I've had problems with my back muscles fishing from the shoreline too. I have noticed that it is usually the worst on cold windy days.
One thing I have tried to loosen up the muscles and ease the pain is a product called Kwan Loong Oil. It is a strong muscle rub similar to Tiger Balm but much more powerful.
It is made in Singapore and and imported by Prince of Peace Ent., Hayward CA, 94545,
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