Core body exercises are those that work the muscles of the abdomen and lower back. These muscles connect the ribcage and vertebrae to the pelvis, and protect the internal organs as well as the lumbar spine. Weak core muscles can result in back pain and poor posture, while strong core muscles provide a foundation for other movements, such as strength training exercises and daily life activities.
Abdominal muscles include the rectus abdominus, the internal and external obliques, and the transverse abdominus — a wide sheath of muscle that surrounds the entire core like a girdle. Muscles of the lower back used during core exercises include the erector spinae and the quadratus lumborum. The number and variety of muscles in the core region demand that several core body exercises be included in a complete workout to adequately target all areas.
Core body exercises intended to improve posture and reduce back injury should typically focus on muscular endurance rather than power, especially in the early stages of a gradual, progressive strength training program. This means that isometric contractions and higher numbers of repetitions should make up the bulk of the core workout, with strength training exercises only being performed once a high level of muscular endurance has been established.
During daily activities, much of the stabilizing and supporting work performed by the core muscles is performed in an isometric fashion. For this reason, it is helpful to employ isometric core body exercises to enhance the muscles' ability to perform this kind of work. This is especially true of the transverse abdominus, which is best worked by the yoga plank.
Yoga planks are a good all-purpose core body exercise that works all the core muscles as well as the stabilizer muscles of the upper and lower body. The exercise involves isometrically holding a pose best described as the top position of a push-up. As a modification, the plank can be turned sideways, with one arm supporting the torso and the other reaching toward the ceiling. This variation tends to add extra activation to the abdominal obliques.
The rectus abdominus is best stimulated by body curling exercises, most notably the common crunch. Full sit-ups tend to employ the hip flexor muscles more than the abdominals. Side crunches and crossover crunches will bring the obliques into play in addition to the rectus abdominus.
The erector spinae run the length of the back and function in the flexion, extension, and stabilization of the spine. The quadratus lumborum spans between the lumbar spine and the iliac crest of the pelvis, and is commonly associated with lower back pain. To isolate these muscles, back extensions are an excellent choice, and allow a wide enough range of modification that they can easily be adapted to all ability levels.
Back extensions can be performed on the floor, an exercise ball, or a specialized bench. Kneeling single leg extensions will work the lower fibers of the muscles, as well as the upper gluteus maximus. With most modifications, holding an isometric contraction at the top of the movement will train the muscles to stabilize the lower back, and add a functional element to core body exercises.