Pennate versus parallel muscle fibres

In skeletal muscle, the arrangement of myocytes (muscle fibres) sets a trade-off between force and range. In a parallel muscle, the myocytes run in the same direction as the long axis of the muscle, as in sartorius, the long strap-like muscle of the thigh. The fibres lie end to end along the line of pull, so their shortening adds up and the joint moves through a large range of movement.

In a pennate muscle (from the Latin penna, a feather), the myocytes attach obliquely, at an angle, to a tendon, like the barbs of a feather slanting onto its central quill. Because the fibres slant, many short fibres can be packed into the same volume.

Maximal force follows the physiological cross-sectional area: the combined cross-section of all the fibres, measured at right angles to the fibres rather than the outer thickness of the muscle. Packing in more fibres means more force. The price is range: each fibre is short and pulls on the tendon at an angle, so the tendon moves only a short way.

When muscles of similar size are compared, fibre direction sets a trade-off between force and range of movement. Rectus femoris is bipennate (fibres on both sides of a central tendon) and the deltoid is multipennate (several tendon strands). Fusiform muscles, such as biceps brachii, have a spindle-shaped belly (a thick middle that tapers at both ends) and behave like parallel muscles.