You curl at the end of every push day. Maybe a dedicated arm day too. Your biceps get a pump, you feel like you trained arms, and months later your arms have barely changed size. It's tempting to blame genetics. The actual answer is usually much simpler.
You've been training the smaller half of your arm.
The triceps are most of your arm, not the biceps
The triceps make up roughly 60 to 70% of your total upper arm mass. The biceps and the often-ignored brachialis underneath it make up the rest. This isn't close to a 50/50 split, it's a muscle that's genuinely bigger than most lifters treat it as.
If your arm-specific training is mostly curls, with triceps work bolted on as an afterthought after chest or shoulder pressing, you're spending your deliberate, focused effort on the smaller portion of the arm and hoping incidental pressing volume covers the larger one. It usually doesn't cover it well enough.
Your biceps aren't one muscle either
The biceps brachii has a short head and a long head, and underneath both sits the brachialis, a muscle that contributes real width to the upper arm but doesn't show up in the classic "biceps peak" the way the biceps itself does.
Grip and wrist position change which head does more work. A supinated grip, palms up, standard curl, biases the biceps more directly. A neutral or hammer grip shifts more of the load onto the brachialis. Lifters who only ever do standard barbell curls are, again, working a narrower slice of the muscle than they think.
This is a smaller effect than the biceps-versus-triceps imbalance, but it compounds the same underlying issue: doing one exercise and assuming it covers the whole muscle group.
The triceps have three heads, and one exercise won't hit them all
Long head
Crosses the shoulder joint, so it's most loaded when the arm is overhead, think overhead extensions and skull crushers with the arm positioned back. This head is also the one most people undertrain, because pushdowns, the most common triceps exercise in most gyms, barely stretch it.
Lateral head
The head most visible from the outside of the arm, heavily involved in pushdowns and any pressing movement with the elbow close to the body.
Medial head
Sits deeper and works throughout most triceps extension movements, less visually distinct but still contributing to overall arm thickness.
A program that's all pushdowns is training the lateral and medial heads reasonably well while barely touching the long head, which happens to be the largest of the three. That gap alone accounts for a lot of "why won't my triceps grow" frustration.
Why biceps and triceps don't recover on the same schedule
Biceps generally recover in around 48 to 72 hours after a genuinely hard session. Triceps, being larger and also loaded indirectly during every pressing exercise you do, bench, overhead press, dips, push-ups, tend to accumulate more total fatigue across a full training week than biceps do, even without a dedicated triceps day.
That means the same "arms: fine" assumption that misleads people about arm size also misleads recovery planning. Your triceps might be carrying real cumulative fatigue from Monday's bench and Wednesday's overhead press by the time Friday's arm day rolls around, while your biceps, only worked directly on arm day, are essentially fresh.
What actually moves the needle
Train arms with a frequency of roughly twice a week, which most hypertrophy research points to as a strong general target once total volume and recovery are accounted for. Weight your triceps volume to match its actual size relative to biceps, rather than defaulting to equal sets of each. And vary triceps exercises across angles, at minimum one overhead-position movement for the long head alongside your pushdowns, instead of relying on a single exercise to cover a three-headed muscle.
Knowing which head or which muscle is actually under-recovered, instead of treating "arms" as one number, is where subzone-level tracking helps. Valen separates biceps and triceps fatigue rather than averaging your whole arm into one score, calculated from your logged training, free, with no wearable required.
Frequently Asked Questions
Why won't my arms grow even though I curl all the time?
The triceps make up roughly 60 to 70% of your total arm mass, more than the biceps and brachialis combined. If your arm training is mostly curls, you're neglecting the larger majority of the muscle.
Do biceps and triceps recover at the same speed?
Not exactly. Biceps typically recover in around 48 to 72 hours. Triceps are larger and also worked indirectly during every pressing exercise, so they often carry more accumulated fatigue across a training week.
How often should I train arms directly?
Most hypertrophy research points to roughly twice a week per muscle as a strong general target, assuming total weekly volume and recovery are managed well.
Does one exercise train the whole triceps evenly?
No. The triceps has three heads that respond differently to arm position. Overhead extensions bias the long head, pushdowns bias the lateral and medial heads. Relying on one exercise leaves part of the muscle underdeveloped.
Your arms are also caught in the middle of a bigger pattern, see why you're probably pushing more than you pull. For how the subzone principle applies across every major muscle group, see the complete guide to muscle subzone tracking.