Transversus Abdominis & Core Stability

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Transversus abdominis and core stability: has the pendulum swung?

British Journal of Sports Medicine 2008;42:630- 631

In the past decade there has been a focus on isolated transversus
abdominis activation and how it contributes to lumbo-pelvic
stability. This rationale has not only influenced the management of
chronic low back pain (LBP); it has also been included in exercises
for many other pathologies of the lower and upper limb and also for
prophylaxis in pain-free subjects.

The rationale that the feedforward bilateral muscle activation of the
transversus abdominis stabilises the segmental lumbar spine is based
on the reports that, unlike other trunk muscles, transversus
abdominis is activated independently of the direction of any spinal
perturbation. 1 2 This finding implies that it plays an important role
in spinal stability. The finding that individuals with low back pain
or normal subjects with anxiety and stress appear to have altered
timing of feedforward onsets of transversus abdominis reinforces the
case for the presence of a motor control dysfunction. 3–6 It is often
inferred that such a dysfunctional pattern corresponds to less than
optimal core stability.

Such an inference – that altered timing of the transversus abdominis
leads to poor core stability – is popular in the literature but on
further inspection fundamental evidence is lacking.
Firstly, the literature very quickly generalises the research
findings of unilateral transversus abdominis activation to a
bilateral pattern. In rapid unilateral arm raising (the preferred
research model) this assumption is not valid. The contra lateral side
preactivates the deltoid but in most normal controls the ipsilateral
side is significantly lagging.1 This critical finding suggests that,
although some studies show that bilateral activation of transversus
abdominis is able to stiffen the spine,7 such findings do not
correspond to the unilateral arm-raising task. That is, although they
may provide some evidence that the bilateral activation of the
transversus abdominis provides some degree of spinal stiffening
(albeit mostly in flexion), bilateral feedforward activation of
transversus abdominis is not the normal activation pattern for
unilateral arm raising.1 2

Secondly, when arm flexion is performed using alternate arms, the
transversus abdominis (left and right) are clearly directionally
specific.1 8 The previous reports that transversus abdominis (left)
is not directionally specific refers to the comparison of flexion and
extension of the same (right) arm. This may reflect the difference in
the strategy of arm movement, not the actual perturbing force acting
on the spine due to the different directions of arm movement. The
feedforward activation of the transversus abdominis on the
contralateral side to arm movement is related to the rotatory torque
acting on the spine and the degree of asymmetry between sides is
related to the magnitude of this torque.2

As the name infers, transversus abdominis holds true to the concept
of form and function since it is most sensitive to torques parallel
to the muscle fibres. Transversus abdominis shows directional
specificity based on the direction of the perturbation, 1 2 and under
certain types of movement it is likely to be synergistically active
with other leg and trunk muscles in a diagonal rather than the corset
action.1 2 Bilateral arm raising that generates a sagittal plane
torque (ie, no significant rotatory torque) makes the transversus
abdominis activation more symmetrical (corset-like) but also delays
the activation.2 Hodges et al9 demonstrated that three of eight
normal pain-free control subjects did not have feedforward responses
in 70% of trials during bilateral arm raising. We propose that this
is not due to "less than optimal stability" but rather a normal
variation of motor control related to the lack of trunk rotation
perturbation. Delayed activation of transversus abdominis in patients
with low back pain may be more related to the lack of trunk rotation
used in the arm raise by these subjects than to specific motor
control problems with transversus abdominis. The activation pattern
and onsets of this muscle just may be a better marker of this change
in movement strategy than other trunk muscles. The isolated bilateral
transversus abdominis activation training strategy, if it does
provide a mechanical stiffness of the spine in pathological
populations, is therefore more likely to be a compensatory control
strategy than a correction of normal patterns of activation. This
compensatory training strategy may be a cortical process to normalise
movement control. This then re-establishes a normal asymmetrical
transversus abdominis action during rotation tasks within a complex
muscle synergy rather than correcting a single dysfunctional muscle.

It follows that, although bilateral transversus abdominis isolation
has demonstrated some clinical utility, the assumption that it plays
a significant and direct mechanical role in stability of the spine is
unclear. Furthermore, the bilateral feedforward response is not a
normal pattern in predictable rotation perturbations. The idea that
this isolated muscle pattern should be taught prophylactically in
normal pain-free athletes is at best controversial. 10 Whatever the
clinical utility of the intervention, the mechanistic rationale
cannot be based on the presumption that the directional invariant
bilateral feedforward response of transversus abdominis is acting as
a corset stabiliser and is the normal pattern for all spinal
perturbations. Similarly, care has to be taken in the interpretation
that all other activation patterns represent motor control
dysfunction and that this can be translated into a mechanical
inference that these individuals have less than optimal core
stability. The evidence is just not there.