The core consists of four primary muscles: the diaphragm, multifidus, transverse abdominis and pelvic floor. More broadly the core or powerhouse is the muscles of the trunk running from the bottom of the ribs to the hips anteriorly and the gluteal fold posteriorly. This includes the internal/external obliques, rectus abdominis, spinal extensors and hip flexors/extensors.
The diaphragm is a muscle of the deep core. Although its primary function is in respiration, the diaphragm can be used to create intra-abdominal pressure used in abdominal bracing. In Pilates, the diaphragm serves to initiate, through the breath, the activation and hollowing contraction of the TVA and other deep core muscles responsible for stabilization.
The transverse abdominis is the deepest of the four abdominal muscle groups. TVA muscle fibers run horizontally and therefore the primary function is stabilization. It acts as a natural girdle for the trunk and pelvis, protecting the spine during movement and supporting internal organs.
The multifidus, a series of muscles attached to the vertebrae, is the deepest of the spinal extensors, running from the sacrum to the axis in the cervical spine. The functions of the multifidus are to remove pressure from vertebral discs so body weight is distributed along spine, maintain upright posture and provide spinal stability. Prior to movement the multifidus is activated to prevent injury. The multifidus and the TVA are widely considered the two most important muscles for core stability.
The pelvic floor is composed of two muscles, the coccygeus and the levator ani (pubococcygeus, puborectalis, iliococcygeus), also known as the Kegel muscles. The pelvic floor spans the base of the pelvic bowl and provides support for the pelvic organs, maintenance in continence, facilitates birth and maintains optimal intra-abdominal pressure.