This is an excerpt from Pilates-3rd Edition by Rael Isacowitz.
The term trunk stabilization describes a stable position of the spine and in this context is synonymous with spinal stabilization, core stabilization, torso stabilization, and lumbopelvic stabilization. It is important to recognize, as noted earlier, that stabilization is dynamic and that the stabilizers are continuously adapting to a changing environment.
I typically use the term trunk stabilization when the trunk is stable and the upper girdle is being mobilized, because of the close relationship between the two regions. The movement of the arms has a direct influence on the thoracic spine and, conversely, certain muscles of the thoracic region play an important role in correct mechanics of the shoulder girdle. Furthermore, achieving good head alignment and shoulder function is impossible without correct alignment of the trunk. Similarly, I use the term lumbopelvic stabilization when the lower limbs are being mobilized due to the close relationship between the lower limbs and the pelvis. If the pelvis is out of alignment or unstable, it will directly affect the function of the limbs as well as the alignment and function of the spine. Essentially, the pelvis acts as a bridge between the lower limbs and the spine. The position of the spine and the direction of the pull of gravity or resistance determine which muscles are recruited and to what extent.
Maintaining a stable position of the trunk and pelvis is fundamental to the success of many Pilates exercises. In order to achieve trunk stabilization, the ISS must be recruited. Depending on the position of the trunk—flexion, neutral, or extension—certain muscles are activated more than others. It is the intricate interplay between the agonists and antagonists that create and maintain the position (shape). For instance, in Mat: Rolling Like a Ball, the abdominals draw the spine into forward flexion; however, the back extensors play an important role in moderating the degree of flexion and creating the gradual curve of the back. In Mat: Front Support, the abdominals, together with the hip flexors, play a vital role in stabilizing the trunk and avoiding collapse through the center. In the Mat: Back Support, on the other hand, although the abdominals are recruited to add support to the structure, it is primarily the back extensors, together with the hip extensors and shoulder extensors, that hold the structure up. Further insight into this topic is found in the individual exercise descriptions.
In a standing position, the gravitational pull is equal in the front and back of the body (ideally). In this case the abdominals and back extensors work in a state of co-contraction to stabilize the trunk. The back extensors hold the body upright and prevent it from folding forward. At the same time, the abdominals play an important role in creating a girdle of support around the midsection, preventing excessive stress on the lower spine and, in effect, functioning as a second spine.
Note that the abdominals tend to decondition more rapidly than the back extensors, and the back extensors tend to respond to conditioning more readily than the abdominals. Add to this the fact that the back extensors are used most of the time to keep us upright, and the result is that they are typically better conditioned than the abdominals. However, the almost constant use of the large back extensors, particularly the erector spinae, can in itself create a problem. These superficial muscles may become hypertonic (overused) and very tight. “Dimming” (inhibiting) them, or in some cases “switching them off” to allow the deep posterior spinal extensors and abdominals to fully engage, is a crucial first step in creating balance in the trunk and achieving healthy trunk and lumbopelvic stabilization.
To stay well conditioned, the muscles of the trunk should be worked in all ranges of motion: flexion, extension, lateral flexion, and rotation. Individual needs vary, but a safe guideline is to allocate 50 percent of the time spent working core muscles to the spinal flexors, 25 percent to the lateral flexors and rotators of the spine, and 25 percent to the spinal extensors. Of course, Pilates exercises often include the combined activity of many muscle groups in various ranges of motion, so this percentage allocation should only serve as a general guideline and not a steadfast rule. It is interesting to note that exercises geared toward endurance, as opposed to strength, appear to have a more prominent effect on enhancing spinal stability. This is not surprising considering they need to function for extended periods of time during our daily activities. The BASI block system helps ensure that all these factors are addressed in a well-rounded Pilates session.