From a very interesting article, which Im sure many will have seen before at
http://www.ksanti.net/free-reed/essays/back7.html
It matters not a hoot that your accordion weighs thirty pounds or twenty pounds (or in the case of the plastic fantastics sixteen pounds). The actual weight of the instrument has absolutely nothing to do with the ease of playing the instrument or getting injured. Provided, however, that you play the instrument in its proper playing position which is sitting down. What most people dont realize (obviously doctors and some manufacturers), is that in the sitting position the accordion weight rests on the thigh or thighs, thereby relieving all the actual weight from the shoulders or neck. There is absolutely no additional strain or compromise placed on the neck, shoulders or the spine when played in an energy efficient upright position.
However, for those of you who stand to play the reverse is true. Unless you are an entertainer, like Dick Contino or Myron Floren, whos solo presentation on stage is part of the show, there is no reason that one should stand. You dont see pianists standing to play to enhance their musical presentation. Why should an accordionist stand? It is a difficult enough instrument to master without creating physical and painful distractions by having the instrument hang off the shoulders and strain the neck and low back. The accordion is balanced nicely in the sitting position with the player assuming a position which is relaxed, upright and energy efficient. The number of accordionists I have seen professionally who have serious neck and shoulder problems over the years is considerable. Almost without exception, they are the performing, standing players. If you want to shorten your playing career, stand-up.