Originally published on March 3, 2015
Strength coaches and physical therapy types are always talking about the types of stresses our bodies undergo. But they usually sprinkle around words such as stress, strain, load, tension, shear, compression, torsion, etc. more like they are decorating a cake than trying to teach us something. I sometimes wonder why so many like to impress us with their vocabulary but so few ever want to take the time to clue us into the fundamental meaning of their jargon. So, here I’ll take the time to explain what all the words mentioned in the title mean.
What Does All That Tension, Compression, Shear, and Torsion Stuff Mean?
Right now, as you read this, gravity is acting on you. So a force is acting on you. But since the ground or chair is pushing up on you with equal force and you happen to have a marvelous body that can maintain its center of mass, you are, at any one time, very stable. This means that you are good at resisting any disruption to your equilibrium.
Despite this, however, the structure of your body is under load. So, there are always various types of stresses acting on it, such as tension, compression, and shear stress, all of which affect the body’s internal structure. These are illustrated on the bone images below.
Tension stress (or tensile stress) occurs when two forces pull on an object in opposite directions so as to stretch it and make it longer and thinner. The primary load a muscle experiences is a tension load. When the muscle contracts it pulls on the tendons at both ends, which stretch a little. So the tendons are under tensile stress. The change in length of the tissues is considered the strain. Although you may think of strain as an injury, in this case, it is the percentage of deformation, or specifically the change in length that the tissues have undergone due to the applied stress. So keep in mind that the word strain does not always indicate a permanent deformation and it does not necessarily happen all at once.
Another example of tension is when you hang from a pullup bar. Gravity pulls down on your body which causes tension in the spine.
Compression pushes or presses an object so as to make it shorter and thicker. Right now, again, whether you are sitting or standing, certain structures in your body are experiencing a compressive load. When you stand, gravity is “pushing down” on your body while the reaction force of the floor is “pushing up” (for lack of a simpler way of putting it). So your intervertebral discs and your sacroiliac joints are experiencing a compressive stress. The soles of your feet as well. In fact, your feet are getting the primary load. When a person weighing around 150lbs (68kgs) stands barefoot, the soles of their feet are exposed to a compressive load of about 3.72lbs per square inch (0.26kg/cm2) If they raise up on the balls of their feet, the compression stress should be about doubled. No wonder Dr. Scholl’s is so popular.
Tension and compression stress are both sometimes referred to as axial stress because the forces act along a structure’s longitudinal axis.
Shear stress is two forces acting parallel to each other but in opposite directions so that one part of the object is moved or displaced relative to another part. The best way to visualize shear is to think of how scissors work. Or better yet, garden shears. Shear causes two objects to slide over one another. This results, of course, in friction. If one vertebra is being caused to slide relative to another then there is a shear stress between them.
You undergo shear stress all the time when you walk. Every time you take a step, for example. As one leg leaves the ground and the other leg takes all your weight this creates a shear stress in the pelvis because the ground is pushing up on one side of the body through the supporting leg while gravity is pushing down on the unsupported side.
Severe shear related injuries of the lumbar spine used to be fairly common in motor vehicle accidents as the lap belt would cause an anteroposterior shear force to the lumbar that was enough to cause what was termed a “chance fracture.” Such injuries are uncommon now that most vehicles are equipped with diagonal shoulder belts.
Bending and Torsion Loads
All these are just examples of simple loads. That is, one specific type of load that results in one specific type of stress. Many times, however, there are different types of loads acting at once to cause a mixture of stresses on the structures of our body. The two primary ones are bending loads and torsion loads.
When I say bending you probably immediately think of bending over. That’s good. Specifically, think of bending or rounding the back, otherwise known as flexion. Bending loads produce tensile and compressive stresses. If you bend a stick or pencil with both hands you are actually creating a compressive stress on one side and a tensile stress on the other.
Similarly, when the spine flexes, the intervertebral discs undergo compressive stress on the anterior side and tension on the posterior side, as the illustration below shows. Repeated cyclic compression-tension loading and unloading of the lumbar spine is a great way to get a messed up back, as I’m sure most of you have heard.
Lateral bending will, you guessed it, cause compression on the side you are bending towards and tension on the other side.
Torsional loading, which we usually just call torsion, is when forces acting on a structure cause a twist about its longitudinal axis. This is what happens in your spine when you twist your body from side to side, for instance. When you bend laterally to pick up an object in one hand there is a bit of torsion going on in the spine. Likewise when you carry something heavy in one hand. Among sports, golf is one big fit of spinal torsion. The stresses that occur during torsion are much more complex and hard to measure, including shear, compressive, and tensile stress. Anatomically, due to the facet joint orientation, the lumbar spine is more susceptible to torsion than the thoracic which has more tolerance to twisting.
Although I have briefly mentioned injury I want to “stress” that all these stresses are a normal part of human movement. They are, in effect, necessary for the body to move. All too often, these days, all facets of movement are discussed as if we are fundamentally broken. Although these stresses can be part of the injury mechanism they are also part of the fundamental process of human movement and of how our bodies adapt to the environment we place them in.
I’d like to get away from this “injury-centric” view as it causes us to look always for deficiencies and this leads us to be on a never-ending vigil against insufficiency. Meanwhile, strengths are ignored. But to get strong there is a little motto I like to use: “Play to your strengths”. We must protect ourselves from injury but to perform better we must also subject ourselves to stress. In fact, it is a frequent complaint of mine that many strength experts have almost completely left behind all aspects of performance besides qualitative aspects. It just will never work.
The very fact that we can deal with all the various forces acting upon our bodies on a daily basis while walking upright on a stack of bones precariously slung between our legs is a testament to how well our bodies are made.