The Muscular System

Introduction

The muscular system is essential for movement, posture, and stability in the human body. Skeletal muscles, which are under voluntary control, play a key role in producing body movements by contracting and pulling on bones.

Muscle Movements, Roles, and Naming

The Five Golden Rules of Skeletal Muscle Activity

Understanding skeletal muscle activity is facilitated by five foundational principles:

• Rule 1: With few exceptions, all skeletal muscles cross at least one joint.

• Rule 2: The bulk of a skeletal muscle typically lies proximal to the joint it crosses.

• Rule 3: All skeletal muscles have at least two attachments: the origin and the insertion.

• Rule 4: Skeletal muscles can only pull; they never push.

• Rule 5: During contraction, a skeletal muscle's insertion moves toward its origin.

Types of Body Movements

Muscle Attachments: Origin and Insertion

Muscles are attached to bones at two main points:

• Origin: The attachment to an immovable or less movable bone.

• Insertion: The attachment to a movable bone.

• When a muscle contracts, the insertion moves toward the origin.

• Body movement occurs when muscles contract across joints.

Flexion and Extension

Flexion and extension are fundamental movements at joints:

• Flexion:

  • Decreases the angle of the joint.

  • Brings two bones closer together.

  • Common in hinge joints (e.g., knee, elbow) and ball-and-socket joints (e.g., hip).

• Extension:

  • Opposite of flexion.

  • Increases the angle between two bones.

  • Typical of straightening the elbow or knee.

  • Extension beyond 180° is called hyperextension.

Visual Example: Muscle Attachments

The brachialis muscle in the arm demonstrates origin (proximal, less movable) and insertion (distal, more movable) points. When the muscle contracts, the forearm moves toward the upper arm.

Additional info:

• Other types of movements (such as abduction, adduction, rotation, circumduction, dorsiflexion, plantar flexion, inversion, eversion, supination, pronation, and opposition) are also essential in describing joint actions and will be covered in further detail in subsequent notes.