Apparel for monitoring optimal body shell temperature during physical activity. The apparel uses thermochromic dyes and thermochromic inks to indicate changes in body shell temperature as physical activity commences and progresses. The thermochromic dyes and inks reveal when the body shell temperature has reached a temperature indicating optimal muscle performance, and inform the wearer of the apparel whether to increase or decrease the intensity of the physical activity to establish or maintain the optimal body shell temperature.
