locomotor system
Health Question Received
Our Response
30 April 2004
This is clearly an extensive topic, and I am not sure how much you already know about the body as you begin your project. What I can do now is give you a few pointers to get started, and then if you need help with more specific aspects you can contact us again.
Lets begin with the lying posture. The position of the body has not been specified, so you may need to take into account possibilities such as supine, prone, and side positions (such as the recovery position). Again this will depend on the depth of study that is required. Imagine someone lying on the floor in a supine position (ie: lying on their back and looking up at the ceiling). Their body is well-supported at several points the back of their head, across their shoulders and upper back, elbows and hands, back of the pelvis, back of the hamstrings and calf muscles, and the backs of the heels. So not much muscular activity will be required to sustain this position. The head would have a tendency to roll to one side or another, so the controlling actions of muscles such as sternocleidomastoid would be needed to stabilise the head. A similar case exists for the feet they would tend to loll outwards, and there may be some muscular activity in the medial rotators of the lower limb to balance this tendency. Elsewhere, stability at joints can be largely left to ligaments. For example, think of the lumbar region of the vertebral column. It is concave posteriorly, and in the lying position there would be a tendency for this curve to flatten as a consequence of the weight of the abdominal region above. No doubt the curve flattens somewhat, but a point will be reached where the ligaments linking the vertebrae take the strain.
If we move now to the seated position, the lower limbs are well supported and would require only limited muscle activity to stabilise their position. Few of the ligaments of the lower limb would be experiencing much tension. The pelvis, although supported below by the seat, has a tendency to sway backwards and forwards, depending on the position of the centre of gravity of the structures above. This sway tendency will be counteracted by flexors and extensors at the hip such as iliopsoas and gluteus maximus muscles. Now, looking at the structures above the pelvis quite a lot of muscular activity will be required to support and stabilise the load-bearing vertebral column, to support the upper limb girdle, and support the head, which now has a tendency to fall forwards. (One way to try and work out where muscle action is needed is to borrow an articulated skeleton and feel for yourself where movements are likely to happen if the skeleton is placed in these different positions. You can then work out which muscles and ligaments can counteract these tendencies.)
Finally, the hardest posture to analyse is standing. It is a very unstable position to maintain, and the centre of gravity of the body has only to shift a short distance for the vertical line from it to lie outside the supporting base provided by the feet. This would make the body fall. So the postural corrections have to keep the CG within a narrow envelope. This is partly achieved by muscles, for example those acting at the ankles (gastrocnemius, the extensor muscles acting on the foot) and those acting at the hip (iliopsoas and the gluteals), and also by ligaments around the knee which hold it locked in the extended position and reduce the amount of muscular action required at the knee. Subtle corrections of the position of the trunk and head and neck will also occur. So you will need to take a lot of factors into account when describing the standing posture.
I hope this will help you to begin your study good luck.