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How Do Injuries Occur?

There are many terms doctors, therapists, and patients use to describe an injury. You may have heard of a pulled muscle, contusion, a sprained this, or strained that. All of these terms can be a bit confusing if not explained properly. Are you reading this saying to yourself, aren’t those all the same thing? Read on for a little bit of insight.

Types of Force

Our muscles, bones, ligaments, and other connective tissues are all in place to allow us to move and respond to the forces placed on our body. Our body counteracts external forces (think gravity, someone hitting you, or a gust of wind) by generating internal forces from our muscles, bones, ligaments, and connective tissues. Injury occurs when the load placed on the tissue either too much, applied too quickly, or the structure undergoing the stress is not anatomically ready to endure the amount of stress placed on it (1). This can occur from an external force (think of a contact injury in sports when two players collide) vs. an internal force (think of a non-contact injury when a player’s knee gives out with no contact).

The main forces which act on our tissues are tension, compression, and shear forces (1). A tension load injury pulls too hard on tissue in opposite directions (1). The perfect example of this is pulling apart taffy. The taffy allows a moderate amount of stretch before it eventually breaks because it became too long and too thin. A compression load injury refers to too much force in the same direction from opposite ends (1). Think of squishing a jelly donut so much that the jelly inside leaks out because the donut became too short and too thick. Shear stress refers to forces acting parallel to each other, but in opposite directions (1). Shear forces are applied while using scissors, the friction created cuts the paper in half.

Applying the Principles of Forces to Injury

Both strains and sprains refer to an injury of overload and overstretch (1). Both are tension load injuries but affect different types of tissues. In short, a sprain refers to a ligament injury. Ligaments connect bones to other bones, providing support and stability to our joints. A strain refers to an injury to a muscle or tendon. Muscles are connected to bones through a tendon and are able to generate forces to allow us to move. Contusions are typically caused by a compressive force, essentially the injury occurs by squishing the tissue. Shear forces tend to cause injury to tissues which are designed to be shock absorbers- our spinal discs or the meniscus of the knees. Shock absorbers are designed to tolerate a lot of compression (like jumping), but do not handle shear forces well (think twisting motions).

The next time you hear about someone straining, spraining, or pulling something, you should have a better understanding of the injury. You can use your new found knowledge on force to help prevent injuries or even promote tissue healing!

1. Magee, David J. Orthopedic Physical Assessment. 2008.
2. Jones. “Laboratory Manual of Physical Geology.” Laboratory Manual of Physical Geology, 3rd ed., 2001, socratic.org/questions/what-is-tension.



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