Hi Rosiegsm,
I appreciate your problem and agree with most of the comments above. Yes, it is important that you should try to get expert help, possibly from a physio or an Alexander teacher. Take your instrument and stool with you. A few pieces of information are missing from your opening statement. How tall are you? German or French bow? Does the pain happen when you are both sitting or standing? Is the pain just below your deltoids (shoulder cap), at the top of your bicep? Can you post a picture or more of how you sit or stand with the instrument?
Some broad suggestions (perhaps already made) -
(1) Posture therapy and ergonomics checkouts.
(2) Exercise with light weights and many repetitions to add strength to your torso and upper body.
(3) Stretches before and after playing, especially for your neck and shoulders.
When sitting, some immediate suggestions to try -
(1) Examine your posture for comfort without your bass. Is your stool about 27 inches or more high? Sit more forward on it with both feet on the floor so that if you picked your feet up you might fall forward. If you have done some Alexander work you will know how to lengthen your spine by extending your head "forward and up". This should help you to not slouch because you will feel your lower back muscles tighten and produce a small curve in your lumbar region. Feel for this curve with your hand behind your back. The act of lengthening your spine will also help pull and roll your shoulders back and align them better with your torso. Bend forward from your hip joints with your spine extended like this. Try to do all this without much tension.
(2) After making yourself comfortable then bring the bass to you. Adjust the end pin so that the bass is a little more vertical, the nut is just higher than your left ear and the neck comes further forward, almost level with your face. Turn the bass more to the right and lean it more across your body and towards the inside of your left knee to create a hands-free stand. Where they rest against your body the bass ribs are lower and should be just below your right ribs. The extent to which you turn in is to just avoid leaving a stripe of rosin on your trousers when bowing the E string. Picture an arrow pointing out from your chest roughly towards the bridge and strings where your hands are working. The bass neck should be far enough forward that you don't pull your elbow up higher than and back behind your shoulder to reach the lower positions. Yet you should still be able to bow comfortably at a good contact point.
This following is my own approach, copied from my contribution to a thread started by MaxJacob titled Standinding vs Sitting, 8 - 31 - 2008. There was also a similar thread started by bassistpatrick in May, 2009.
Standing or sitting I try to keep an arrow from my chest pointing towards the bridge or close to it. I also position the bass upright enough to have the strings forward about level with my face. The combined effect is to have my hands and arms doing the work in front of me and weighing back towards my body. My left elbow is forward of and lower than my shoulder to avoid neck and shoulder problems and give enough room between hand and shoulder for relaxed ease of shifting and vibrato. Also, standing or sitting, I set height of bass so that the nut is about 1 inch higher than the top of my left ear when using lower positions, which a compromise of posture between ease of use of left hand and ease of maintaining good bow contact points with the bow arm.
When standing (my feet are comfortably spaced with my left foot slightly forward of my right and left toe turned out slightly, like a sporting stance). The bass leans towards my body so that the corner between back and upper rib is resting in my left groin and the bass balances around to the right so the left thumb carries little weight and shifting and vibrato are as free and relaxed as possible. If you check this stance out you will see that the arrow from your chest is pointing at the bridge and the French bow will just miss your trouser leg when playing on the E string.
When sitting I have the back corner (between rib and back) further across my body to the right and tucked in under my right ribs where the bass ribs are lower. The bass is angled in towards my right leg and body and leans back across my body towards the support of my left knee. Both my feet are on the floor and, at 5 feet 7 inches tall, I am half sitting half leaning on the stool (a taller person can sit right down on top of the stool) The lower right rib is against my right calf, the arrow still points towards the bridge and the bow on the E string still misses my trouser leg. The height of the nut is again 1 inch above the top of my left ear, the bow contact point reachable and the left arm free to shift and vibrate. Both arms still weigh in towards the body.
This seating position allows pretty good posture and works for me when practicing or doing orchestral playing but I would choose to sit differently to play high solo material. I would also choose a smaller bass than mine, with more dropped shoulders too!!!!!
Much depends on your height and shape as well as the size and shape of your bass, and whether your are a French or German user.
DP
Ps Using a block under your left foot is fine. I believe lifting your left knee too high(on a high stool rung) risks distorting posture and putting too much strain on your lower back. Someone once told me your thighs should be hanging down at about 45 degrees to the floor when sitting on a stool for best posture. Can an Alexander teacher confirm this?
