The Spine 107is described as the pressure that can be generated if
the muscle walls around the abdominal cavity are con-
tracted (abdominal muscles, diaphragm, pelvic floor
muscles). The function of IAP was originally described
by Bartelink (1957) as that of acting like a fluid ball
that resists deformation as soon as the pressure within
is raised as seen in figure 3.31A. This was theorized to
provide an additional route to help transfer forces from
the torso to the pelvis such that an unloading effect
estimated to be as high as 30% to 50% is offered to the
spine (Morris, Lucas, and Bresler, 1961).
Although this original model has been challenged
by recent studies, there is still support for the concept
that the abdominal muscles play a role in protecting
the spine. One alternate explanation is that the con-
traction of the abdominal muscles (particularly the
transverse abdominis) reduces shear forces by push-
ing back against the front of the lower spine, resisting
lumbar hyperextension and anterior sliding of the
vertebrae as seen in figure 3.31B. Another theory is
that contraction of the abdominal muscles may help
increase stiffness of the trunk so that the spine does
not buckle when loads are applied (Hall, 1999). A
related theory is that contraction of the abdomi-
nal muscles (specifically the transverse abdominis
and internal oblique abdominal muscles) and the
resultant IAP act by tensioning the thoracolumbar
fascia as seen in figure 3.31C. The thoracolumbar
fascia is a complex structure that can be divided into
three layers (Smith, Weiss, and Lehmkuhl, 1996).
It has connections to the ribs, vertebrae, sacrum,
select posterior ligaments of the spine, and various
muscles including the transverse abdominis, internal
obliques, erector spinae, multifidus, quadratus lum-
borum, latissimus dorsi, and gluteus maximus. Due to
its posterior attachments onto the spine, this thoraco-
lumbar fascia tensioning theory holds that when the
thoracolumbar fascia is tightened it tends to produce
extension that could assist in movements such as lift-
ing, or at least in stabilizing the spine (Richardson,
Hodges, and Hides, 2004), thus decreasing the load
on the lumbar spine (Levangie and Norkin, 2001).
Still another theory (muscle fusion) holds that con-
traction of the abdominal muscles has the opposite
effect, that is, that it tends to pull the lumbodorsal
fascia laterally and to lessen lumbar lordosis, reduc-
ing shear and allowing more efficient support of the
spine (Plowman, 1992; Saal, 1988b).
Whether IAP, shear reduction, thoracolumbar
tensioning, or a combination of these mechanisms
is operative, strengthening the abdominals and
focusing on using them prior to rigorous move-
ments (such as lifting a partner) should theoretically
increase their protective effect for the spine. Studies
of IAP have shown the amount of IAP that can be
generated is much greater in athletic individuals and
elite weightlifters than in slight, nonathletic individu-
als (Bartelink, 1957; Eie and Wehn, 1962; Grieve,
1978). Studies suggest that the transverse abdominis
most importantly, and obliques secondarily, are vital
for the generation of IAP (Bartelink, 1957; De Troyer
et al., 1990; Grillner, Nilsson, and Thorstensson,
1978; Kumar and Davis, 1978). It appears that the
rectus abdominis does not play much of a role inFIGURE 3.31 Potential roles of abdominal muscles in protecting the low back. (A) Original intra-abdominal pressure
(IAP) theory, (B) shear reduction theory, (C) thoracolumbar tensioning theory.