Musculoskeletal Levers

First Class Levers

Definition: axis of rotation between IMA and EMA with external and internal torque equal but producing opposing rotations

Mechanical Disadvantage: extensors of head and neck control posture in sagittal plane and must, therefore, produce more force to maintain force against gravity

• Ex. Forward head posture strains extensors creating a mechanical disadvantage with the EMA and external force of gravity being greater and extensors having to work towards failure

MA =/</> 1

Second Class Levers

Definition: axis of rotation located at one end of bone and IMA greatly exceeds EMA of body weight resulting in internal force possessing greater leverage than external force

Mechanical Advantage: Calf muscles produce a plantar flexion torque in sagittal plane that overcomes force of body weight easily

MA > 1

Third Class Levers

Definition: axis of rotation located at one end of bone creating a shorter IMA and, generally, the external weight has a greater leverage than muscle force

Mechanical Disadvantage: Flexors of elbow produce flexion torque in sagittal plane to support weight of forearm and hand pulling back into elbow extension by gravity

MA < 1

Mechanical Advantage

Mechanical Advantage = IMA / EMA