Deep branch of radial nerve
The radial nerve divides into a superficial (sensory) and deep (motor) branch at the cubital fossa. The deep branch of the radial nerve winds to the back of the forearm around the lateral side of the radius between the two planes of fibers of the supinator, and is prolonged downward between the superficial and deep layers of muscles, to the middle of the forearm.
Source: Wikipedia — Deep branch of radial nerve (CC BY-SA 4.0)