The Sienna schooling shirt with 37.5® technology gives ever rider that classic, yet sporty look that will never go out of style. The patented thermoregulation technology has proven to help your body manage its core temperature and therefore, increase stamina. It keeps you comfortable by helping your body to maintain an optimum core temperature of 37.5° Celsius (99.5° Fahrenheit) which is ideal for peak athletic performance. The particles are naturally derived and work for the life of the product (it will never wash out!).