Abstract: An apparatus includes: a variable hair length module configured to adjust the lengths of hair strands that are rooted in a head by a hair rooting machine from a supply of hair thread, the variable hair length module including two arms configured to move relative to each other along respective continuous paths; and a control system configured to input programs of instruction to the variable hair length module to automatically control the relative movement of the two arms during operation of the hair rooting machine to root hair strands into the head. The hair thread is configured to engage with each of the two arms such that a relative movement of the two arms varies the length of each hair strand continuously within a hair length range that is defined by two extreme lengths.