The controversial issue of whether or not the upper and lower abdominal
muscles can be recruited preferentially often arises.
I fully concur with the thesis that the upper and lower abdominals cannot be
recruited in total isolation of one another. If the trunk is actively
flexed, then the entire abdominal musculature becomes involved. Following
the principle of "reductio ad absurdum", we would be forced to
conclude that
if one part of the abdominal musculature were to be entirely quiescent during
an extended period of active trunk flexion, then this would produce
instability somewhere in the trunk and increase the potential for injury.
The uninjured body does not behave like that.
However, there are other issues to consider if we extrapolate this deduction
to mean that the different parts of the abdominal musculature cannot carry
out functional activity to different degrees and in different patterns. In
this letter, I wish to highlight such issues.
Many of the deductions that the upper and lower abs are recruiited in exactly
the same manner and to the same degree are based upon surface EMG recordings.
Now, one problem with the interpretation of EMGs is that greater
potentials
measured over any region do not necessarily imply that the muscle locally or
over its entire volume is producing 'functional' motor output. Some of the
activity may be spurious (due to sub-optimal motor skill) or stabilising in
nature and not contributing solely to the dynamic action being studied.
For
example, it has been found that skilled performers recruit less muscle tissue
than less skilled individuals for executing the same activity.
Fatigue can also confound the results. In this case, a highly trained,
strong subject in executing a given abdominal exercise uses a far smaller
proportion of his maximal trunk flexion strength than a person who is less
strong, so that fatigue even in a single repetition plays a smaller role in
the production of EMG or trunk torque.
Thus, in using the EMG to analyse human movement, it can be very helpful to
concurrently measure joint torque to examine the nature of the relationship
between motor action and EMG from different locations over the abdomen during
exercises that take the trunk over its full range of movement (ROM). In
this
respect, crunches, V-sit-ups, and posterior pelvic tilts do not involve the
entire ROM from prestretch (i.e. about 10-15 degrees of spinal extension to
full contraction with the lumbar spine maximally flexed.
Lest these comments be regarded as somewhat academic, it is interesting to
take the analysis out of the laboratory into the folowing few practical and,
sometimes most unlikely, situations.
Case 1: Anyone who has watched high level bodybuilding contests will have
noticed that some contestants are able to tense, twist, contort and rotate
different parts of the abdomen.
Case 2: Several years ago, one of the TV learning or discovery channels
featured a belly dancer in a club who performed an amazing feat of abdominal
muscle control. She lay supine on a table with a four coins on either side
of her lower abdomen and she contracted her abs sequentially until she had
flipped the coins over until they reached the upper abdomen. She then
repeated the sequence from top to bottom and then left to right, right to
left and then in diagonal sequences. As a grand finale she called for a
dollar note to be placed upon the centre of her abdomen and she proceeded to
fold the note in two movements to a quarter of its size.
This act was witnessed by millions of TV viewers around the world, so it is
not simply anecdotal - if someone out there has kept a video copy or knows
where to obtain a video copy of this exceptional act, I would be most
grateful to know. It demonstrated to an even greater degree than the
bodybuilders' posing routines that it is possible to functionally utilise
different parts of the abdominal musculature.
Would anyone care to explain how this act is possible if there is no change
in EMG and much of the current theory seems to suggest that localised muscle
control like that is mythical?
This is certainly not to suggest that the average person executing all of
those gross (as opposed to fine motor control) abdominal exercises is capable
of voluntarily activating different parts of the abdominal musculature for
aesthetic 'body sculpting'. Rather, this example has been raised to
stimulate us to look deeper at the use of EMG alone to analyse muscle
activity insofar as it relates to functional motor activity.
Regarding
this, has anyone used EMG or myotensiometric biofeedback to examine to what
extent one can voluntarily alter activation of different parts of rectus
abdominis?
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FURTHER INFORMATION
The following article gives further unseful information on this topic. It
examined differences between upper and lower abdominal activation in skilled
and less skilled subjects. Interestingly, one conclusion was that,
among
incorrect performers of abdominal exercises, either exercise indistinctly
activates the muscle portions - and this is precisely the sort of finding
that is frequently reported by various researchers who are adamant that there
can be no differentiated mucsle activity over the length of rectus abdominis.
In other words, though various abdominal exercises may appear to be very
similar as performed by different individuals, the pattern of muscle and
recruitment and kinesiology may differ significantly. One again, we note
the
fundamental role played by the superordinate cortical and subcortical neural
patterns in determining motor output even in apparently rather gross
activities.
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Sarti M et al Muscle activity in upper and lower rectus abdominis
during
abdominal exercises. Arch Phys Med Rehabil 1996 Dec; 77(12):1293-7
OBJECTIVE: To compare the intensity of the upper versus lower rectus
abdominis (RA) muscle activity provoked by each of two different abdominal
exercises and to contrast the intensity of contraction elicited by two
different abdominal exercises on each RA muscle portion.
Subjects who had
practiced endurance or strength training activities (1.5 hours 3 days a
week
for 3 years) and those who had not accomplished that criterion comprised a
high and a low physical activity group, respectively. Each of these two
groups was divided by the ability to perform the exercises into two
subgroups: correct and incorrect performers (cp, ic).
Average surface
integrated EMG was compared between upper and lower RA and on each muscle
portion performing curl-up (CU) and posterior pelvic tilt (PT) exercises. The
coefficient of variation, a two-way analysis of variance, and the t test were
calculated.
RESULTS: The upper RA showed significantly greater activity during
performance of CU exercise by the cp subgroups of both high and low
activity
groups. Only the cp subgroup of the high activity group showed that PT was
significantly more strenuous than CU exercise on lower RA.
CONCLUSIONS: Among correct performers, CU produces greater activity on upper
RA. For persons who have a high level of activity, PT is more strenuous than
CU on lower RA. Among incorrect performers, either exercise indistinctly
activates the muscle portions.
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Dr Mel C Siff
Denver, USA
mcsiff@aol.com