

The following references were also suggested for further reading on NMES MU

Knaflitz et al. (1990) J Applied Physiology, 68:4, 1657-1667
Prochazka, A (1993) IEEE Transaction on Rehabilitation Engineering, 1:1, 7-16
Sinacore et al. (1990) Physical Therapy, 70, 416
Trimble and Enoka (1991) Physical Therapy, 71:4, 273-282

2) Regarding muscle fatigue, it was the consensus of the respondents that
fatigue is more rapid with NMES than it is with volitional contractions.
Some have suggested that this is because MU recruitment is reversed (large
MUs serving fast-fatigable fibers are activated first) with NMES. However,
as stated earlier, this concept is still under debate. Another reason for
rapid fatigue is because the same MUs are synchronously stimulated each
time NMES is applied, and in order to maintain smooth tetanic tensions,
stimulation can only be done at higher frequencies. In other words, there
is no selective activation of MUs that would allow stimulated MUs to rest
while others are depolarizing, and there's insufficient time for MUs to
recover at higher stimulation rates. Such asynchronous activation is only
observed with volitional contractions. Hence, fatigue sets in more rapidly.


Once again, I would like to thank all those who have responded. I have
honored the wishes of those respondents who've requested anonymity by
excluding their names from this list:

Beverly Burke

Warren G. Darling
University of Iowa

A.D. Pandyan
University of Newcastle upon Tyne

William S. Quillen, PhD, PT, SCS
Indiana University School of Medicine

Joanne Riess, MS
University of Kentucky

David Russ, MPT
University of Delaware

Chad Starkey, PhD, ATC

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