Swim coach had said that I need to develop my shoulder muscles which is why last time I was at the gym I tried working on them.  I did side raises with 10 pound dumbbells and I could hardly complete ten reps.  My hands just felt tremendously heavy - they just want to drop to my sides and stay there.  Dumbbell shrugs I did a little better on but any exercise where I lift my hands past my head is just, eh, tricky.  I then tried an exercise sans dumbbells - basically jumping jacks without the jumping - but even that was difficult - doing it in a slow controlled manner was no help either.  After just twenty reps, I could feel the tightness on my shoulders.  I didn't realize how weak my shoulder muscles were.  So I guess for the next few weeks I'll be hitting my shoulders harder. 

Shoulder Muscles: Delts & Rotator Cuffs, Traps & Scapulae.