Dwayne Johnson isn’t exactly known for being undersized. But when director Benny Safdie told him he needed to “get bigger” to play real-life UFC legend Mark Kerr in The Smashing Machine, Johnson and his longtime trainer, Dave Rienzi, had just six months to deliver a new level of mass and athleticism.

According to an interview Rienzi gave to GQ, the transformation meant pushing Johnson into the largest physique of his career—outmuscling even his WWE days. The payoff? A performance critics say captures both Kerr’s bruising physical presence and his emotional battles outside the ring.

Heavy Shrugs, Bigger Shoulders, and Fight-Ready Training

Rienzi, an IFBB Pro bodybuilder and founder of Rienzi Strength & Conditioning, focused on mimicking Kerr’s signature silhouette: massive traps and rounded shoulders. That meant layering in high-volume shoulder work and multiple weekly shrug sessions.

“We hadn’t done shrugs in years,” Rienzi told GQ. “But for this role, we needed that thickness.” On set in Vancouver, former MMA coach Bas Rutten even remarked that Johnson looked nearly identical to Kerr.

The training went beyond hypertrophy, says GQ. To maintain explosiveness, Rienzi programmed heavy posterior-chain work—glutes, hamstrings, and lower back—while Johnson also trained with MMA stunt coordinators to prepare for fight choreography.

Dwayne Johnson Gained Muscle With a High-Protein, High-Carb Diet

The physical grind was matched in the kitchen. Rienzi oversaw six to seven whole-food meals a day, plus a shake, with an emphasis on protein and carbohydrates to build size without resorting to “dirty bulking,” as reported by GQ. Cardio intervals kept conditioning sharp, while rest days included mobility work and recovery.

Despite the intensity—18 fight-sequence shoot days left Johnson battered—his consistency impressed Rienzi most. Regardless of travel or schedule, he never misses, Rienzi told GQ.

With The Smashing Machine set to cement Johnson’s most dramatic role yet, the Rock’s dedication proves once again that muscle—and discipline—are built one rep, one meal, and one day at a time.

This story was originally published by Men’s Fitness on Oct 14, 2025, where it first appeared in the News section. Add Men’s Fitness as a Preferred Source by clicking here.