The aim of this study was to investigate the influence of restriction of movement on the functional characteristics and myosin heavy chain composition of m. vastus medialis. The subjects of this study were 13 arthroscopically treated patients with meniscus lesions (both men and women, mean age 28 years). All measurements were performed and biopsies were taken bilaterally before and 6 months after knee arthroscopy. Thigh girth was measured to determine the degree of atrophy. A tendency towards decreased thigh girth in the injured leg was noted. As an estimate of functional characteristics, maximum voluntary contraction (MVC) was measured. The MVC of the injured limb was lower than that of the non-injured limb (mean values 548.2 +/- 54.1 and 705.4 +/- 48.2 N respectively; P < 0.05). The myosin heavy chain (MHC) composition was determined electrophoretically. The proportion of MHC I isoform decreased from 59.35% to 47.97% (P < 0.05).