None. Muscles only contracts. In order to extend, the muscle on the opposite movement has to contract. For example, the biceps help flex the elbow, but it does not extend the elbow; the triceps does that.
The biceps muscle relaxes!!!!!=)
The Neck Muscle
The knee joint is an example of a body joint that opens and closes as it flexes and extends during movement.
trapezius muscle
Triceps Brachii extends the forearm at the elbow joint.
extensor digitorum
extends your forearm
The erector spinae
The posterior deltoid is one extensor of the upper arm.
cranial tibial muscles
Abductor muscle - moves a limb away from the midlineAdductor muscle - moves a limb towards the midlineExtensor muscle - increase the angle at a joint - extends a limbFlexor muscle - decreases the angle at a joint - flexes a limbPronator muscle - turns a limb to face downwardsSupinator muscle - turns a limb to face upwardsRotator muscle - rotates a limbIn the human body, these are the responsible muscles.*Muscles which move the thigh and their action*Gluteus maximus - Extends and rotates thigh laterally.Adductor longus - Adducts, medially rotates and flexes the thighAdductor brevis - Adducts, laterally rotates and flexes the thighAdductor magnus - Adducts, flexes, laterally rotates and extends the thigh.Rectus femoris - Extends knee and flexes hipVastus lateralis - Extends kneeVastus medialis - Extends kneeVastus intermedius - Extends kneeSartorius - Flexes knee. Flexes hip and rotates femur laterallyBiceps femoris - Flexes leg and extends thighSemitendinosus - Flexes leg and extends thighSemimembranosus - Flexes leg and extends thigh
the shoulder muscle extends the arm