The Sienna Jr. with 37.5® technology is the perfect schooling style for even our youngest of R.J. Riders. The patented thermoregulation technology has proven to help your body manage its core temperature and 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!). Some colors feature a unique patterned mesh, while others feature a tonal mesh.