Triangular (convergent) muscle

Skeletal muscles are classed by how their fibres are arranged. The four main shapes are parallel, circular, fusiform (spindle-shaped) and triangular. A triangular muscle, also called a convergent muscle, has fibres that start from a broad attachment and converge towards a narrower tendon. Picture a hand fan: wide where it opens out, narrow where it is gathered at the handle.

Because the fibres are spread over a wide area before they gather onto one tendon, the pull of many fibres is gathered at a single point. Pectoralis major, the large muscle across the front of the chest, is a standard example. Its fibres come together on one tendon that attaches to the humerus (the upper arm bone). Temporalis, a muscle of the skull, has the same convergent design.

It helps to pair each shape with a familiar muscle. Sartorius, a thin, rectangular muscle, is parallel, with fibres that all run the same way. Orbicularis oris, around the mouth, is circular, with fibres surrounding an opening that narrows when they contract. Biceps brachii is fusiform, with a spindle-shaped belly that tapers at each end.

Pennate muscles are a separate description of how fibres attach to a tendon. Their fibres attach obliquely to a tendon, like the barbs of a feather, as in rectus femoris, where fibres slant onto both sides of a central tendon. That is a different idea from fibres fanning in from a broad attachment.

Convergence describes how the muscle is built. The force a muscle can produce depends on the contractile fibre area, the fibre length and how strongly it is activated.