Muscle Fiber Arrangements (Penniform vs. Fusiform)

There are two major kinds of muscle fiber arrangement (Ellison, 1991):

1. Penniform — fibers lie diagonally to the line of pull. Generally:

• has greater cross-section
• produces more force (e.g., Quads)

Subtypes:

• a. Unipennate — muscle fibers on one side of the tendon only
• b. Bipennate — muscle fibers on both sides of a central tendon
• c. Multipennate — muscle fibers on both sides of a number of tendons

2. Fusiform — long and thin, with parallel fibers running in the same direction as the length of the muscle. Generally:

• allows for speed of contraction
• produces a small force of contraction because of its small cross-section (e.g., Sartorius)

Remember the "direction" of muscle fibers to understand how they contract.

• Penniform: fibers diagonal to line of pull; greater cross-section; more force (e.g., quads).
• Unipennate (one side of tendon), bipennate (both sides of a central tendon), multipennate (both sides of multiple tendons).
• Fusiform: long, thin, parallel fibers along the muscle length; favors speed; small force from small cross-section (e.g., sartorius).

• skeletal-muscle-structure — fibers, fascicles, and tendons.
• active-tension — fiber arrangement affects force production.
• quadriceps — penniform, high-force example.
• sartorius — fusiform, speed-favoring example.