My two cents:

In my rock band chord organ days I stood most of the time.

As I am really an organist, Hammond in the old days and Lowrey now, I always sit. I am a concert, dance music, jazzy type player. I don't sing, but sometimes I play trumpet with my right hand and keyboard with my left hand transposed down to Bb. I've had no problem for me or with any audiences because of sitting down. But, it probably is the reason I really prefer not to use a music rack. I think its important especially in my case that the audience can see my hands move and also I kind of move around on my seat, moving my legs tapping my feet, swaying back and forth etc. I have considered a video camera pointed at my hands with a TV monitor so the audience can really see what I do. I have done this before in concerts and people tend to be quite impressed with it.

As far as everybodys back is concerned, I learned a few things the hard way too. Make sure your wallet is not too thick, filled with a bunch of stuff. Empty it(send me the cash), or make sure you push it to the side so you don't sit on it. You would be amazed how many back aches come from that.

Make sure the heels on your shoes are not worn too much. I can vouch for this one big time.

Since I sit all the time, the trick to doing that for hours and feeling good and comfortable with no discomfort is to sit up straight, or you will have back discomfort, your lungs will not have enough room to get oxigen, meaning less fuel to the brain and body, so you will start to tire very rapidly. Found this out the hard way too. In my work (selling Lowrey organs) we used to spend a lot of time at the store front, standing and playing. At first this was tiring. But once you learn how to balance etc. it gets easy. At one time we had an organ in a Morrisons cafeteria at the mall. I was to play it every day for an hour or so. But I found myself getting tired, sleepy, I experienced lack of concentration and generally didn't play that great and wasn't having fun. After I analyzed it, I found I was leaning forward hunching over (very easy to do on a two manual organ), so I just tried to straighten up for an experiment and this took care of it.

Another example of posture. I have a student with a severe back problem. She was going to give up the organ because she could only play for about 15 minutes before she would have to quit. I spent some time with her shifting positions moving the bench forward and backward, having her sit closer to the front of the bench and then sliding back on the bench. I also worked with her arm and hand position, and helped her find her center of balance. The next day I called to see how it went. She had played for two hours with no back discomfort.

As far as lifting things in and out of a trunk, I can guarentee, the older you get you will not even question that this is very hard on your back, not to mention the joints in your arms and shoulders.

Scott Langholff