Not a single back muscle has a selective function: nearly all movements of the spine are the sum of various muscles acting in different regions.
The following functions can be distinguished in a person standing upright:
- stabilisation of the posture
Besides the joints and ligaments of the lumbar spine, lumbar muscles mainly facilitate stabilisation during movement of the centre of gravity, especially forwards and sideways. When the centre of gravity moves behind the lumbar spine, the abdominal muscles are engaged. The centre of gravity in a person at rest who stands upright is normally ventrally from the spine at level L4 (in 75% of all cases). In this instance there is minimal activity of the back muscles. In a seated position (without support of the arms and back) the activity of the back muscles is equal to that when standing upright (at rest).
- initiation of small active movements
Especially during the start of the extension, the back musculature is active. During the lateroflexion the ipsilateral paravertebral musculature is involved in initiation of the movement and the contralateral paravertebral musculature ensures balance and control of the movement. When lifting the left leg the (left) ipsilateral paravertebral musculature tenses and when lifting the right leg the (right) ipsilateral paravertebral musculature tenses (e.g. when ‘walking on the spot’)
- initiation of large active movements
During flexion the contribution of the lumbar muscles (excentric contraction) is proportional to the degrees of flexion. It is mainly this movement which causes an increase in pressure in the disc. Extension from the flexion position is accompanied by high activity of the lumbar muscles.
- lifting a heavy object
During this activity, the lumbar muscles are assisted by the hip extensors as lifting is a combined action of extension of the spine and hips. The contribution of the abdominal muscles is unclear and subject to research.
Generally speaking, it is accepted that lifting with a straight back (extension) is better than lifting with a curved back (flexion). During flexion of the back the nucleus inside the disc moves in a dorsal direction. Hence, when lifting with the back bent forward, the risk of a rupture of the posterior anulus fibrosus is increased, possibly resulting in a HNP (herniated nucleus pulposus).
In addition, the distance between the object that is to be lifted and the body is also hugely important: the closer the weight (object) is to the body, the better it is for the back – from a biomechanical viewpoint.