Muscle Fiber Arrangements (Penniform vs. Fusiform)

There are 2 major kinds of muscle fiber arrangement (Ellison, 1991):

1. Penniform — fibers lie diagonally to the line of pull. Generally has a greater cross-section and produces more force (e.g., quadriceps). Subtypes:
   • Unipennate — muscle fibers on one side of the tendon only.
   • Bipennate — muscle fibers on both sides of a central tendon.
   • Multipennate — muscle fibers on both sides of a number of tendons.
2. Fusiform — long and thin, parallel fibers running in the same direction as the length of the muscle. Generally allows for speed of contraction but produces a small force because of its small cross-section (e.g., sartorius).

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

• 2 arrangements: penniform (diagonal fibers, greater cross-section, more force — e.g., quads) and fusiform (parallel fibers, speed, small force — e.g., sartorius).
• Penniform subtypes: unipennate, bipennate, multipennate.

• quadriceps — penniform example (greater force).
• sartorius — fusiform example (speed, small force).