Exercise apparel, suitable for monitoring optimal body shell temperature in a wearer, comprising a plurality of thermochromic dyes that independently change colour at a specific temperature along a temperature continuum so as to indicate a temperature of the body shell in an area of the body. Using the apparel during physical activity, the thermochromic dyes and inks reveal when the body shell temperature has reached a temperature indicating optimal muscle performance, and thus 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. The thermochromic dyes or inks may be impregnated in or coated on the fibres of the fabric of the apparel. Methods for dyeing apparel using thermochromic dyes are also described in the application.
