Watching the stableboy grow into a man had turned Mike on incredibly. He looked down at the strong young man, tied down with ropes that cut into his flesh, and had an idea. He felt his own horsecock swell as he grabbed a brush with "Muscle +" written on the handle. Just to test it out, he gave himself a quick brush. He felt his body tense, as every muscle seemed to flex at once, and then relax. Looking down, he could see that his already strong body looked a little different. Just barely, but he thought his pecs were just a bit more defined, just a bit bigger. He smiled, and his horsecock swelled.
Mike turned back to his captive, and began to run the brush over the man's body. He watched as the stableman's skin became taut, stretching over muscles that grew in size and hardness. Veins began to appear, pushing up under the skin, pushed up by the growing bulges of hard flesh. The stableman groaned, and tried to thrash about, as the ropes bit into him hard. "Stop! I'm going to be strangled by these ropes!" he cried, his voice getting lower as he spoke.
Mike just kept brushing, excited to see the body on the table shake and shudder as the growing man struggled against his bonds. The rope was beginning to fray, the rough strands of fiber having more and more trouble containing the growing stableman. Mike brushed faster, his breathing quick with excitement, his eyes wide with delight as the stableman's muscles bulged and swelled spectacularly, growing more quickly now, strength filling them with each stroke of the magic brush. With a deep growl of anger, the stableman pushed himself upward, causing the ropes to snap away from him. He took deep breaths, savoring his freedom from the painful bindings.
"Spectacular!" Mike panted in approval, still running the brush over the stableman's body, more turned on than ever. The now quite powerful young man regained his senses, and pushed Mike away, rolling off of the table and onto the floor. He slowly stood, his body a powerful 275 pounds of muscle packed onto a six foot frame, his musculature a match for Mike's own impressive human-half physique. "E- enough!" the newly built stableman gasped. Mike gazed at him in appreciative study, noting everything from the new muscles to the hard foot-long dick that bobbed between strong, muscular legs.