Charcot-Marie-Tooth disease is a genetically heterogeneous disorder, characterized by length dependent degeneration of motor and sensory nerve fibers. The variability in clinical severity is typical, and is considered the result of environmental factors interacting with the genotype. We report the case of 2 sisters who present the same muscle involvement, with complete atrophy below the knees, but a very different degree of disability: the milder affected sister can walk independently for 1 km, the more severe for 50 m only. The factor, responsible for the different functional impairment, is the persistence of ankle dorsiflexion. This is the result of prolonged stretching of the heel cords in the more severely affected sister, which destabilizes stance and ambulation. On the contrary, the milder affected sister presents a slight equinus deformity well compensated by wearing shoes with a bit of heel. The presented cases and biomechanics suggest that the persistence of range of motion in dorsiflexion, when the triceps surae muscles weaken beyond a certain degree, represents an important negative factor of clinical variability in Charcot-Marie-Tooth disease; this can be averted by correct rehabilitation management.