Your arms are too small. Or at least you probably feel that way, if you’re like the average guy. But chances are it’s not because you’re neglecting those muscles.

In 15 years of training clients, I’ve yet to come across a would-be arms dealer who hasn’t tried every biceps curl and triceps extension in the book. So what’s the problem? Your upper back. And your core. And your glutes. (Yes, butthead, your glutes.) When those muscles are weaker than they should be, they act like brakes on the size and strength of everything else. Especially your arms.

Step 1: Call in the reinforcements

Physiology 101: your biceps bend your arms, and you work them with curls. You probably figured this out as a kid when Dad told you to “make a muscle” for one of those photos parents use as blackmail for when their sons start dating. Chances are you did a curl the first time you ever picked up a dumbbell, and you’ve been doing them ever since. Now ask yourself: when’s the last time I increased the weight I use on my arm exercises?
Physiology 201: if you want your arms to grow, you need to create overload and challenge them with progressively heavier weights . They’ll adapt by growing bigger and stronger. Since they aren’t receiving that overload from curls, you need to recruit bigger muscles to help them grow in tandem. Start with the chin-up. This move forces you to lift your entire body – several times the amount of weight you could curl – on each repetition. Your lats, which are the fan-shaped middle-back muscles that run from your armpits to your spine, do a good deal of the work. But your biceps are more than just bystanders. They’re working as hard as they can. Without their help, you couldn’t do a single rep. Target your triceps the same way. Do body weight dips or close-grip bench presses with a loaded barbell. You’ll be able to lift multiples of the weight you use for extensions. Make these heavy, multi-muscle exercises the focus of your upper-body training. After you’ve performed them, add curls and extensions to give your arms some extra oomph.

Step 2: Restore your core

Next time you hit the gym, try this test: go to the triceps push-down station, select the heaviest weight you can use for 10 reps, and do a set. Rest a few minutes, then repeat the set… standing on one leg. You won’t be able to knock off 10 reps, and the reason is obvious. Your triceps didn’t become weaker; they simply lost some of their support base. A weak or unstable core limits how efficiently your central nervous system controls your muscles; this can raise your injury risk. So even though your triceps aren’t part of your core, their performance is affected by it. A weaker or less balanced base of support limits the strength and power your arm muscles can generate, while a stronger core enables all your muscles to work harder, longer and more productively.

Step 3: Shore up your weak links

Face a mirror with a 15kg dumbbell in your non-dominant hand (your left, if you’re right-handed) and do a set of curls. Watch your body closely, especially as you tire.
Notice how your upper arm pinches in against the side of your torso after a few reps? That’s caused by muscles in your shoulder called external rotators. They’re pulling your arm into a stronger and more stable position.
Deeper into the set, you’ll need more momentum to start each lift. Your shoulder blade will rise up and drop down as you curl. That’s your trapezius – the diamond-shaped muscle that runs from your neck to your middle back and out to the edge of each shoulder, helping your biceps do their job. Finally, as your biceps near exhaustion, notice how you squeeze your butt cheeks and lean backwards to complete your last two or three reps. That’s the action of the muscles in the back side of your body, including your hamstrings, glutes and spinal erectors.
You won’t find this mentioned in the standard bodybuilding playbook, but these are the muscles that enable or limit the potential growth of your biceps and triceps.

Weak link 1: External rotators
Your upper body assigns two of its biggest and strongest muscles, the lats and the pectoralis major, the task of internal arm rotation – turning your upper arm inwards, like when you’re arm wrestling. For external rotation, it assigns a couple of relatively small muscles (infraspinatus and teres minor, in case you’re wondering). If your external rotators are disproportionately weak compared with your internal rotators, your body will be reluctant to increase your upper-body strength and power, which would set you up for injury. Your solution? Exercises such as YTWL raises, which directly target your external rotators, balance your strength and allow you to lift more weight.

Weak link 2: Trapezius
Your biceps and the main part of your triceps originate on your shoulder blades, the triangles of bone at the corners of your upper back. The actions of your shoulder blades are largely controlled by your trapezius. The stronger your traps, the more solid the platform you have to build everything else attached to your shoulder blades (that is, your arm muscles). By adding farmer’s walks, dead lifts and inverted rows, you can build the strength needed to add weight when you train your arms directly.

Weak link 3: Glutes
So how are your glutes connected to your arm muscles? Here’s how: your glutes are part of a chain of muscles and other tissues that also includes your lats. They work together to stabilise your spine when you move. So when you do an exercise like a chin-up, your biceps and lats work to move your body while your lats and glutes protect your spine and prevent sudden twists that might cause an injury. That’s why lat pull-downs are a mediocre substitute for chin-ups. Standing and lifting your body (or free weights) train your core to work harder to stay stable.
When your glutes and lats are strong and your spine is safe, chin-ups enable your biceps to work as hard as you want to push them. That’s why the prone cobra, which strengthens your glutes and back (among other muscles) is another key exercise for building great arms.
Enough explanation. Now it’s time for you to put your butt in gear… along with your traps and your core and all the other muscles I just mentioned. All you have to lose is the empty space in your shirtsleeves.