I got whiplash years ago and sometimes have muscle spasm problems in my shoulders and neck, so am glad to try new stuff.
Did you know that good old Listerine can be used for a temporary relief too? My doctor said it was probably because it has eucalyptol and menthol in it.
When I had to drive out of town for work I would splash some on my shoulders and then when it was dry, used a damp washcloth to get rid of the majority of the smell. Worked for me for the time I was driving.
If you ever try it though, watch which one you buy. I had gotten Natural Citrus Listerine one time (free with rebate) and it was sticky. Yuck!