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functions of the same muscle or muscle group simultaneously. If muscle activations patterns that
underlying the switching from one task demand to the other are different, this could create a
disruption of the ongoing task, and such conflict may lead to motor errors, increasing postural
instability and subsequent risk for development of LBD (Ebenbichler et al., 2001; Kollmitzer et al.,
2002; Oddsson et al., 1999).
1.2.
Low Back Anatomy
The trunk skeletal system is comprised of the pelvis, the vertebral column and the rib cage. The
vertebral column is composed of 24 vertebrae, the sacrum and the coccyx. Vertebrae are connected
to form the vertebral column by articulations that are formed by two adjacent vertebrae, which
are interconnected by ligaments and are separated by the intervertebral disk. The intervertebral
articulation is formed of two joints, one between vertebral bodies and the zygapophyseal joint.
The nature of the intervertebral disk allows each vertebra to move in 6 degrees-of-freedom (DOF),
which take place by compressing and stretching the disk. The zygapophyseal joints guide spine
movement in particular planes and prevent movements in other planes. The movement of the
vertebrae is coupled, and any spine movement combines the concurrent translation and rotation
of the vertebrae, that gives to the whole spine a great range of motion. Rib cage comprises the 12
thoracic vertebrae with the corresponding ribs and the sternum. Rib cage provides attachment
points for many of the muscles supporting the back. The sacrum articulates inferiorly with the
coccyx, laterally with the two ossa coxae (hip bones) and together these bones form the pelvis
(McKinley and O’loughlin, 2006).
The muscles of the low back can be divided into local and global systems according to their
architecture (Figs. 1.1 and 1.2). Local system comprises the deep muscles that act on lumbar spine
and deep portions of some muscles that have their attachment onto the lumbar vertebrae, either
the insertion or origin, or both. Global system comprises the large, superficial muscles that tranfer
load directly between the thorasic cage and the pelvis (Bergmark, 1989). The muscles of the local
system contribute to spinal mechanical stability by controlling muscle stiffness and spine curvature
(intersegmental motion). However, local muscles cannot control spinal orientation and thus local
system alone cannot stabilizes the spine system effectivelly (Hodges, 2004). The global muscle
system contribute to trunk movements and its mechanical role is to balance external loads applied
to trunk in order the residual force transfered to the lumbar spine can be handled by the local
system. Although global muscles can attenuate the force applied on the lumbar spine and control
trunk orientation, they cannot fine-tune control intervertebral motion (Hodges, 2004). Activation
patterns of local system muscles are mostly determined by the posture of the lumbar spine and
the magnitude of the external load, whereas activation patterns of global system muscles are also
dependent on the distribution of the external load (Bergmark, 1989). Regarding WRLBD, global
muscles control intervertebral motion only by augmented activations, resulting in co-contractions,
higher spine loads and reduced normal movement of the spine (Hodges, 2004). Therefore, the
activity of the global muscles was used as a measurable compensation for poor passive or active
(by local muscles) segmental support (Cholewicki, Panjabi, and Khachatryan, 1997).
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