Atrophy of the supraspinatous and infraspinatous muscles is quite common in the equine, especially of the draft breeds. This condition is commonly referred to as sweeney. In most cases the atrophy is due to pressure on the suprascapular nerve at the point where it crosses the neck of the scapula. A common cause of this pressure is an improperly fitted collar. Another common cause is working a green colt on a heavy load until he becomes tired and in resting himself he pulls sideways.
Lewis, R. Vaughn
"Atrophy of the Supraspinatous and Infraspinafous Muscles in a Young Colt,"
Iowa State University Veterinarian: Vol. 7
, Article 12.
Available at: https://lib.dr.iastate.edu/iowastate_veterinarian/vol7/iss4/12