The Muscular System

Overview of the Skeletal Muscular System

The muscular system consists of all the voluntary skeletal muscles in the human body. It is responsible for producing body movements and stabilizing the body, such as maintaining posture or joint position. Approximately 700 muscles make up the muscular system.

• Skeletal muscles are under voluntary control and are attached to bones.

• The primary functions include movement, posture maintenance, and joint stabilization.

Skeletal Muscles: Structure and Attachments

Muscle Attachments and Actions

Skeletal muscles are attached to bones by tendons. When a muscle contracts, it pulls on the tendon, which then pulls the bone, resulting in movement at a joint.

• Most muscles cross at least one joint.

• The origin is the proximal attachment (usually stationary), and the insertion is the distal attachment (usually moves).

• All muscles provide a specific action or joint motion.

Muscle Belly

The muscle belly is the fleshy portion of the muscle between the proximal and distal tendons. This is where the majority of muscle contraction occurs.

Fascicle Arrangements

Definition and Functional Significance

Fascicles are bundles of muscle fibers within a muscle. The arrangement of fascicles affects the muscle's range of motion (ROM) and power.

• Longer fascicles provide greater range of motion.

• Thicker and shorter fascicles provide more power.

Types of Fascicle Arrangements

There are five main types of fascicle arrangements, each providing different functional characteristics:

1. Parallel: Fascicles are parallel to the longitudinal axis of the muscle and terminate at either end in flat tendons. These muscles provide a large range of motion but little power. Example: Sartorius.

2. Fusiform: Fascicles are nearly parallel to the longitudinal axis, but the muscle tapers toward the tendons, making it thinner at the ends than at the belly (cigar-shaped). Example: Biceps femoris.

3. Circular: Fascicles are arranged in concentric circles, forming sphincter muscles that enclose an orifice. Example: Orbicularis oris (mouth).

4. Triangular (Convergent): Fascicles spread over a broad area and converge at a thick central tendon, giving the muscle a triangular shape. Example: Pectoralis major.

5. Pennate: Fascicles are short and attach obliquely to a central tendon. There are three types:

   • Unipennate: Fascicles on one side of the tendon (e.g., extensor digitorum longus).

   • Bipennate: Fascicles on both sides of a centrally positioned tendon (e.g., rectus femoris).

   • Multipennate: Fascicles attach to several tendons (e.g., deltoid).

   Pennate muscles provide greater power due to the high number of fascicles.

Summary Table: Fascicle Arrangements

Additional info: The arrangement of fascicles is a key determinant of a muscle's mechanical advantage, influencing both the force and the range of movement a muscle can produce.