A type of mint that is often used for addressing the discomfort of sore muscles, but spearmint oil can be very sensitizing due to its high volume of an ingredient known as methyl salicylate.
References for this information:
Toxicon, March 2007, pages 407-409
PDR for Herbal Medicines. 1st ed. Montvale, NJ: Medical Economics Company, Inc., 1998.