bed rest on the truck muscles. This study showed that the
cross-sectional area of certain muscles decreased or were
unaffected by bed rest as one would imagine but surprisingly
found that the psoas major and rectus abdominis actually
increased in cross-sectional area. The authors
attributed this increase or hypertrophy to increases in muscle
tone and to the possibility that the subjects maintained
a flexed truck position during bed rest, resulting in a psoas
muscle shortening.
More recently Dickx et al.32 used muscle functional
magnetic resonance imaging (mfMRI) to evaluate changes
in lumbar muscle activity with induced muscle pain.
This study was one of the first to examine patients with
acute low back pain and how it affects activity of the
trunk musculature. mfMRI was obtained under three different
conditions: a resting MRI was obtained after the
subjects laid supine for 30 minutes; an MRI was obtained
after trunk extension at 40% of one-repetition maximum
without pain; and an MRI was obtained after the subjects
were injected with hypertonic salt into the right longissimus
muscle to induce pain and then subjects were required
to again perform the back extension exercise
while experiencing low back muscle pain. There were no
significant changes in the psoas major muscle recruitment
between resting and exercise leading the authors to
conclude that the psoas major was not significantly recruited
during trunk extension exercises. During the
trunk extension exercises with pain induced, the authors
reported that there was a statistically significant reduction
in the psoas major activity bilaterally and at multiple
levels whereas previous studies found it to be ipsilateral
and on the symptomatic side.28,30