The present invention relates to a method for estimating the relative orientation between a tibia (11a) and a femur (11b) of a lower limb (10) of a person (1), the method being characterised in that it comprises the steps of: (a) During at least one reference movement of said lower limb (10) from a reference position, acquiring by first inertial measuring means (20a) integral with the tibia (11a) and by second inertial measuring means (20b) integral with the femur (11b) reference accelerations and angular velocities; (b) Estimating a plurality of morphological parameters of said lower limb (10), as a function of said measured reference accelerations and angular velocities; (c) Determining a relative orientation in said initial position between said tibia (11a) and femur (11b), as a function of said estimated morphological parameters of said lower limb (10); (d) During a working movement of said lower limb (10) from an initial position, acquiring by the first inertial measuring means (20a) and the second inertial measuring means (20b) working angular velocities; (e) Estimating the relative orientation between said tibia (11a) and femur (11b) as a function of said measured working angular velocities, and of said relative orientation in said initial position.