I’ve heard others hint at this, but have never seen an explanation. Why are my shoulders/rotator cuff’s more susceptible to soreness and pain when swimming with a swim buoy vs. swimming without one?
My guess is that your body position and stroke mechanics are different when using a pull buoy. For most swimmers the load up front is lessened by the bouy as it raises your feet and legs. However, you may find that you rotate your body less with the buoy. By swimming flat, your arm is sent wide on recovery and this puts added strain on the small muscles of the shoulder (infraspinatus, supraspinatus, teres major, teres minor, deltoids). As well, as you catch and pull without rotation you lose the added strength of the lastissimus dorsi and rely more on the shoulder to move water. I would have someone watch your stroke with and without the bouy.
KP
If you have a powerful kick that is removed, you may find additional load on the shoulders using a buoy.