Introduction and objectives: The aim of this study was to analyze antagonist coactivation (biceps femoris) during maximal isometric leg extension in road cyclists, and its relationship with agonist activation (rectus femoris, medial vastus, lateral vastus) and the force developed. Gastrocnemius activation was also monitored. Methods: Sixteen road cyclists carried out a maximum unilateral isometric contraction on a leg-extension machine, using their dominant leg. The test lasted 5 seconds and the isometric force and electromyographic (EMG) activity of 6 lower limb muscles was monitored. Results: Antagonist and gastrocnemius average EMG activity (EMG med) was 37.4% and 18.2% of the agonist EMG med respectively. Analysis of the EMG med of each of the 3 agonist muscles separately showed that the rectus femoris EMG med was significantly higher than the vastus medialis and vastus lateralis EMG med. Biceps femoris EMG med was negatively correlated with maximal isometric force. However, neither agonist nor gastrocnemious activation was related to the force developed. Conclusions: Antagonist coactivation limits the maximum isometric force on knee extension in road cyclists. This finding and excessive rectus femoris activation could be related to biomechanical differences between pedaling ­ the movement practiced by cyclists ­and leg extension on the machine.