The direct association between improved upper leg mus- cle strength and the outcome of exercise therapy is in line with three previous studies [10–12]. In a fourth study [14], the association between improvement in quadriceps strength and treatment response was only of borderline significance (P = 0.06), which can be attributed to the short duration (6 to 8 weeks) and low frequency (1 session/week) of the intervention, resulting in minimal improvement in strength.