Muscles: Structure, Function, and Actions at the Shoulder Joint

Muscles

This section introduces the concept of skeletal muscles, their anatomical features, and their significance in human movement and physique. The image of a professional bodybuilder (Phil Heath, Mr. Olympia 2017) highlights the development and visibility of major muscle groups.

• Skeletal muscles are responsible for voluntary movements and are attached to bones via tendons.

• Muscle size, definition, and symmetry are key aspects in bodybuilding and anatomical study.

• Major muscle groups visible in the image include the pectoralis major, deltoids, biceps brachii, rectus abdominis, and quadriceps femoris.

Actions of Muscles – Shoulder Joint

Overview of Muscle Actions at the Shoulder

Muscles produce movement at joints by contracting and pulling on bones. The action produced depends on the muscle's location relative to the joint.

• Flexion: Movement that decreases the angle between two body parts (e.g., raising the arm forward).

• Extension: Movement that increases the angle between two body parts (e.g., moving the arm backward).

Muscle Actions Based on Position Relative to the Joint

• Muscle crossing the anterior side of a joint produces flexion.

• Muscle crossing the posterior side of a joint produces extension.

Examples of Shoulder Joint Actions

Muscle	Position Relative to Joint	Action	Example
Pectoralis major	Anterior	Flexion of the shoulder	Raising the arm forward
Latissimus dorsi	Posterior	Extension of the shoulder	Moving the arm backward

Antagonistic Muscle Pairs

• Muscles often work in pairs called antagonists. When one muscle contracts to produce a movement, its antagonist relaxes and lengthens to allow the movement.

• Example: The pectoralis major (flexion) and latissimus dorsi (extension) are antagonists at the shoulder joint.

Key Terms

• Flexion: Decreasing the angle at a joint (e.g., arm forward).

• Extension: Increasing the angle at a joint (e.g., arm backward).

• Antagonist: A muscle that opposes the action of another muscle.

Summary Table: Actions at the Shoulder Joint

Action	Muscle Group	Example Muscle
Flexion	Anterior muscles	Pectoralis major
Extension	Posterior muscles	Latissimus dorsi

Additional info: In anatomical terminology, the direction a muscle crosses a joint determines its action. Muscles that cross the lateral or medial side of a joint may produce abduction or adduction, respectively, but these are not shown in the provided images.