The film stars Craig Fairbrass as Terry, a hands-on personal trainer that gradually takes over the life of Simon (Cavan Clerkin), a jaded office worker.

He becomes drawn into a gym that he passes on his work commute, and quickly comes under the influence of Terry (Fairbrass), a hands-on personal trainer with a military background who offers to mentor Simon, introducing him to bodybuilding and growing his self-esteem.

[2] Empire gave it a four-star review: "Muscle takes the trappings of a macho crime flick and informs them with sensitivity and smarts, perfectly played by Cavan Clerkin and Craig Fairbrass.

"[1] The Guardian gave the film a three-star review, praising the casting in this "laudably gruelling descent into a macho underworld of dumbbells and sexual debauchery.

"[3] Sight and Sound described the film as "a striking and atmospheric piece of work that crackles with tension" and that "Johnson has a keen eye and ear for performative machismo and the rituals of male-dominated spaces.