Body Type Doesn’t Predict Muscle Gains Blog Banner
It’s one of the most common misconceptions trainers hear from clients, usually mumbled while pinching a bicep or eyeing the guy curling next to them: “I’m just not built for muscle.” Or worse, “I’ve got skinny bones. It’s not worth trying.”
If you’ve been coaching long enough, you’ve probably heard variations of this. Maybe you’ve even thought it yourself when assessing someone new with narrow shoulders, slim wrists, or a frame that doesn’t scream “powerlifter”. But here’s the thing, while skeletal structure affects how someone looks when they walk into the gym, it doesn’t say much about how they’ll respond to training once they get started.
Thanks to a new study by Tiede and colleagues (2025), we’ve got solid evidence that helps debunk the idea that your bone structure determines your potential for muscle growth. And no, we’re not talking about those somatotypes like ectomorph, mesomorph, or endomorph. We’re talking about actual measurable bone characteristics, like shoulder width and bone mineral content. So let’s dig into what the research really says and how you can use it to coach more confidently and effectively.
Before we go any further, let’s clear up a common point of confusion. When we say “body type,” we don’t mean somatotypes. What we’re focusing on here is skeletal structure. That’s actual physical measurements like the width between the acromions (shoulder width), the distance across the greater trochanters (hip width), and bone mineral content in the arms, legs or pelvis. These are things we can measure accurately with tools like DXA scans, and which don’t change much over time.
It’s easy to assume that someone with broader shoulders or thicker bones has an advantage when it comes to muscle gain. But assumptions aren’t data. That’s where this study comes in.
Tiede et al. (2025) recruited 119 untrained adults, ranging from 18 to 70 years old, and put them through a 10–12 week supervised resistance training programme. Each participant trained twice per week with a full-body routine including exercises like hex bar deadlifts, leg press, extensions, hamstring curls, chest press and cable pulldowns. Training intensity was monitored using reps in reserve (RIR) to make sure everyone was working appropriately hard.
Before and after the programme, participants underwent DXA scans to measure bone-free lean tissue mass (that’s muscle), as well as skeletal features like bone mineral content and joint widths.
The big question was “Do your pre-training bone characteristics predict how much lean mass you’ll gain from lifting”?
The researchers found very strong associations between bone structure and starting lean mass. In other words, yes. People with broader frames or denser bones typically started out with more muscle. That’s not particularly surprising.
But when they looked at who gained the most muscle from training, those same skeletal traits weren’t much help in predicting the outcome. Across the board, the correlation between bone structure and muscle growth was weak. Shoulder width, hip width, bone area and even bone mineral content barely explained the variability in lean mass gains. After adjusting for sex, those relationships got even weaker.
What’s more, other factors like training volume, age, protein intake or even BMI also had only modest associations with hypertrophy. That doesn’t mean they’re unimportant, but it does suggest there’s no single, clear-cut predictor of how well someone will respond to a 10–12 week programme.
So what actually does drive muscle gain?
The real take-home here is that hypertrophy is complex and mostly influenced by what happens inside the muscle, not how the bones underneath are shaped. Mechanisms like mechanical tension, satellite cell activation and ribosome biogenesis are key players. These are molecular processes triggered by progressive overload, not shoulder width.
In fact, several other studies have shown that the best predictors of hypertrophic response are things like:
These are things we can’t see with the naked eye or measure with a tape measure, but they’re what actually drive adaptation. And crucially, these mechanisms are responsive to training, not predetermined by your skeleton.
For coaches and S&C professionals, the implications are huge. First and foremost, it means we can stop making assumptions about a client’s potential based on their structure. That slender client who thinks they’re “not built for lifting” might just need good programming, consistent training and a bit of encouragement. The stocky guy who looks like he lifts might already be near his ceiling.
We also need to be mindful of the psychological side of this. Clients often carry limiting beliefs about their bodies. This is particularly true if they’ve been labelled “skinny” or “bony” in the past. If a trainer subconsciously reinforces those labels, it can lead to low motivation or even drop-out. Instead, we must build confidence by focusing on progress, effort and the controllable factors that actually drive results.
Use skeletal assessments to inform movement screening, not hypertrophy predictions. Joint widths and limb lengths are useful for things like squat stance, deadlift setup and risk management, not for guessing who will grow.
Track what matters. This includes effort, recovery, consistency and progressive overload. Encourage clients to focus on their own journey rather than comparing themselves to others with different frames.
And when a client tells you they’re “just not built for muscle”, remind them that the science says otherwise.
Bone structure might shape how someone looks, but it doesn’t decide how they’ll respond to training. The new research confirms what good coaches have long suspected. That’s with the right approach, anyone can build muscle, regardless of how broad or narrow their frame is.
So next time a client says they’ve got “bad genetics” or “thin bones,” you’ve got the perfect response. “It Doesn’t matter. Let’s train.”
If you’re looking to start a rewarding career in fitness, the Gym Instructor & Personal Trainer Practitioner, Specialist & Master Diplomas™ are the perfect place to begin. Backed by decades of research, resistance training has been shown to deliver real, measurable results. In fact, a 2025 study of untrained adults showed that participants gained up to 773g of lean muscle in just 10 weeks of structured training. This is proof that the right programme, support and consistency make all the difference. Our Diplomas give you the skills and qualifications to deliver exactly that. Whether you’re just starting out or want to specialise in areas like strength & conditioning, nutrition or women’s health, you’ll gain the knowledge and practical experience to train clients with confidence. With flexible study options, daily support, and our Qualification Guarantee™, now’s the time to become a personal trainer.
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