FUNCTIONAL ANATOMY & BIOMECHANICS · CHAPTER 1 · LEVERS, MOMENT ARMS & TORQUE
Why does the same weight feel harder farther away?
You pick up the same object, but reaching out makes it feel harder to hold. The object didn’t get heavier. The demand on the joints changed. FABIO gives us a way to explain why.
1. Which joint are we talking about?
The first step is to choose the joint and the task. Suppose a patient holds a small object with the elbow flexed. We can start by examining the external moment about the elbow. If we change the shoulder or wrist position, the analysis may change too.
Your FABIO notes separate kinematics (describing motion) from kinetics (studying forces and their effects). Neither tells the whole story alone. A movement can look identical at two speeds but involve different forces.
Quick terms from the original class
Force has magnitude and direction. A vector represents both. A lever rotates about an axis; forces can create a moment about that axis.
2. Find the perpendicular moment arm
A common mistake is to measure any convenient length. For torque, use the perpendicular distance from the axis to the force’s line of action. With an external force of 20 N acting perpendicular to a lever, a 0.20 m moment arm gives 4 N·m. Increasing that moment arm to 0.40 m produces 8 N·m.
The general relation is torque = force × perpendicular moment arm. If a force is applied at an angle, the perpendicular component matters; for a straight lever the same relationship may be written as F × r × sin(θ). Watch your units: centimeters must be converted to meters when reporting N·m.
This explains the external turning demand, not a directly measured muscle force. Internal muscle moment arms, joint geometry and co-contraction are separate parts of the picture. A longer external moment arm often increases muscular demand, but the actual muscle force is not simply the weight in the hand.
Quick check: the load is unchanged, but the perpendicular moment arm doubles. What happens to the external torque?
Reasoning: It doubles, assuming the force magnitude and direction stay the same. A 20 N load at 0.20 m produces 4 N·m; at 0.40 m it produces 8 N·m.
3. How does that help with a patient task?
Imagine a patient says carrying a bag close to the body is manageable, but placing a similar load onto a shelf with the arm extended is difficult. Before deciding that a particular muscle is weak, describe the movement, support, load position and relevant joint demands.
Try changing one variable at a time: how far the object is from the joint, the shelf height, the amount of load or the available support. Recheck the actual task. Force analysis helps explain the demand; it does not establish the diagnosis or choose treatment by itself.
Quick check: a patient moves a box closer to the trunk while carrying it. Did the weight decrease?
Reasoning: No. The gravitational force on the box is essentially unchanged, but the external moment about relevant joints can decrease because the horizontal/perpendicular distance changes.
Sources & learning scope
Original course sources: FABIO Principles.pdf and FABio Sample Calculation Problems.pdf (lever systems, vectors, torque and perpendicular moment arms). The 20 N examples are newly written instructional numbers, not copied exam problems. This is a conceptual lesson, not a complete patient-handling procedure. Cross-check actual equations and diagrams with your class notes. Coming next: how changing the angle of a force changes torque, followed by levers and mechanical advantage.
Try the mechanics on a real task
Example: A 15 N object is held perpendicular to the forearm. The horizontal distance from the elbow axis to the load’s line of force increases from 0.15 m to 0.45 m.
Work through the reasoning
Answer: The external moment increases from 2.25 N·m to 6.75 N·m—three times as large—if the force direction and magnitude are unchanged. This doesn’t tell you the exact muscle force because the internal moment arm and other forces still matter.
Common trap: Confusing external torque with a measured muscle force. Remember: Draw the perpendicular distance before interpreting effort.
From My PT School Notes · Companion learning content. Examples are for study, not individualized medical advice or a substitute for supervised lab instruction.
