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The Physics of Movement

The way humans move has always intrigued me. In fact, this interest was one of the underlying reasons I went to physical therapy school. I can equally assure you that a love of physics was NOT a reason I continued onto PT school. It was during my time at the University of Nebraska Medical Center that I really came to appreciate just how important physics is in our daily lives, and how we can use its principles to modify the way we do things. A physical therapy professional's understanding of physics, and how to use that knowledge pertaining to the human body, is arguably the greatest treatment tool we have. For the sake of this article I'd like to highlight some examples of how I, or another physical therapist, could use the principle s of physics to enhance a patient's outcome.

Let's pretend that your friend challenges you lift a gallon of milk and hold it off the table for an entire minute. On first attempt, you lift the full milk container at arms length directly in front of you. After a few seconds the weight would likely overcome your strength, causing you to return the container to the table in the shame of defeat. On the second attempt, you bend your elbow and tuck it in at your side lifting the jug to shoulder height but only inches in front of your body. As a result, the jug seems to weigh less and you can easily surpass the one minute mark and perform your favorite victory dance. What changed? Simply by bending your elbow and shortening your arm length, you were able to decrease the affects of the gravity force vector (downward pull) on your shoulder joint, and therefore required less strength to lift the milk jug. Now I realize that not too many of you routinely participate in milk container challenges like this, but the principles are true of anything you're lifting. For this reason, when you're lifting something heavy, you should keep it close to your body. Even light-weight objects lifted routinely away from your body can cause injury due to the load it places on your muscles.

Let's switch gears and talk about a walking pattern that I routinely see. Have you ever noticed that some people almost seem to waddle side to side when walking? Have you ever stopped to think about what could be causing such a movement? Oftentimes, the cause is a muscular weakness on the outside of the hip. When all the weight is on one leg during the stance phase of walking, a person with weak hip muscles will shift their bodyweight over the stance leg to reduce the force required by those muscles. This is visualized as a "waddle." In addition to strengthening and balance exercises, if the weakness is only on one side, the addition of a cane in the opposite hand can quickly remedy this movement pattern by counteracting the downward forces. This is physics hard at work again.

Running athletes who suffer lower extremity injuries may also benefit from a quick physics lesson as well. As a runner's stride increases, the foot lands further ahead of the body. While this may sound harmless, there's a direct relationship between stride length and what is known as the braking impulse. Essentially, the longer the stride, the higher the impact forces on the body when the foot makes contact with the ground. For someone struggling with stress fractures, or lower extremity pain, a physical therapy professional may recommend changes to a runner's stride length as a quick way to decrease the stresses placed upon the body.

These examples are just scratching the surface of how a physical therapist may use physics to enhance outcomes for their patients. If you have questions about how these principles could be used to help you, reach out to your local physical therapist.

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