Mechanical Principles in Exercise Therapy
💡 Core Concept: Exercise Therapy is applied physics. Understanding forces, levers, and gravity allows a therapist to modify exercises (make them easier or harder) and analyze human movement efficiently.
1. Force, Gravity & Stability
Force
A push or pull that alters the state of rest or motion of a body. It is a Vector Quantity (has magnitude and direction).
- Composition of Forces: Applying multiple forces to produce a resultant force.
- Resolution of Forces: Breaking a force into components (e.g., Rotary vs. Stabilizing).
Gravity (The Constant Force)
- Center of Gravity (COG): The point where the mass of the body is concentrated.
- Human COG: Anterior to the S2 vertebra (Sacral segment 2) in anatomical position.
- COG shifts up when arms are raised, and down when knees are bent.
- Line of Gravity (LOG): An imaginary vertical line passing through the COG to the ground.
- Base of Support (BOS): The area beneath an object or person that includes every point of contact with the supporting surface.
Equilibrium (Stability)
| Type | Definition | Example |
|---|---|---|
| Stable | COG is low, BOS is wide. If disturbed, COG rises and returns to position. | Lying flat, Sitting with wide BOS. |
| Unstable | COG is high, BOS is narrow. If disturbed, COG falls and stability is lost. | Standing on one leg, Handstand. |
| Neutral | COG remains at same level when moved. | A ball rolling on the floor. |
Fixation vs. Stabilization (Exam Critical)
- Fixation: Preventing motion mainly at the origin of a muscle so the force is transmitted to the insertion. (e.g., Holding the scapula to allow deltoid to work).
- Stabilization: Controlling movement to maintain a posture or background for movement (often dynamic).
2. Axes and Planes
Movement occurs in a Plane and around an Axis. The axis is always perpendicular to the plane.
- Sagittal Plane (Frontal Axis): Flexion / Extension (e.g., Walking, Bicep Curl).
- Frontal/Coronal Plane (Sagittal Axis): Abduction / Adduction (e.g., Jumping Jacks).
- Transverse/Horizontal Plane (Vertical Axis): Rotation (e.g., Turning head).
3. Laws of Motion (Newton)
- 1st Law (Inertia): A body remains at rest until acted upon. PT App: Weak muscles struggle to start a movement (overcoming inertia).
- 2nd Law (Acceleration): F = ma. Acceleration is proportional to Force. PT App: More strength needed to move a limb faster.
- 3rd Law (Action-Reaction): For every action, equal and opposite reaction. PT App: Ground Reaction Force (GRF) during gait.
4. Physics of Movement
| Term | Definition |
|---|---|
| Speed vs Velocity | Speed is scalar (how fast). Velocity is vector (speed + direction). |
| Work | Force × Distance. (Joules). No work is done in Isometric contraction (Distance = 0). |
| Power | Rate of doing work. (Work / Time). Essential for plyometrics. |
| Friction | Resistance between surfaces. Advantage: Grip. Disadvantage: Blisters, Joint erosion. |
5. Levers (The "FLE" Rule)
A rigid bar moving around a fixed point (Fulcrum).
Components: Fulcrum (F), Load/Weight (W), Effort (E).
Memory Trick: "FLE" (1-2-3)
The letter in the middle determines the class.
1st Class: F in middle.
2nd Class: L (Load) in middle.
3rd Class: E (Effort) in middle.
The letter in the middle determines the class.
1st Class: F in middle.
2nd Class: L (Load) in middle.
3rd Class: E (Effort) in middle.
| Class | Arrangement | Advantage | Human Body Example |
|---|---|---|---|
| 1st Class (See-saw) | E - F - W | Variable | Atlanto-Occipital Joint (Nodding head). F: Joint, W: Face, E: Neck Extensors. |
| 2nd Class (Wheelbarrow) | F - W - E | Force (>1) | Standing on Toes (Calf Raise). F: Toes, W: Body weight, E: Calf Muscle. |
| 3rd Class (Forceps) | F - E - W | Speed/ROM (<1) | Biceps Curl (Elbow Flexion). F: Elbow, E: Biceps insertion, W: Hand. *Most common lever in the body. |
6. Angle of Pull & Pulleys
Angle of Pull
- The angle between the muscle tendon and the bone it inserts into.
- 90 Degree Rule: A muscle is most efficient at moving a bone when the angle of pull is 90 degrees. All force is rotary.
- < 90 Degrees: Most force goes to Stabilizing the joint (compressing).
- > 90 Degrees: Most force creates a Dislocating component (distraction).
Pulleys
- Fixed Pulley: Changes direction of force only. No mechanical advantage (MA = 1). Example: Cervical Traction, Theraband attached to door.
- Movable Pulley: Halves the effort required. MA = 2.
- Anatomical Pulley: A bony prominence that changes the direction of muscle pull to improve the angle of insertion (Increases Mechanical Advantage).
- Example: Patella (for Quadriceps), Malleolus (for Peroneals).
📝 20 High-Yield MCQs
Test your knowledge for AIIMS/JIPMER.
Q1. The Center of Gravity (COG) in an anatomical standing position is located at:
Rationale: In an average adult human standing anatomically, the COG lies slightly anterior to the second sacral vertebra (S2).
Q2. Which class of lever is the most common in the human body?
Rationale: 3rd class levers (Effort in the middle) are the most common. They sacrifice force advantage to gain speed and range of motion, which is essential for human limbs.
Q3. The Patella acts as an anatomical pulley to improve the efficiency of which muscle?
Rationale: The patella pushes the quadriceps tendon away from the axis of the knee joint, increasing the moment arm and improving the mechanical advantage for extension.
Q4. At what angle of pull is a muscle most efficient at producing rotation (movement)?
Rationale: At 90 degrees, the entire force vector is rotary (perpendicular to the lever). There is no stabilizing or dislocating component wasted.
Q5. "Every action has an equal and opposite reaction" is Newton's:
Rationale: This is Newton's 3rd Law. In PT, this is visualized as Ground Reaction Force (GRF) when walking.
Q6. Elbow flexion (Biceps Curl) is an example of which lever?
Rationale: The Effort (Biceps insertion) is between the Fulcrum (Elbow joint) and the Load (Hand/Dumbbell). This is the definition of a 3rd class lever (FLE).
Q7. If the COG is raised and the Base of Support is narrowed, the equilibrium becomes:
Rationale: High COG and narrow BOS facilitate easy movement but reduce stability (Unstable equilibrium), e.g., standing on tiptoes.
Q8. Nodding the head (Atlanto-Occipital Joint) represents which lever?
Rationale: The Fulcrum (AO Joint) is between the Load (Face weight anteriorly) and Effort (Neck extensors posteriorly). This is a 1st Class lever.
Q9. What is the mechanical advantage (MA) of a single fixed pulley?
Rationale: A fixed pulley only changes the direction of the force. It does not reduce the effort required. MA = 1.
Q10. The rate of doing work is defined as:
Rationale: Power = Work Done / Time. It requires both strength (force) and speed.
Q11. Standing on your toes (Heel raise) is conventionally described as which lever?
Rationale: Fulcrum is the toe, Load is body weight (through tibia), Effort is the calf muscle (Achilles tendon). Load is in the middle.
Q12. Movements of Flexion and Extension occur in which plane?
Rationale: Sagittal plane divides the body into left and right. Forward/backward movements (flexion/extension) happen in this plane around a frontal axis.
Q13. If the angle of pull is less than 90 degrees, the major component of force is:
Rationale: When the angle is acute (<90), the muscle pulls the bone *into* the joint, creating stability or compression.
Q14. Which law explains why a weak patient struggles to START a movement?
Rationale: An object at rest stays at rest. Overcoming the initial inertia requires more force than maintaining the movement.
Q15. Which class of lever always offers a Mechanical Advantage > 1 (Force advantage)?
Rationale: In 2nd class levers, the Effort Arm is always longer than the Weight Arm (E > W), meaning you can lift a heavy load with less effort.
Q16. Fixation is primarily used to:
Rationale: Fixation holds the bone of origin steady so that the muscle contraction moves the insertion bone effectively.
Q17. No work is done during which type of muscle contraction?
Rationale: Physics Definition: Work = Force × Distance. In isometric contraction, there is force but zero distance moved. Therefore, Work = 0.
Q18. A cone balancing on its tip is an example of:
Rationale: The COG is high and the BOS is a single point. Any slight disturbance will lower the COG and the object will fall.
Q19. The perpendicular distance from the line of action of the force to the fulcrum is called:
Rationale: The moment arm determines the torque. A longer moment arm (e.g., using a wrench) creates more torque with less force.
Q20. Which plane does Abduction and Adduction occur in?
Rationale: The frontal plane divides the body into front and back. Lateral movements like abduction/adduction occur along this plane.
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