From the limited understanding of anatomy/physiology and biomechanics I have, I am understanding that that ligaments for the most part are responsible for attaching bone to bone and for aiding in joint stability.
Having said the above, the UCL at least from my limited perspective would serve the same function, joining the humerus to the radius and ulna medially and providing stability medially.
We know that the anterior muscles of the forearm are responsible for wrist flexion and pronation and originate for the most part on the medial aspect of the humerus.
The posterior muscles of the forearm are responsible for extension of the wrist as well as supination.
If I were to take an educated guess I would say that the “stress” would be medial” as the anterior muscle group is responsible for pronation and that is where where for the most part the origin of the muscles that perform that function originate.
I am a bit confused, however, about how the “stress on the UCL” would occur if tendons pulling on bone cause movement and not ligaments?
I understand that excessive forces such as in throwing can lead to a UCL tear, however, I don’t see how simple pronation and supination of the forearm causes stress at the UCL.