Body Posture & Muscles Functions: (Part 4)

Does any of you encountered low back pain? How about inability to perform full push up exercise? How about doing excessive knee bent sit up exercise regularly? In today's post, I will discuss about the trunk, primarily about vertebral column.

Innervation to the trunk muscles does not include an intervening plexus between the spinal cord and the peripheral nerves like the cervical, brachial, lumbar and sacral plexus. Abdominal muscles receive their innervation from the thoracic branches of the ventral divisions of the spinal nerves.

Based on the skeletal structures, the trunk is composed of two parts. The thoracic spine and the ribcage constitute the upper part; the lumbar spine and the pelvis constitute the lower part. The spinal column, together with the extensor muscles govern, to a great extent, the posture and movements of the trunk. This chapter examines the role of trunk muscles in movement and support of the trunk, and the role of hip muscles that act simultaneously with trunk muscles in movements and support of the pelvis.

It is interesting to note that muscles that act in unison for certain movements act in opposition to each other in support of good alignment. For example, in the prone position during the movement of spine extension, hip extensors assist by stabilizing the pelvis to the femur. In the supine position during the movement of spine flexion, the hip flexors act to stabilize the pelvis. On the other hand, in support of good postural alignment in standing, the hip extensors act with the abdominal muscles, and the hip flexors act with the back extensors. In the movement of raising the trunk up sideways, lateral trunk muscles laterally flex the trunk as the hip abductors stabilize the pelvis. In support of good alignment in standing, lateral trunk muscles are assisted by hip abductors on the opposite side.

In relation to the trunk, one of the major concerns is the painful low back. It is common knowledge that a high percentage of the adult population has suffered from low back pain at one time or another. For many, the treatment of choice consists of restoring good postural alignment and muscle balance.

The lungs and diaphragm are located in the trunk. Faulty alignment of the skeletal structures and problems of muscle imbalance can adversely affect the respiratory system. The Respiratory muscles lists the twenty three muscles (each of which has a right and left component) plus the diaphragm as the muscles of respiration. Most of these muscles also have a function related to posture and muscle balance.

The trunk, or torso, is the body excluding the head, neck, and limbs. The thorax, the abdomen, the pelvis, and the low back are all parts of the trunk. The term trunk raising may be used to describe raising the trunk against gravity from various positions, from face-lying (prone position), trunk raising backward, from side lying, trunk raising sideways, and from back lying (supine position), trunk raising forward. The term may also apply, in standing, to raising the trunk from positions of forward bending, side bending, or backward bending to the erect position.

The thorax is elevated (chest lifted upward and forward) by straightening the upper back, bringing the rib cage out of a slumped position. The thorax is depressed when sitting or standing in a slumped position, or it may be pulled downward by the action of certain abdominal muscles.

The trunk is joined to the thighs at the hip joints. The movement of hip flexion means bending forward at the hip joint. It may be done by bringing the front of the thigh toward the pelvis, as in forward leg raising, or by tilting the pelvis forward toward the thigh, as in the sit up movement.

The forward-bending, long-sitting position involves hip joint flexion along with back flexion. One must try to disregard the hip joint movement when observing the contour of the back. Range of motion and contour of the back may also be observed by having a subject bend forward from the standing position. As a test position, however, this has certain disadvantages. If the pelvis is not level or is rotated, the plane of forward bending will be altered, and the test will not be as satisfactory as that in the longsitting position in which the pelvis is level and rotation is better controlled.

Because the low back muscles are seldom weak, the range of back extension may be determined by the active strength test in the prone position. Please see photo below. 

Whether the range of motion is normal, limited, or excessive, the subject is capable of moving through the existing range. The anterosuperior-iliac spines should not be lifted from the table during back extension, because doing so adds hip extension to the back extension range of motion. Please see photo illustration below.

Back extension often is checked in the standing position. The test is useful as a gross evaluation, but it is not very specific. Swaying forward at the hips is almost a necessity for balance when bending backward, but doing so adds the element of hip extension to the test, or the knees must bend somewhat if the hip does not extend.

Similar to the test to determine range of motion in spine flexion, a test can be done to determine the range in spine extension. The subject lies prone on a table, resting on the forearms with elbows bent at right angles and arms close to the body. If the subject can extend the spine enough to prop up on the forearms with the pelvis flat on the table, the range of motion in extension is considered to be good.

Now, let's talk a little bit about flexion. In the cervical region, flexion of the spine is movement in the direction of decreasing the normal forward curve. Movement continues to the point of straightening or flattening this region of the spine, but it normally does not progress to the point of the spine curving convexly backward.

In the thoracic region, flexion of the spine is movement in the direction of increasing the normal backward curve. In normal flexion, the spine curves convexly backward, producing a continuous, gently rounded contour throughout the thoracic area. Next, in the lumbar region, flexion of the spine is movement in the direction of decreasing the normal forward curve. It progresses to the point of straightening or flattening the lower back. Normally, the lumbar spine should not curve convexly backward, but excessive flexion in the low back is not uncommon. Certain types of activities or exercises  such as knee bent sit ups, can cause flexion beyond the normal range and make the back vulnerable to strain from heavy lifting movements. I've wrote about this before in previous post. 

Moving on, let's discuss about extension. Extension of the spine is movement of the head and trunk in a backward direction, while the spine moves in the direction of curving convexly forward. In the cervical region, extension is movement in the direction of increasing the normal forward curve. It occurs by tilting the head back, bringing the occiput toward the seventh cervical vertebra. It may occur, in sitting or standing, by slumping into a round upper back, and forward-head position that results in approximating the seventh cervical vertebra toward the occiput.

In the thoracic region, extension is movement of the spine in the direction of decreasing the normal backward

curve by straightening the upper back. Movement may progress to, but normally not beyond, the straight or flat position. In the lumbar region, extension is movement in the direction of increasing the normal forward curve. It occurs by bending the trunk backward or by tilting the pelvis forward. 

In the next post, I will discuss about the posterior muscles including hamstring and spine, as well as pelvis. Stay tuned.