In osteoarthritis (OA) of the knee, arthrogenous muscle inhibition (AMI) is considered to be one of the reasons of quadricep muscle weakness. Its influence on functional impairment such as gait alterations, however, still remains unclear. Fourty-seven patients with knee OA (mean age 64 +/- 5.2 years, 36 women, 11 men) and 47 age- and gender-matched healthy controls were investigated for voluntary activation (VA) and maximum voluntary contraction (MVC) of the quadricep muscle. In addition, these two parameters of AMI have been correlated with the extent of gait alterations. Measurement of VA and MVC were performed by a twitch interpolation technique; for gait analysis an optoelectronic motion analysis system (ELITE, Italy) was used. Quadricep MVC and VA were significantly lower in OA than in control knees. Moreover, in addition to reduced walking velocity and step cadence, gait analysis revealed significantly lower stance phase re-extension angles in OA patients (mean 2.7 degrees +/- 2.6) than in control knees (mean 10.7 degrees +/- 4.9). Hereby, in OA knees there was a significant correlation between MVC and VA deficits and the reduction of re-extension angles. The work shows that knee OA is characterized by severe alterations of the quadricep motor function due to arthrogenous muscle inhibition. Hereby, quadricep AMI represents one major reason for functional impairment in knee OA.