8 male subjects were tested to elucidate the organization of EMG activities in mono- and bi-articular thigh muscles when hip and knee extension are combined. 2 types of isometric movement, single and dual joint movements, were studied: 1) 20% of maximal voluntary contraction (MVC) in separate hip extension (HE20) and knee extension (KE20), 2) simultaneous HE20 and KE20, combined voluntarily (HE20.KE20). In HE20.KE20, the value of the integrated EMG (IEMG) from the muscles tested was normalized as a percentage (%IEMG) of the IEMG of each muscle in HE20 for gluteus maximus (GM) and semimembranosus (SM), and KE20 for vastus medialis (VM) and rectus femoris (RF). The average %IEMG was 50.5 +/- 16.9% for GM, 42.1 +/- 6.1% for SM, 153.4 +/- 22.8% for VM and 66.6 +/- 18.7% for RF. These data suggest that the EMG activities of GM, SM and RF are inhibited and the EMG activity of VM is facilitated by combining hip extension with knee extension.