9.6F: Lever Systems

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Arrangement of muscles allows them to move relative to one another, while the insertion joint acts as the pivot point for a lever system.

Learning Objectives

Differentiate between flexors and extensors and between adductors and abductors

Key Points

A simple lever system can be used to describe skeletal muscle action. Levers are composed of a pivot, a fixed rod, and a load to which a force is applied.
The relative positions of the pivot, load, and force determine the type of lever and subsequent action.
Flexors and extensors work to adjust the angle between two body parts. Flexion decreases the angle and extension increases the angle.
Abduction and adduction are movements relative to the midline of the body. Abduction is the movement away from this midline while adduction is movement towards the midline.
Internal rotation brings the distal portions of the limbs inward towards the midline. External rotation is the opposite movement, pointing the distal portion of limbs away from the midline.
The elevation of a limb or body part moves it in a superior direction, while depression moves body parts in an inferior direction.

Key Terms

abduction: Movement away from the midline of the body.
rotation: The act of turning around a centre or an axis.
first class lever: Force and load to either side of the pivot.
second-class lever: Load between force and pivot.
third-class lever: Force between pivot and load.
origin: The relatively immovable point of attachment for a muscle.
insertion: The movable point of attachment for a muscle.
flexion: Reduces the angle between two body parts.
extension: Increases the angle between two body parts.
adduction: Movement towards the midline of the body.

Skeletal muscle is usually attached to a relatively immovable part of the body at one end and more mobile region on the other end of a joint. The attachment at the immovable end is referred to as the origin and at the moveable end, the insertion. Upon contraction, the insertion is pulled towards the origin. This movement can be described using a simple lever system. Muscles can have multiple origins and insertions which modulate the type of movement they produce.

Levers

A lever is composed of three parts: a fixed rod which is attached to a fulcrum (pivot) and a load. Depending on the relative position of the three components, levers can move heavy loads or move loads further or faster when a force is applied.

If a load is close to a pivot and the force is applied far from the pivot, then the lever is said to operate at mechanical advantage. A large but relatively small force can move a heavy object. The classic example of such a lever is a car jack. With big movements of the lever, the heavy car is lifted in small increments off the ground.

If a load is far from a pivot and a force is applied near to the pivot, then the lever is said to operate at a mechanical disadvantage. A large force is required to move a relatively small load, but the speed and distance at which that load can be moved is greatly increased. An example of this is a spade combined with forceful movements of the muscles in the arm, which results in large movements of the spade head.

In muscles, the joints are the pivots and the bones are the fixed rods. The load is the weight of the bone, associated tissues, and other objects being moved, and the force is applied by the muscle at its insertion point.

Classes of Levers

Levers can also vary based on the relative position of the load, pivot and point of force application. Classes of levers include:

First-class Lever

In a first class lever, the load and force sit on either side of the pivot like a seesaw. First-class levers are relatively uncommon in the body, but one example is the triceps brachii muscle of the upper arm which acts to extend the forearm. The force is applied at its point of insertion on the ulna in the forearm, the elbow is the pivot, and the load is the humerus in the upper arm. Thinking of the relative distance between the points of attachment, the triceps brachii can be said to act at a mechanical disadvantage.

Second-class Lever

In a second-class lever, the force is applied at one end with the pivot at the other and the load in between. Second-class levers are also relatively uncommon in the body. One example is raising yourself up on your toes. The pivot is based at the front of the foot, the load is the weight of the body, and the force is applied through the Achilles tendon in the heel. All second-class levers in the body act at a mechanical advantage since the force is always applied closer to the load than to the pivot.

Third-class Lever

In a third-class lever the force is applied between the load and the pivot. The majority of muscles in the body are third-class levers and all act at a mechanical disadvantage as the force is applied closer to the pivot than the load. An example of a third-class lever in the body is the biceps brachii, which flexes the forearm towards the shoulder. Originating at the scapula, the pivot is the elbow, with the force applied immediately after the elbow at the point of insertion on the radius of the forearm. The load is the forearm and any objects a person carries.

Types of Movement

Working together using the lever principles discussed above, skeletal muscles can induce a wide range of movements.

Flexors and Extensors

Flexors and extensors adjust and change the angle between two body parts. Flexion decreases the angle and extension increases the angle. For example, extension of the arm opens the angle of the elbow joint, while flexion allows for the bending of the arm. Flexion can also move inwards towards the body or forward, such as with hips or shoulders. Extension in this case moves the limbs towards the posterior side of the body lever.

Abduction and Adduction

Abduction and adduction are movements relative to the midline of the body. Abduction is the movement away from the midline of the body and adduction is movement towards this line. For example, moving arms or legs laterally away from the body is abduction, and bringing the limbs back to the midline is adduction.

Internal Rotation

Internal or medial rotation is specific to the shoulder or hip and brings the distal portions of the limbs inwards towards the midline. Internal rotation can also move the humerus and femur inward. External or lateral rotation is the opposite movement, pointing the distal portion of limbs as well as the humerus and femur away from the mid-line.

Depression and Elevation

A limb or a body part can be moved upward (or in a superior direction) through elevation. For example, the trapezius elevates the apex of the shoulder upward. Depression is the opposite of elevation, or moving body parts in an inferior direction.

This diagram depicts various muscle movements. From left: Angular movements, flexion and extension at the shoulders and knees; Angular movements, flexion and extension of the neck; Angular movements, flexion and extension of the vertebral column; Angular movements, abduction, adduction, and circumduction of the upper limb at the shoulder; Rotation of the head, neck, and lower limb.

Types of body movements: Muscle positioning around a joint determines the type of movement that is produced.