Let us now take a look at the muscles of upper arm crucial in lifting weights. There are four muscles in this group viz. coracobrachialis, biceps brachii, triceps and brachialis. Brachium in biology is branching, fork etc. and therefore is used for arms which look like branches striking out of the trunk- the torso. Brachialis is the muscle in arm nearer elbow which helps flexion that is bending of forearm.
Coracobrachialis is a muscle arising from the scapula anterior or ventral i.e. front side. The word coraco probably refers to a crow/ raven/ jackdaw (Corvus corax/ splendens/ Corvus monedula) beak and 'oid' is for like. The origin of the muscle is from the coracoid processs of the shoulder blade. If you stand before a mirror and lift up your arm sideways you can see the coraclobrachialis muscle in between biceps and triceps. This muscle helps in bringing the forearm nearer the body from frontside.
Biceps brachii is the two headed, big bulky muscle on front side of the arm. It is the one youngsters like to and easily can flaunt as they get into weight training. Biceps bring the forearm up near the body and turn palm upwards. Biceps originate from humerus, the arm bone and are inserted on to the ulna, the thin bone in forearm.
Triceps brachii is a three headed, big bulky muscle on backside of the arm. It originates in the scapula, i.e. the shoulder blade and is inserted on ulna. This muscle is responsible for extension of forearm that is in taking away the forearm brought near body by the biceps brachii.
Since muscles can only pull and not push they work in pairs. Biceps and triceps are antagonistic that is have exactly opposite actions and form a pair controlling bending - straightening movement of arm at the elbow. When biceps contract the forearm is pulled nearer the body but for taking the forearm back into its position mere relaxation of biceps is not enough, the triceps must contract. Only then the arm straightens at the elbow which is called extension of the arm. The pair of biceps and triceps brings about the movement of flexion-extension at the hinge or ginglymus joint.
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