Animals are endowed with the capacity to execute a large spectrum of motor behaviours. Numerous motor tasks result from changes in joint angles through contractions of flexor or extensor muscles. However, many motor behaviours, such as holding an object, require joint stabilisation, produced by co- contraction of flexors and extensors. And others, such as the follow-through when throwing a ball, may require co-inhibition of both. To allow the simultaneous control of multiple muscles, the motoneurons innervating them must receive synchronised inputs from as yet unidentified premotor circuits. We recently discovered that a fraction of premotor spinal interneurons are divergent, i.e. innervate more than one synergist or antagonist motor pools. Therefore, these neurons form an anatomical substrate for mediating joint stabilisation or relaxation. In this project, I aim to build a taxonomy of spinal interneurons innervating pairs of synergist or antagonist motor pools based on their molecular, anatomical, and physiological properties. I will then compare the synaptic strengths of the innervation of synergist versus antagonist motoneurons by divergent interneurons. Lastly, I will manipulate divergent premotor interneuron activity to determine their roles in movement. Thus, this project will shed light on the identity of these neurons and their functions in motor behaviour.