Physics and Physical Therapy

For this blog post I decided to investigate the physics involved in physical therapy. I have spent hundreds of hours observing in physical therapy settings and have noticed that many of the recovery exercises take into account physics. 

The body’s skeletal muscle system can largely be thought of as a system of levers and pulleys. For example, the elbow joint can be thought of as a lever and the knee joint can be thought of as a pulley. It is important in both of these scenarios to take into account torque when performing exercises. Torque (T) is the force required to cause a rotation and is related to the motion of joints. Torque is equal to the distance from the axis of rotation to where the force is exerted (lever arm) multiplied by the force perpendicular to the lever arm: T=rFsinΘ. It also equals the moment of inertia multiped by the angular acceleration: T=Iα. When a person is performing strengthening exercises, for example on their hip, a person lies down on their back with one leg out straight and strap a weight around their ankle. The person then lifts their straight leg to about a 45° angle, then lower it again. 

The moment of inertia of the system is equal to the moment of inertia of the leg, which can be thought of as a uniform rod, plus the moment of inertia of the weight, which can be thought of as a point mass. A person’s hip cannot withstand a very high torque without becoming even more injured. So the physical therapist must take into account the moment of inertia of the system to ensure that the person does not accelerate their leg too quickly and injure their hip again. One way to make it less likely that the torque on the person’s hip is not great enough to injure the person is to lower the mass that is strapped to their ankle, lowering the moment of inertia of the system.

Physics also comes into play in physical therapy when performing aerobic exercises. These exercises should be performed on rubber mats instead of on wood or concrete floors. For example, when jumping up and down, the rubber mats move slightly when a person’s feet hit the mat. The mat absorbs more of the energy from the person’s jump than a wood or concrete floor would. This lessens the amount of force that the person feels when landing. By lessening the amount of force that the person’s knees or hips experience when landing lessens the chance of further injuring themselves.