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STENSDOTTER, A.-K., P. W. HODGES, R. MELLOR, G. SUNDELIN, and C. HÄGER-ROSS. Quadriceps Activation in Closed and in Open Kinetic Chain Exercise. Med. Sci. Sports Exerc., Vol. 35, No. 12, pp. 2043–2047, 2003.For treatment of various knee disorders, muscles are trained in open or closed kinetic chain tasks. Coordination between the heads of the quadriceps muscle is important for stability and optimal joint loading for both the tibiofemoral and the patellofemoral joint. The aim of this study was to examine whether the quadriceps femoris muscles are activated differently in open versus closed kinetic chain tasks.Ten healthy men and women (mean age 28.5 ± 0.7) extended the knees isometrically in open and closed kinetic chain tasks in a reaction time paradigm using moderate force. Surface electromyography (EMG) recordings were made from four different parts of the quadriceps muscle. The onset and amplitude of EMG and force data were measured.In closed chain knee extension, the onset of EMG activity of the four different muscle portions of the quadriceps was more simultaneous than in the open chain. In open chain, rectus femoris (RF) had the earliest EMG onset while vastus medialis obliquus was activated last (7 ± 13 ms after RF EMG onset) and with smaller amplitude (40 ± 30% of maximal voluntary contraction (MVC)) than in closed chain (46 ± 43% MVC).Exercise in closed kinetic chain promotes more balanced initial quadriceps activation than does exercise in open kinetic chain. This may be of importance in designing training programs aimed toward control of the patellofemoral joint.