Rehabilitating Low Back Disorders - Fundamentals and Assessment Tools for Occupational Ergonomics - page 460

robust spine models to document the ability of each component to stiffen and stabilize. Second,
electromyographic recordings of all muscles (even deep muscles requiring intramuscular electrodes)
are necessary to confirm the extent that the motor control system involves each muscle to ensure
sufficient stability. For some time our limited intramuscular EMG and modeling studies, and those
of others, suggested that virtually all torso muscles play a role in stabilization. (Our most recent
quantification breakthroughs appears at the end of this section.) However, while multifidus,
the other extensors, and the abdominal wall, have been highlighted before, the architecture of
quadratus lumborum (QL) suggests that it can be a stabilizer. This notion is further strengthened
by some earlier observation that the motor control system involves this muscle together with the
abdominal wall when stability is required in the absence of major moment demands. The fibers
of QL cross-link the vertebrae, they have a large lateral moment arm via the transverse process
attachments, and traverse to the rib cage and iliac crests. Thus, the quadratus could buttress
shear instability, and be effective in all loading modes, by design. Typically, the first mode of
buckling is lateral — the quadratus can play a significant role in local lateral buttressing.
Further, activation profiles support the notion of the stabilizing role of quadratus. It is active
during a variety of flexion dominant, extensor dominant and lateral bending tasks. Specifically,
Andersson et al. 4 found that the QL did not relax with the extensors during the flexion-relaxation
phenomonon. The flexion-relaxation phenomonon is an interesting task since there is no substan-
tial lateral or twisting torques and the extensor torque appears to be supported passively —
suggesting some stabilizing role for QL. Other very limited data suggest (our laboratory techniques
to obtain QL activation were rather imprecise at the time) that in an experiment where subjects
stood upright, but held buckets in either hand, where load was incrementally added to each
bucket, the QL appeared to increase its activation level (together with the obliques) as more
stability was required. This task forms a special situation since only compressive loading is
applied to the spine in the absence of any bending moments. The three layers of the abdominal
wall are also important for stability together with muscles, which attach directly to vertebra —
the multisegmented longissimus and iliocostalis and the unisegmental multifidii. Cholewicki 18 has
also presented an argument for the role of the small intertransversarii in producing small
but critical stabilizing forces. On the other hand, psoas activation appears to have little relationship
with low back demands — the motor control system activates it when hip flexor moment is
required (data is presented by Andersson et al. 3 and Juker et al. 23 ).
Most recently we have completed evolution of our model to quantify the role of individual muscles
to contribute to stability. Once again the conclusion is that all muscles are important and that the
most important muscle at any instant or task is a transient variable — they continually change their
relative contribution (see Figure 20.4 and Figure 20.5 for the ranking in a selected group of exercises).
So which are the wisest ways to challenge and train these identified stabilizers?
20.6 Training QL
Given the architectural and electromyographic evidence for QL as a spine stabilizer, the optimal
technique to maximize activation but minimize the spine load appears to be the side-bridge
(Figure 20.6) — beginners bridge from the knees while advanced bridges are from the feet.
When supported with the feet and elbow the lumbar compression is a modest 2500 N, but the
quadratus closest to the floor, appears to be active upto 50% of MVC (the obliques experience
similar challenge). Advanced technique to enhance the motor challenge is to roll from one elbow to
the other while abdominally bracing (Figure 20.7) rather than repeated “hiking” of the hips off the
floor into the bridge position. Higher levels of activation would be reached with the feet on a labile
surface. 35