The heaviness for me was due to lack of blood flow , lack of coordination followed if I continued the activity.

Ongoing heaviness , swelling & blue or mottled color changes in arms can be Venous TOS related..

In the past some had both NTOS & VTOS & Pec minor syndrome, or other combination of factors..
Like scalene issues, Extra C rib, large transverse process, large neck muscles or larger than normal bones..
A tricky process to figure out all of it.. if you can find out & work on the worst symptoms, the rest might follow.