An assembly for coupling a valve stem to an actuating member of an actuator in a desmodromic valve actuation mechanism comprises a spherical bearing having two portions each of which defines a respective bearing surface which is complementary to the bearing surface defined by the other portion, at least one of the surfaces being part spherical, one of the portions being arranged to be coupled to the actuating member and the other portion being arranged to be coupled to the valve stem; and a resilient arrangement which exerts a biasing force on one of the bearing portions and provides resilience in the coupling provided by the assembly between the valve stem and actuating member.