The greatest loading moment of force at the lumbar intervertebral
joints occurs in the initial portion of a lift,21
when
the body must overcome the inertial forces necessary to
complete the lift successfully. The discussion, therefore, will
focus on this segment of the lift, when the muscle strategy
may be most important in protecting the lumbar spine. As
discussed earlier, contraction of the back extensor musculature
is important in overcoming the spinal bending moment
resulting from the load; however, its action will generate
undesirable compressive forces at the lumbar intervertebral
joints.16
Whereas the spine can withstand a considerable
degree of compressive forces, the anterior shear forces that
are generated by the upper body and the load cannot be
tolerated readily. Back extensor muscle contraction may have
a role in offsetting these damaging anterior shear forces.17
Abdominal muscle contraction may help support the spine
during lifting by both increasing the intra-abdominal pressure
and tensing the thoracolumbar fascia through the attachments
of the internal oblique and transversus abdominis muscles.12,18
As demonstrated in this study, the ES muscle activity was
greater during the crucial initial period in the BBI lift as
opposed to the BBO lift, and the OA muscle activity was
greater during the crucial initial period in both styles of lift.
The muscle recruitment pattern that occurs in the BBI styl