How is the physics in the human body

Forces, moments, intervertebral discs: Mechanics in the human body

Physics is in everyone's bones - and in their muscles. Laws of mechanics help explain what causes a herniated disc, for example. So that such overloads do not occur, both the strength and the interaction of the muscles play an important role.

Usually we do not think about the movements that we are making. Even complex processes of daily life such as writing, drying dishes or tying shoelaces take place automatically. We only actively deal with movements when they are new, for example when they are learning to ride a bike. Even with severe pain, such as a herniated disc, we suddenly perceive simple processes such as bending and stretching very clearly. How can such painful overloads occur? And how does our body have to work so that they don't even arise?

Physically, movements essentially take place in the area of ​​the musculoskeletal system. The fact that mental processes, the cardiovascular system and other factors also play a role is not taken into account here. The total of more than two hundred bones of the human skeleton, its passive elements, are held together, moved against each other and stabilized by 656 muscles - as active elements - and a large number of ligaments and tendons - as elastic elements.

Cross section through the back of a person

The movement of the bones is guided by joints. In the simplest case, the two ends of a muscle must be connected to one of the bones via a tendon so that two bones connected to one another by a joint can be moved against each other. If the muscle contracts, the joint angle is changed. Since one muscle can only exert traction, at least a second muscle is required, for example to straighten the arm or leg again. The effect of the second muscle is opposite to that of the first - the two work as so-called antagonists.

In fact, every joint is surrounded by a multitude of muscles that work both with opposite effects and as so-called agonists - with concurrent effects. This not only enables more complex movements such as simple bending and stretching, the muscles also stabilize joint positions and postures. In most joints, small, short muscles near the joints are responsible for stabilization (see info box: Biomechanical models). For this stability, which is particularly required when forces are transmitted via the joint, not only the strength and endurance of the muscles are decisive. The coordination between the muscles plays an important role so that a well-balanced amount of tension and counter-tension can protect the stressed joint. When picking up an object, for example, the human body has to perform a very complex movement of the entire trunk, at least one arm and the legs. The main role is played by the trunk and, within the trunk, the spine. When looking only at the skeleton, it is this that ensures the upright posture.

Muscles provide stability

Even standing upright requires constant muscle activity. This becomes immediately clear when you look at the picture of a person standing from the side. On the one hand, the spine in the trunk is well behind the center of gravity. Therefore, the back muscles must be activated continuously while standing in order to prevent tipping forward. On the other hand, the mobile spine is also in a steady, unstable equilibrium laterally, which must be maintained through continuous muscle action. That is why the stability of any posture depends to a large extent on the trunk muscles being resilient and well coordinated.

Power compensation through the back muscles

For example, the maximum strength of the abdominal muscles that can bend the body forward should be about seventy percent of the maximum strength of the back muscles. However, stabilizing postures through muscle work also puts a strain on the internal structures of the human body from the outset: For example, the intervertebral discs, primarily in the lumbar spine area, are more stressed when one is leaning forward and to the side than when standing upright. How much stress these structures are placed on during everyday movements , can be estimated with a simple calculation: One thinks of a point-shaped center of rotation in the area of ​​the lower lumbar spine for bending the upper body back and forth. Then you combine the mass of the upper body in a single, central point. The result for an average, athletic person (1.75 meters tall, 70 kilograms) is a weight of about 45 kilograms, which pulls the upper body down about three centimeters from the pivot point. The tensile force and this distance together cause a forward torque that has to be compensated for by the back muscles.

Both forces - the weight of the upper body and the compensatory force of the muscles - press the spine together under the imaginary pivot point. The total load is around 720 Newtons, which is 1.6 times the weight of the upper body. For a person of the same size and weight of 100 kilograms, the upper body mass on the one hand and the lever on the other hand would be larger. Correspondingly, the torque and thus the compensatory force of the muscles increase. The compressive force acting on the lumbar spine already corresponds to 2.2 times the upper body mass, so the load is significantly greater. That is of course a very rough estimate. However, calculations with very precise biomechanical models for different loads have already been carried out. The resulting values ​​are consistently even higher than the results of the estimation carried out above.

Pressure on the intervertebral discs

The results of direct measurements are even more precise: as early as 1966, the physician Alf Nachemson measured the internal disc pressure in test subjects in different postures by implanting pressure sensors in the nucleus of the disc. The value measured when standing upright, to which all other information is based, was 4.5 bar. The orthopedic surgeon Peter Neef received similar test results from self-experiments in the same year. While the results when standing and lying on your back were comparable to those of Nachemson, he gave lower values ​​for sitting and lying on your side, but much higher values ​​for lifting with a hunched back. Here, at 25.5 bar, five times the pressure occurred when standing upright. It is only about the pressure load, other forces are not taken into account. Nevertheless, it becomes very clear that the curvature of the spine in particular leads to an increase in pressure in the intervertebral discs.

Loads on the intervertebral discs

Stresses on the musculoskeletal system, but also noise, stress or heat that affect the human body, are converted into stresses on internal structures. Whether one of these structures is subsequently damaged depends exclusively on the level of stress, so that the stress itself can initially be viewed as neither good nor bad. What is of decisive importance is what conditions the load meets. It plays an important role whether the receiving and transmitting bones, ligaments and muscles work in a physiologically favorable position during physical stress. This actually mainly means joint angle positions. How effectively muscles can stabilize these positions depends largely on the current muscle length - and this in turn on joint angles.

Joint positions that effectively absorb or pass on stresses are beneficial. In healthy people, the processes take place automatically and so quickly that they are over before the brain even knows anything about them. The muscles seem to activate themselves, reflexes cause their contraction. The further away the corresponding body parts are from a favorable physiological position at the time of loading, the more likely this loading becomes to overload. This can be explained very well using the example of an intervertebral disc. The intervertebral discs connect the individual vertebral bodies with one another. They consist of a soft core of gelatinous mass, which is surrounded by a multi-layered ring of fibers. The direction of pull of the fibers of superimposed layers of the fiber ring is crossed. In addition, the angle of the fibers changes from the inside to the outside. This cross braid is extremely strong mechanically. The intervertebral disc is therefore designed from the outset to withstand very high compressive forces.

Vertebral body with intervertebral disc

However, this strength of the intervertebral disc is based on an even distribution of tension in the fiber ring. By tilting or twisting the two vertebral bodies against each other, this structure can be preloaded on one side. Twisting leads, for example, to the fact that the fibers running in the direction of rotation are stretched, while those running counter to the direction of rotation are compressed. As a result, the cross braid loses part of its strength: the system leaves its physiological working area. An additional external load - for example the weight of the upper body when picking up an object - can then easily overstrain the entire complex. Parts of the gelatinous nucleus can escape into the space between the vertebral bodies and affect nerves running there: a classic herniated disc.

As a rule, it is possible for us to pick up an object without suffering a herniated disc immediately. A healthy and productive body is naturally set up to absorb stress and prevent excessive strain. The already mentioned stabilization of joints by the surrounding muscles plays an important role. This is illustrated by studies of the body's reaction to sudden stress: If a person standing upright is thrown off balance by a sudden pull on the arm, the entire body, down to the legs, tries to maintain balance. A movement response runs through the entire body from the arm over the shoulder, the trunk and the pelvis to the legs.

Coordination of the muscles

In healthy people it can be proven that the muscles in a certain area of ​​the body, for example at a certain height in the back, are already active before the movement even arrives there. This indicates that the joints, including the spine, are stabilized in advance. This protects the joints from overload. The same studies on patients with chronic back pain deliver the opposite picture: Here the movement precedes the muscle activity, the protective activation is missing.

Interaction between muscles and skeleton during movements

Another example from another region of the body supports the picture: A common injury in the shoulder girdle area is the so-called rotator cuff tear. As a rule, the tendon of the relatively small one tears Supraspinatus musclethat runs from the top of the humerus to the shoulder blade. A comparatively low weight to be absorbed when falling can cause such a rupture, if only the Supraspinatus muscle goes into action. The risk of injury is immediately reduced considerably if the significantly larger and stronger one in good time Deltoid muscle intervenes in a supportive manner. However, this requires an appropriate coordination in the muscles.

In summary, it can be said that the human body is exposed to high mechanical loads, even with seemingly light everyday movements, which are usually well tolerated. The prerequisite for this is that the interaction of the muscles, tendons, ligaments and bones involved works and that the individual components are properly used. Sport or manual work can maintain or even improve the resilience of the musculoskeletal system.

In modern society, however, the burden is decreasing more and more, for example through frequent work at the desk. This certainly has its advantages. On the other hand, the associated lack of exercise reduces the resistance of the musculoskeletal system in many people, which, for example, causes muscles to be broken down. In order to maintain the body's resilience and thus health, it is therefore necessary to counteract the lack of exercise with a healthy amount of stress.