The intermediate fulcrum lever is the one most used in physiology and, in order to clearly explain what happens, it is necessary to resort to some elementary concepts of physics.
The triangle represents the joint support point (for example intervertebral disc), we consider that gravity (G) falls exactly on the support point. For the system to be in balance, the strength of the anterior (Fma) and posterior (Fmp) muscles must be equal. mutual tensions.
If G falls far from the point of support, the force Fmp must increase to safeguard the balance. For this reason the antigravity tonic muscles will be more numerous and strong in this district. In physiology this force is defined strength of erection.
This indispensable organization causes the compression of the support point and is called crushing component.
Demonstration
The example proposed corresponds to the intermediate fulcrum system of the dorsal region. The spines, arranged on the opposite side of the support point, represented by the intervertebral disc, contrast the weight of the thorax.
We depict the bone with a lever.
The bone (or the lever) is therefore subject to:
Ft: action due to the weight of the chest
Fd: action of the disc on the bone
Fmp: muscle actionFor equilibrium we will have: Ft + Fd + Fmp = 0
Or
Case 1 Fd = -Ft-Fmp
Case2 Fd = Ft + Fmp
In this situation it can be deduced that the intensity of the action on the bone on the disc is the sum of the weight of the chest and the action of the muscle.
These examples want to demonstrate how, in the specific case of the dorsal column, a situation of "unfavorable levers" is highlighted: the point of application of the posterior muscles is close to the point of support (vertebra), while the weight to be countered (thorax) is away from the hub.
with the strengthening of the abdominal and paravertebral muscles, as this type of work will lead to further compromise of the intervertebral disc. The more rigid, hypertonic and shortened a muscle is, the more the joint crushing component increases. Furthermore, the further the weight is from the point of support, the greater the antigravity muscle strength must be.Another thing we should reflect on is the patient with dorsal hyperkyphosis: also in this case, it is incorrect to strengthen the paravertebral muscles as the abnormal movements of the masses anteriorly require greater activity of the spinal muscles by increasing the joint crushing component.
Another mistake is to consider the rectum of the abdomen as a lumbar de-lordosizer. The rectum of the abdomen has no relationship with the lumbar spine, its action only causes the lowering of the ribs. its insertion at the level of the pubis, it is forgotten that the iliacs are in relationship with the sacrum by means of the sacro-iliac joint, for this reason the iliac bone moves independently without modifying the lumbar curve.
In conclusion, the posterior erector muscles must counter not only weight but also their complementary antagonists. The dorsal spines are opposite to the abdominals which lower the thorax and to the anterior musculo-fibrous system; their reinforcement causes a crushing of the intervertebral disc.
