For sprains and strains you need to use RICE Rest Ice Compression Elevation
Although, one of the simplest heating pads that can be made: fill a new tube sock 1/2 way up with about rice and tie the open end. Then Microwave it for about 7 min. (I know it will smell funny but it works. You can add herbs to it like Lavender or Rosemary) It will keep the heat for a long time and conform to what ever you need it to do. Its great for shoulders or around feet.
RICE
Rest
Ice
Compression
Elevation
Although, one of the simplest heating pads that can be made: fill a new tube sock 1/2 way up with about rice and tie the open end. Then Microwave it for about 7 min. (I know it will smell funny but it works. You can add herbs to it like Lavender or Rosemary) It will keep the heat for a long time and conform to what ever you need it to do. Its great for shoulders or around feet.