It's pretty common knowledge that magnesium can help against cramps; I didn't really know about muscle soreness;
But the thing is: it helps only if the cramp, or the muscle soreness, is caused but a magnesium deficiency. There can be many causes; if the cause is something else and your body already has enough magnesium, taking extra magnesium won't help. I guess in the case of magnesium it doesn't really hurt if you take somewhat more than you need, so it's probably fine to experiment within reason.
But the thing is: it helps only if the cramp, or the muscle soreness, is caused but a magnesium deficiency. There can be many causes; if the cause is something else and your body already has enough magnesium, taking extra magnesium won't help. I guess in the case of magnesium it doesn't really hurt if you take somewhat more than you need, so it's probably fine to experiment within reason.