Uncovering the FUTREX-6100XL prediction equation for the percentage body fat.

Based on the near infra-red (NIR) interactance method, the FUTREX company has developed a series of instruments, for the estimation of the body fat percentage (%BF). %BF is estimated through prediction equations incorporated in the instruments, which for the newest models (FUTREX-6100XL and FUTREX-6100A/ZL) are proprietary and they are not published anywhere. This missing knowledge may lead to several misunderstandings and confusion and degrades those instruments to 'black boxes'. The present work uncovers and presents the prediction equation of FUTREX-6100/XL and discusses the contribution of each term of that equation to the %BF. Furthermore, this study presents the method used, which can be used to uncover equations incorporated in other instruments. This method is based on the idea of firstly uncovering the dependence of the equation on each parameter separately and then combining those dependencies to uncover the unknown equation.