The length of the femur is measured in several ways, often from it's femoral head at the proximal end to its medial condyle at the distal end. Issues arise if their is a high degree of body fatness or muscle mass that may make these bony landmarks difficult or impossible to access. Some trainers measure from the the midpoint of the inguinal ligament to the proximal edge of the patella, which also has it's issues. The simplest way to derive an approximation of what the average femur length of a given population is to divide height by 4.
The Michigan State Basketball team's athletes are much taller than the general population and they wanted their Pendulum Machine to reflect their average femur length and had Rogers customize their Pendulum 3-Way Row.