Final answer:
The muscle that is located medial and deep to the biceps brachii is the brachialis muscle, which plays a key role in flexing the forearm.
Step-by-step explanation:
The muscle that is located a bit medial and deep to the biceps brachii is the brachialis muscle. This muscle is pivotal in providing the strength required for the flexion of the forearm. Deep to the biceps brachii, the brachialis muscle acts as a workhorse when it comes to elbow flexion, lying underneath the more superficial biceps brachii and contributing to the overall power of the movement.
While the brachialis serves a primary role in forearm flexion, other muscles such as the brachioradialis also assist in this motion. The brachioradialis is capable of flexing the forearm at a faster pace or aiding in lifting a load more gradually. It's interesting to note that the prefix 'brevis' refers to short, which may become relevant when studying muscle anatomy, but it does not directly apply to the muscles under consideration in this instance.