biceps and triceps

Biceps and triceps muscles in the upper arm. Left: biceps contracted. Right: triceps contracte.

The biceps is either of two muscles that are split in two in their upper part to form a Y-shape.

Biceps brachii is the chief upper arm muscle; it runs along the humerus – the large bone of the upper arm – and is connected to the radius at one end and the scapula, or shoulder-blade, at the other. Contraction of the biceps causes the arm to flex (see flexor at the elbow joint. It works antagonistically with the triceps, which contracts to extend the arm (see antagonist).

Biceps femoris is a thigh muscle, attached to the pelvis, the femur, and the fibula.