Why the biceps alone cannot make arm straighten?

The biceps brachii is a muscle that is located on the front of the upper arm. It is one of the two main muscles responsible for flexing the elbow joint (the other being the brachialis muscle). The biceps brachii is also responsible for supinating the forearm, which means turning the palm of the hand upward.

The triceps brachii is the muscle that is responsible for straightening the elbow joint. It is located on the back of the upper arm and is the largest muscle in the arm. The triceps brachii is also responsible for extending the forearm, which means turning the palm of the hand downward.

So, the reason why the biceps alone cannot make the arm straighten is because it is not the muscle responsible for this action. The triceps brachii is the muscle that is responsible for straightening the elbow joint.