The spasms are more than likely due to poor body mechanics. Your elbows should be at a 45* angle when you are sewing. If your machine is sitting on your table you will need to raise your chair height. If your chair isn't adjustable then I would invest in an office task chair without arms. To make do with what you have you can put one or two pillows on your chair seat and see if that raises you up enough. If the problem is the opposite ie table is too low you can either get bed risers to put under the legs or place some 2x4s under them to raise the table up. Do some stretching exercises often while you are quilting. Take frequent breaks stand up and stretch. Make sure you are properly hydrated as well.