The motor unit recruitment control strategy of the antagonist thigh muscles was determined during linearly increasing isometric flexion and extension of the knees. The median frequency (MF) of the power density spectrum of surface electromyograms recorded from eighteen subjects was used as an indicator of motor units recruitment. The results showed that different recruitment strategies were used by the muscles that are antagonist to each other. When quadriceps and hamstrings acted as agonist most of the motor units were recruited in a linear manner up to 60% of maximal voluntary contraction (MVC). When the muscles acted as antagonist, quadriceps motor units were recruited up to 40% MVC during flexion while during extension the antagonist hamstrings recruited motor units up to 60% MVC. In both muscles, when acting as antagonist, the MF decreased after the recruitment phase, whereas when functioning as agonists, the MF remained relatively constant past the recruitment phase. The results suggested that a single muscle can employ different motor units recruitment strategies in accord with the type of contraction it performs and while acting as an agonist muscle as well as an antagonist.