Af. Mannion et al., THE INFLUENCE OF MUSCLE-FIBER SIZE AND TYPE DISTRIBUTION ON ELECTROMYOGRAPHIC MEASURES OF BACK MUSCLE FATIGABILITY, Spine (Philadelphia, Pa. 1976), 23(5), 1998, pp. 576-584
Study Design. This was a cross-sectional study carried out on a group
of 31 healthy, consenting volunteers with no history of low back pain
(17 men, 14 women). Objectives. To evaluate the relationship between e
lectromyographic measures of erector spinae fatigability and the muscl
e's fiber type characteristics. Summary of Background Data. Using elec
tromyographic techniques, a pronounced fatigability of the muscles of
patients with low back pain has been identified. It has been postulate
d that this is the result of an unfavorable back muscle fiber type dis
tribution, although an association between electromyographic measures
of fatigue and the muscle's fiber type characteristics has never been
established. Methods. Two tests of back extensor fatigability were per
formed (on separate days), each to the limit of endurance: 1) maintena
nce of 60% total maximum voluntary contraction of the back extensors,
and 2) performance of the Biering-Sorensen test. Pairs of surface elec
trodes were attached to the skin overlying the belly of the erector sp
inae, bilaterally, at T10 and L3. The median frequency was computed fo
rm the electromyographic power spectrum, and fatigability was given by
the slope of the linear regression of median frequency on time (MFgra
d; %.s(-1)). One week later, two percutaneous erector spinae muscle bi
opsy samples were obtained from the same sites described for electromy
ography (left side only). Samples were prepared for histochemistry for
the identification of muscle fiber types. Fiber sizes (cross-sectiona
l areas) were quantified using computerized image analysis. Results. T
he mean fiber size at each erector spinae region showed a significant
correlation with maximum back extensor strength. In the thoracic regio
n, the relative area of the muscle occupied by Type I fibers (which ac
counts for the relative size and distribution of the fiber types) show
ed a significant relationship with MF-grad recorded during each fatigu
e test. A similar relationship was observed for the lumbar region, but
for the Biering-Sorensen test only. Conclusions. The electromyographi
c changes recorded in back muscles during fatigue appear to be related
to the underlying muscle fiber type area distribution. This confirms
the usefulness of electromyography in reflecting such muscle character
istics in a noninvasive manner, when monitoring changes in function co
nsequent to the development of, or rehabilitation from, low back pain.