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  • Dr. Piotr Solowiej PT, DPT

Force Vectors and Knee Pain

Outdoor basketball

What's the best exercise for knee pain? That is very difficult and likely impossible to determine. When it comes to causes of knee pain, the variables to control for are immense:

  • Training history

  • Injury history

  • Irritability

  • Stage of tissue healing

  • Comorbidities

  • Weight-bearing status

  • Fear/apprehension

  • Sleep quality

  • Nutrition

  • Goals

  • Beliefs, etc.

In this post I'm going to discuss one in particular: the force demands of a movement task.

Specifically, I'm talking about the direction of force vectors involved in a task. Let's explore how doing a biomechanical analysis helps asses someone's pain/symptom experience.

Not Just Body Position

Biomechanics are the study of mechanical laws (physics) in relation to human (biology) movement. This often involves free body diagrams with force vectors (see diagram) and joint torques (not pictured):

Free body diagram

Biomechanics are sometimes used interchangeably with 'body mechanics.' This second term is often used to specify and emphasize joints positions. Sometimes joint positions are vital to consider. Other times the most important variable may be less about joint position than the direction of force across the joint.

Force Direction Matters

Take the knee joint during a basketball game. Boxing out, and jumping up for a rebound, the forces across the knee are pretty much exclusively vertical. Much like the figure above.

A great exercise to prepare and build capacity for vertical jumping is a barbell back squat. The load is added above the center of mass and moved in purely up/down fashion.

Now let's consider the forces across the knee joint in some other scenarios when hooping:

  • Cutting

  • Transitioning from a fast break to jumping up for a layup/dunk/block/jump shot

What changes in these scenarios? The direction of the force.

Here's what happens:

  • A significant reduction in vertical forces

  • An accompanying increase in horizontal forces

While on offense, running from the top of the key into the paint at full speed, will require a significant horizontal force in the opposite direction of movement to brake momentum for either a layup or jump shot.

Here's a visual example of a fast break pullup:

To stop running from right to left, a significant horizontal force must be created in the knees (and other joints).

Rehab Implications

As a rehabilitation professional, this is very important. Jumping and cutting involve similar joint positions: knee flexion. If joint position were the only consideration, a squat seems like a perfectly reasonable exercise to develop and rehab both.

However, when considering the direction of forces, a patient complaining of symptoms when jumping from a standstill should be guided very differently from someone complaining of knee pain when transitioning from a run to jump, or when cutting.

For cutting and transition movements, a WAY more applicable and prudent strategy would be to use a leg extension machine.

Erik Meira is a Physical Therapist on the west coast, teaches a course on the hip and knee, and has a detailed blog post about why. Rather than duplicate the information I linked it in the previous sentence to continue to spread the knowledge, as he does a fantastic job. For more detail on why a leg extension is more beneficial than a squat for cutting movements, definitely check out his post.

Thanks for reading,

Dr. Piotr

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