The fitness world has been gripped by a powerful trend: training muscles in their stretched position for maximum growth.
From influencers to research-backed coaches, the message has been clear—if you want optimal muscle development, prioritize the stretch. But Jeremy Ethier, a popular fitness YouTuber, just invested over $40,000 in a comprehensive study that challenges this widely-accepted principle. The results are turning heads across the industry, as explained in his recent YouTube video.
The stretch-focused training movement gained momentum from decades of research. Studies dating back to the 1970s showed chickens’ lat muscles growing 170% just from hanging weights on their wings. Human trials on extreme loaded stretching produced similar impressive results.
More recent research comparing exercises like tricep pushdowns versus overhead extensions, and bottom-half leg extensions versus top-half, consistently showed that challenging muscles in stretched positions led to significantly more growth—sometimes up to twice as much.
However, Ethier identified three critical gaps in this research. First, nearly all studies focused exclusively on biceps, quads, and calves because they’re easier and cheaper to test. Second, many compared completely different exercises, making it impossible to isolate whether the stretch itself was responsible for superior results. Third, most studies used ultrasound measurements, which only capture muscle thickness at specific points rather than providing complete growth data.
To address these limitations, Ethier assembled an elite research team and designed a groundbreaking study. They tested commonly trained but rarely studied muscles: chest, glutes, side delts, and rear delts.
Instead of ultrasound, they invested roughly $20,000 in MRI scans, providing complete 3D models of each muscle. Custom machines from Prime Fitness USA allowed them to adjust resistance profiles with a simple knob turn, making exercises harder either in the stretched or squeezed position while keeping everything else identical.
The study used a within-subjects design, splitting 20 participants so one side of their body trained with stretch-focused resistance and the other with squeeze-focused resistance. This eliminated variables like genetics and diet that typically complicate results. After 10 weeks and 97% adherence across 400 possible workouts, muscles grew approximately 20% bigger—excellent growth that would reveal any meaningful differences between methods.
The conclusion? Every single muscle showed identical growth on both sides. Chest, side delts, rear delts, and glutes all responded the same whether challenged hardest in the stretch or the squeeze.
Ethier clarified that these findings don’t invalidate stretch training entirely. The research reveals three distinct aspects of “the stretch” that shouldn’t be conflated.
First, certain biarticular muscles crossing two joints—like hamstrings, quads, calves, and triceps—do benefit from exercises that stretch them further. Second, full range of motion remains crucial for growth. Third, and most importantly, the resistance profile—where an exercise feels hardest—doesn’t seem to matter as long as there’s adequate tension throughout the full range.
These findings align with a recent 2024 study comparing cable and dumbbell lateral raises, which produced similar shoulder growth despite different resistance profiles. Even Jeff Nippard, another prominent science-based coach, acknowledged the study’s implications when reviewing the results, noting it provides valuable flexibility in exercise selection.
The takeaway isn’t that previous recommendations were wrong, but rather that effective training has more room for variation than many believed. Hip thrusts can match squats for glutes, reverse dumbbell flies can equal cables, and traditional exercises remain highly effective.
The key principles remain unchanged: train hard, use full range of motion, and maintain progressive overload.
