Human Engineering: Dynamic Stability/Body Mechanics – Lower Extremities

Human Engineering: Dynamic Stability/Body Mechanics – Lower Extremities

(see part 1 of this series here)

(see part 2 of this series here)

by Sanjeev Joseph

Physical Therapist, CEO, Innovator, Educator passionate about fixing bodies and changing lives

I recommend you read Part 1 and Part 2 of this series before learning the upcoming concepts. If you have done so, congratulations, you are one step closer to understanding the secrets to human movement and the physical laws controlling your body. Use the following concepts as tools to help preserve your body, while at the same time increasing output. In other words, enjoy the efficiency!

Dynamic Stability

In Part 2, you learned about Static Stability, or as we know it better, “Posture”. Once you master the concepts of posture, you can now hold your body in positions like sitting, standing and lying without causing damage to structures like the spine, shoulders and hips/knees. If you apply these same concepts into movement, you have now engaged in Dynamic Stability. The way you control your body parts through coordinated muscle engagement during movements if known as “Body Mechanics”.

What To Focus On

You will now learn how to harness the power of your own body to create movements that are not only functional, but safe and healthy. By following these principles, you will be able to increase your workload, but reduce wear and tear of your precious body parts. Think of it this way: If even of of your car’s tires are off angle by even a degree, that deviation from the normal line will result not only in wearing of the tread on the tire, but eventually the power train, motor, and transmission will wear out faster. If that can happen to a car, why can’t it happen to the human body, which is nowhere near as durable as mechanical parts of a car? And unlike a car, you can’t simply replace your body parts without permanent loss (talk to person with a knee replacement, and you’ll know what I mean).

So from now on, your job is to look within and create a self awareness by “listening” and “paying attention” to the feedback from your body as you engage in what you learn here.

Physics is the Playing Field

To improve body mechanics, you will need to understand some basic mechanical concepts of physics. First Gravity is a constant downward force that pulls your body down toward the earth. To be able to overcome these forces, your muscles must have the capacity to lift your physical structure against these force. This is why strength is important.

Posture is the Platform

In Part 2, I went over posture. This is the basis of all the movements that will be described in this module. I recommend referring back to it if there are any questions. Here is a quick review:

  1. Contract your core, hip and pelvic stabilizers.
  2. Engage the axial stabilizers by contracting the upper abdominals and spinal extensors.
  3. Squeeze the shoulder blades together (scapular stabilization), and bring the neck to a relaxed and neutral position.
  4. Press your arches into the shoes or floor. (If you have flat feet, consider using orthotics before attempting any movements, as it will be impossible to overcome bony deformities if they are present.)
  5. Now you’re ready to MOVE YOUR BODY!


Probable the most basic of movements, but also the most misunderstood, and the most likely to make mistakes with. Most simple are usually where mistakes happen…because their importance is overlooked. This results in a systemic set of errors in all other motions. So how do we “fix the squat”. The science behind it is complex, but the application is easy.

  1. Engage in great posture.
  2. While keeping the weight through the arches of the feet, and keeping the core, axial, pelvic and hip stabilizers engaged, bend the knees and hips. You may keep your hand out in front of you if you want.
  3. Go as far down as you can. Only stop when there is pain, physical limitations or incoordination.
  4. Return all the way back up while keeping all of the postural muscles engaged and the weight through the arches of the feet.
  5. Once up, relax the body. That is one rep. I recommend doing 10 of these daily, taking time through the motions to avoid mistakes in the biomechanics.
  6. Congratulate yourself: You have now learned antigravity movements of the lower extremities.

Sit to Stand/Stand to Sit

Once you have mastered the squat, you just need to put a chair behind you to do a sit to stand/stand to sit.

  1. From a chair, engage the postural stabilizers.
  2. Lean the head forward and once the nose moves ahead of the knees, use the lower extremities to ascend into standing. (Trick: think of the nose as an airplane. You need to gain speed on a runway before standing up, so use speed and momentum as you go forward, then ascend up…like a plane taking off for flight).
  3. Keep your weight through the arches until you are in the standing posture.
  4. Descend to the seat by keeping the head, arms and upper body forward; hips backward and weight through the arches.
  5. Gently sit, don’t plop down, and relax the body.
  6. Congratulate yourself again.

Step Up

Here the calves play a big role in the smooth execution of this motion. Use the balls of your lower foot by pushing off quickly (like you’re springing up to the next step) to avoid excess wear to the upper leg.

  1. Stand behind the step with great posture.
  2. Move your head, trunk and hips forward (like an airplane) while lifting up one foot onto the top step.
  3. While doing step 2, propel yourself upward by pushing off through the big toe of the bottom foot.
  4. Before reaching the apex of the upward motion, transfer weight from the bottom foot to the top foot smoothly.
  5. Use the top extremity’s antigravity muscles to gently bring the entire body to the upper step.
  6. Congratulate yourself.

Step Down

What goes up must come down. The sequence is a little different going down, but the concepts and principles all apply here. The key is that the top foot now, has to work to lower the body down to the next step.

  1. Start with great posture.
  2. Placing weight through the arch of one foot, pick up the other foot and move it forward and downward to the bottom step.
  3. Use the antigravity muscles of the top extremity to lower the body, while pointing the toes of the bottom toward the bottom step.
  4. Gently transfer weight to the bottom foot by starting from the toes, through the arch and then the heel.
  5. Bring the top foot down to the bottom step by using the bottom extremity’s antigravity muscles until both feet are on the bottom step, returning to great posture.
  6. Congratulate yourself.


Phew, now you’ve made it through, and acquired another set of skills. It’s time to practice. Consider going back to earlier modules if the movements aren’t just right. Just doing these once or twice is not enough. It is recommended that if you’re not already doing of these activities a minimum of 10 times a day, then it should be part of your daily exercise program.

In the next module, I will go over upper extremity mechanics in reaching, lifting, carrying, pushing and pulling.

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