Its contraction greatly enlarges the chest and draws air into the lungs with each inspiration. This forceful compression is required to expel urine from the bladder, the contents of the colon and rectum during bowel movements, and the baby from a woman's uterus. In each of these cases, smooth muscles in the walls of the hollow organs also take part in the expulsion of the contents. The vertebral column has a great number of muscle bundles that help to position and move it. When caught off guard, these muscles can be strained, causing spasms and pain.
Large muscles radiate from it to the chest wall, spinal column, and skull.
The shoulder joint is capable of many movements, because it has flexors, extensors, abductors, adductors, and rotators. The emphasis in these areas is on opposing groups of flexor and extensor muscles on the front and back of each limb. In addition to the long muscles that come from above and provide force, the digits have many small local muscles that produce fine movements. The combined functions of the two groups of muscles provide both power to grip and precision manipulation in the hand. The human foot lost most of these functions when it became highly specialized for bi-pedal posture and walking, which are actually much more human traits than manipulation.