The relationships between electromyographic (EMG) activity and force as well as muscle blood flow and work have been well established. However, the association between muscle blood flow and EMG activity remains unsolved. Thus, to test the hypothesis that muscle EMG activity relates to muscle perfusion in different compartments of the quadriceps femoris (QF) muscle, 12 healthy male subjects were studied. During two very submaximal exercise bouts, at different exercise intensities, oxygen labelled radiowater and positron emission tomography were used to measure muscle perfusion. In addition, produced force of knee extensors and muscle EMG activity in the vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) muscles were recorded during both exercise bouts. Although the exercise intensity and average force production was higher during the second exercise bout (38 ± 15 versus 51 ± 17 N; P = 0·007), the mean EMG activity was lower (RF; P<0·001) or unchanged (VL; P = 0·722 and VM; P = 0·640). During the second exercise period, perfusion also remained unchanged in the entire QF muscle (P = 0·223) and in its separate muscles (VL, P = 0·703; VM, P = 0·141; RF, P = 0·113) in a group level. However, the individual changes in muscle perfusion were tightly related to changes in muscle EMG activity in VL (r = 0·84; P = 0·002) and VM (r = 0·68; P = 0·015) but poorly in the RF muscle (r = 0·40; P = 0·257). In conclusion, the different associations between muscle perfusion and EMG activity in different QF muscles suggests specific functional role of the vasti muscles and the RF muscle.