The back extensors have several important roles Longissimus, lliocostalis, and Multifidus Croups

Most anatomy books address the extensors in three groups although athletes are probably better served considering the thoracic group as one group and the lumbar portions as another group. The lumbar and thoracic portions are architecturally (Bogduk, 1980) and functionally different (McGill and Norman, 1987). First, in terms of fiber type, the thoracic sections contain approximately 75% slow-twitch fibers while lumbar sections are generally evenly mixed (Sirca and Kostevc, 1985). The thoracic components of longissimus and iliocostalis follow a line of action parallel to the compressive axis of the spine and just underneath the skin. This provides the greatest amount of extensor moment with a minimum of compressive penalty to the spine (see figure. 3.7). This means that these muscles extend the entire thoracic and lumbar spine. A wise lifter uses them - they would never isolate the lumbar muscles. The great ones have visible bulk in the extensors of the thoracic area (see figure 3.8).

Figure 3.8 This world-class power lifter exemplifies the hypertrophied bulk of the iliocostalis and longissimus muscles seen in trained lifters. While this muscle bulk is in the thoracic region, it creates extensor torque over the entire thoracic and lumbar spine.

B Figure 3.7 Longissimus thoracis pars

B thoracis (inserting on the ribs atT6) with

B its tendons lifted by probes course over

B the full lumbar spine to its sacral origins.

Fl They have a very large extensor

Fc ¦¦ mm

B moment arm (just underneath the skin)

B to provide the maximum mechanical

B advantage.

The lumbar components of these muscles (iliocostalis lumborum pars lumborum and longissimus thoracis pars lumborum) are very different from the thoracic portions and are special in their anatomy and function. Their line of action is not parallel to the compressive axis of the spine, but rather has a posterior shear component (see figure 3.9). These posterior shear forces support any anterior reaction shear forces of the upper vertebrae produced as the upper body is flexed forward in a typical lifting posture. This is critical for injury avoidance. This important line of action is modulated by posture. These muscles lose their shearing line of action and become more aligned to the compressive axis of the spine with lumbar flexion (McGill et al., 2000). In this way, lifters with a flexed spine are unable to resist damaging shear forces (see figure 3.10, a and b).

Figure 3.9 Iliocostalis lumborum pars lumborum and longissimus thoracis pars lumborum originate over the posterior surface of the sacrum, follow a line of action under the skin (muscle force (MF)) and then turn obliquely toward their vertebral attachments. This force pathway supports anterior shear (S) forces and extensor moments on each successive superior vertebrae. The compressive axis (C) is indicated.

Figure 3.10 Posture determines the ability of the iliocostalis lumborum and longissimus thoracis to protect the spine against large anterior shear forces. When the spine is neutral the oblique angle of these muscles as viewed with an ultrasound imager is about 45 degrees (a). This means that about 70% of the force helps to support shear. When the spine is flexed this angle is reduced to about 10 degrees so that anterior shear forces cannot be supported (b). Thus the load bearing ability of the spine in shear is dependant upon posture - for many athletes a neutral spine Is essential. Reprinted with permission Clinical. Biomechanics, McGill, S. M., Hughson, R. L., and Parks, K., 15 (1): 777-780, 2000.

The multifidus muscles span two or three segments (see figure 3.11) and perform a different function from those of the longissimus and iliocostalis groups. These shorter muscles tend to run parallel to the compressive axis. The multifidus muscles are involved in producing extensor torque (together with very small amounts of twisting and side-bending torque), but only provide the ability for corrections, or moment support, at specific joints that may be the foci of stresses. Being short, they have a different force-stiffness profile than their longer, more lateral, extensors which will come into play in the stability discussions. Interestingly, many

People discuss multifidus when they actually mean the full erector spinae group. Further, we have not seen evidence for the ability of people to activate just multifidus. All muscles of the extensor group appear to activate together. Some therapy groups advocate trying to activate multifidus - in essence activating the extensors from medial to lateral in order. We do not think that this is possible. It is wiser to think about training the extensors up and down the spine (lumbar and thoracic), rather than from medial to lateral - or multifidus, then longissimus and iliocostalis. As will be shown in the third section of this book, this is how some of the strongest backs train.

Figure 3.11 Multifidus is a relatively small lumbar extensor with shorter fibres that can span 1 to 3 vertebral segments. Its line of action does not support anterior shear of the superior vertebrae but actually contributes to it. Right panel courtesy of Primal Pictures.

Figure 3.12 Latissimus dorsi originates from each lumbar spinous process via the lumbodorsal fascia and inserts on the humerus to perform both lumbar extension and stabilization roles. It is a key muscle for strength enhancing "superstiffness". It also helps in building the torso "corset". Image courtesy of Primal Pictures.

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