There are many contributing factors to an effective core including the skeletal structure and breath. Jan has already discussed the breath and Elly is going to talk about the placement of the pelvic and shoulder girdles.
The Pelvis is like a bowl of bones made up of two iliac bones and the sacrum that supports the weight of the upper body and head and it absorbs the shock transmitted up from the lower body. The positioning of the pelvis effects the 3 curves of the spine. When exercising with a neutral spine the 3 curves are still present, however if the pelvis is tilted anteriorly or posteriorly this directly impacts the spinal curves and therefore the ability of the core to contract.
The shoulder girdle sits on top of the pelvic girdle and equally has an impact on the spinal curvature and the core. If the shoulders are excessively rounded and internally rotated this creates a kyphotic posture, whereas the counter causes a lordotic stance.
When the pelvis is in neutral and the shoulder girdle is sat equally on top of the pelvis then the weight of the upper body is loaded evenly through the spine and the discs.
The core is made up of two corsets – an outer and inner corset. The outer corset is a vertical and diagonal corset running from the ribs to the hips and includes the rectus abdominus and the external obliques. The stablising internal corset runs horizontally around the spine and includes the transverse abdominus. The two corsets are connected at the top by the diaphragm and the bottom by the pelvic floor. The pelvic floor is a hammock of muscles running from the pubic bone to the tailbone and impacts on intra-abdominal pressure.
As you take a breath in, the lungs expand with the breath causing the diaphragm to drop down into the abdominal cavity- at the same time, the pelvic floor relaxes slightly. The reverse happens as you exhale, the pelvic floor contracts and draws up towards the abdominal cavity and the diaphragm relaxes back up towards the lungs. This autonomous contraction and relaxation causes the internal organs to rise and fall slightly. Knowing how the corsets work together can have an impact on core work during exercise.