Conversation so far:
which muscle inserts on the lesser tubercle of the humerus
The subscapularis is a large triangular muscle which fills the subscapular fossa and inserts into the lesser tubercle of the humerus and the front of the shoulder-joint capsule.
what is the function of the subscapularis muscle
The subscapularis rotates the head of the humerus medially and adducts it; when the arm is raised, it draws the humerus forward and downward.

Question: where is the origin
Self-contained rewrite: where is the origin of the subscapularis muscle