Levers and Force Couples

Levers

• Principle of a rigid bar, rod, or segment rotating about its pivot point or fulcrum when acted upon by forces that produce rotation.
• Requires at least two linear forces, each acting at a distance from the pivot point or axis.

Simple Levers: 4 Variables

• Muscle force/Effort (EF).
• Moment arm for the muscle force/Effort (EA).
• Resistance Force in the form of gravity/external force/load pulling on segment/system (RF).
• Moment arm for the Resistance (RA).

First Class Lever

• The forces are applied on opposite sides of the fulcrum or axis.
• The forces will attempt to cause rotation in opposite directions.

Second Class Lever

• The (external) resistance force/load is in between (muscle) effort force and axis.
• Both forces act at a distance from the same side of the axis.
• Strength lever.

Third Class Lever

• The (muscle) effort force is between the (external) resistance force/load and axis.
• Fast (speed) lever.

Lever Examples

• First-class lever: load-fulcrum-effort arrangement (e.g., scissors).
• Second-class lever: fulcrum-load-effort arrangement (e.g., wheelbarrow).
• Third-class lever: load-effort-fulcrum arrangement (e.g., tweezers or forceps).

Levers in the Body

• First-class lever: raises your head off your chest (posterior neck muscles provide the effort, the atlanto-occipital joint is the fulcrum, and the weight to be lifted is the facial skeleton).
• Second-class lever: standing on tip-toe (effort by the calf muscles pulling upward on the heel, joints of the ball of the foot are the fulcrum, and the weight of the body is the load).
• Third-class lever: flexing the forearm by the biceps brachii muscle (effort exerted on the proximal radius of the forearm, the fulcrum is the elbow joint, and the load is the hand and distal end of the forearm).
• Mnemonic: FLE-123 (Fulcrum, Load, Effort and their positions for lever classes 1, 2, 3, respectively).

Mechanical Advantage

• Mechanical Advantage (MAd) = Effort Arm (EA) / Resistance Arm (RA) MAd = \frac{EA}{RA}
• If MAd > 1.0, the internal (muscle) force has the advantage; a small muscle force can overcome a larger resistance.
• If MAd < 1.0, a larger amount of internal force is needed to overcome a smaller resistance, or a small amount of external resistance can overcome a larger amount of internal force.

Mechanical Advantage by Lever Class

• Determining which lever class has a mechanical advantage depends on the ratio of Effort Arm (EA) to Resistance Arm (RA).

Clinical Application

• Considering mechanical advantage/disadvantage during manual muscle testing (e.g., Quadriceps and Deltoids during knee extension/shoulder abduction).

Force Couples

• Forces acting together to move an object around a pivot point.
• Example: Scapular force couple (Lower trapezius, Upper trapezius, Upper serratus anterior, Lower serratus anterior).

Levers - Exercise

• The drawing represents the lower extremity with the joint axis at the left side of the bar.
• HF = Hamstrings Force, G1 is gravity pulling on the straight leg, W1 is an ankle weight added to the leg
• Most joints are third class levers where the Resistance has the advantage.
• Segment/system weight does not change (RF).
• Moment arm for muscle at a given joint angle does not change.
• Only 2 variables left are EF (muscle force) and RA (moment arm for resistance RF).
• If want to have muscle (EF) work harder (create more force): Increase (RA).
• If want to have muscle (EF) work less (decrease force): Decrease (RA).

Resistance Moment Arm

• Point of application for resistance moment arm, RA, is at system Center of Mass (CoM).

Hand Positioning

• Hand positioning with Proprioceptive Neuromuscular Facilitation (PNF) diagonals to change torque.
• Resistance applied at the wrist yields greater moment/torque than the same resistance applied at the forearm or elbow.

Quantifying Basic Kinetics

• Calculations for 2D and 3D linear forces, moments, powers, etc. are more involved.
• Terms such as “forward solution”, “inverse dynamics approach”, cross-products, and matrices may be used in articles calculating kinetics.
• Simplified examples are used to conceptually understand the type of loading the body undergoes.