Pennate muscles

Rectus femoris
Muscles of the iliac and anterior femoral regions. (Rectus femoris highlighted in red.)
Details
Origin	anterior inferior iliac spine and the exterior surface of the bony ridge which forms the iliac portion of the acetabulum
Insertion	inserts into the patellar tendon as one of the four quadriceps muscles
Artery	lateral femoral circumflex artery
Nerve	femoral nerve
Actions	knee extension; hip flexion
Antagonist	Hamstring
Anatomical terms of muscle []

A pennate or pinnate muscle (also called a penniform muscle) is a muscle with fascicles that attach obliquely (in a slanting position) to its tendon. These types of muscles generally allow higher force production but smaller range of motion When a muscle contracts and shortens, the pennation angle increases

From the Latin pinnatus “feathered, winged,” from pinna “feather, wing.”

In muscle tissue, 10-100 endomysium-sheathed muscle fibers are organized into perimysium-wrapped bundles known as fascicles. Each muscle is composed of a number of fascicles grouped together by a sleeve of connective tissue, known as an epimysium. In a pennate muscle, aponeuroses run along each side of the muscle and attach to the tendon. The fascicles attach to the aponeuroses and form an angle (the pennation angle) to the load axis of the muscle. If all the fascicles are on the same side of the tendon, the pennate muscle is called unipennate (Fig. 1A). Examples of this include certain muscles in the hand. If there are fascicles on both sides of the central tendon, the pennate muscle is called bipennate (Fig. 1B). The rectus femoris, a large muscle in the quadriceps, is typical. If the central tendon branches within a pennate muscle, the muscle is called multipennate (Fig. 1C), as seen in the deltoid muscle in the shoulder.

One advantage of pennate muscles is that more muscle fibers can be packed in parallel, thus allowing the muscle to produce more force, although the fiber angle to the direction of action means that the maximum force in that direction is somewhat less than the maximum force in the fiber direction. The muscle cross sectional area (blue line in figure 1, also known as anatomical cross section area, or ACSA) does not accurately represent the number of muscle fibers in the muscle. A better estimate is provided by the total area of the crossections perpendicular to the muscle fibers (green lines in figure 1). This measure is known as the physiological cross sectional area (PCSA), and is commonly calculated and defined by the following formula (an alternative definition is provided in the main article):