Muscles can have two kinds of fibre arrangement, fusiform and penniform.

Fusiform: Fibres form the shape of a spindle or cigar and taper at both ends. The fibres run along the length of the muscle, and as a result, the amount that the muscle shortens when it contracts is proportional to the movement it causes. This is an advantage in terms of speed of movement. An example of a fusiform muscle is the brachioradialis of the arm.

Penniform: Fibres are arranged diagonally to the direction of pull, which means that more fibres can be used in a contraction. However, the range of motion is not so great as the fusiform type muscle. Penniform muscles are therefore to be found where the force is applied through short ranges of motion. An example of a penniform muscle is the rectus femoris muscle at the front of the thigh.