The evidence-led answer
NST verdict
It is mostly a false contest. For the muscle it actually trains, a single joint exercise grows it about as well as a multi joint one, so a biceps curl builds the biceps as much as a pulldown does when the effort matches. Compounds are the efficient backbone of a programme because each set trains several muscles at once and lets you load heavy, not because they grow any one muscle more. Isolation exercises earn their place for muscles the big lifts hit only lightly or that lag behind, and for adding targeted volume without much whole body fatigue. Build around a handful of compounds, then add isolation where a muscle needs it. If time is short, compounds alone get most people most of the way.
Do compounds build more muscle in the muscle they train?
No. In a 10 week trial, untrained men trained their elbow flexors with either lat pulldowns or biceps curls. Both groups added about 6% to biceps thickness and about 10 to 12% to elbow flexor strength, with no difference between them. A review of the wider evidence reaches the same conclusion for the arms, and notes that the working muscle fires about the same in single joint and multi joint exercises. For the muscle being worked, calling the exercise "compound" is not doing anything extra.
Does adding isolation exercises help?
A little, at most. Two trials added single joint exercises on top of a multi joint programme. Neither found any extra strength gain or extra fat loss from the added isolation. One did find a small extra increase in flexed arm circumference, 5.2% with the isolation added versus 4.0% without. That is one small finding with a crude measure, so read it as a hint that isolation can add a bit of size to a target muscle, not proof that it always does.
So why are compounds the backbone?
Efficiency. A squat, a row or a press trains several muscles in one set and lets you handle heavier loads, so you get more muscle building stimulus per minute in the gym. That is why sensible programmes are built on a small number of compounds. Not because a compound grows the quads or the lats more than a dedicated exercise would, but because it grows several things at once while you are there.
What are isolation exercises actually for?
Specific jobs. They target muscles the big lifts hit only lightly or that lag behind, side and rear delts, biceps, hamstrings, calves, which presses and rows work only indirectly. They add volume to a priority muscle without piling on the whole body fatigue that another heavy compound would. The review suggests the lower back extensors in particular benefit from direct single joint work. And they let you train around an injury, loading one muscle when a compound is off the table. One trade off: the review found isolation exercises caused more local fatigue and soreness than compounds.
What this means in practice
Build the programme around a handful of compounds: a squat or leg press, a hip hinge, a horizontal push, a vertical push, a horizontal pull and a vertical pull. Then add roughly two to six isolation exercises for the muscles that lag or that the compounds do not hit hard, commonly side delts, biceps, triceps, hamstrings, calves, rear delts and abs. If your time is limited, the compounds alone will get most people most of the way. If a particular muscle is a priority or is clearly behind, isolation work is the right tool to bring it up.
Practical meaning
Build the programme around a handful of compounds, a squat or leg press, a hip hinge, a horizontal and a vertical push, a horizontal and a vertical pull, then add roughly two to six isolation exercises for muscles that lag or that the compounds do not hit hard (side delts, biceps, triceps, hamstrings, calves, rear delts, abs). If time is tight, compounds alone get most people most of the way. If a muscle is a priority or lagging, isolation is the right tool.
Limitations and uncertainty
The head to head trials are small (about 20 to 30 people), short (8 to 10 weeks), in untrained men, and mostly about the elbow flexors and upper body, so they show parity for a trained muscle over a short block in beginners, not the full picture for trained lifters or every muscle group. 'Compounds are efficient, not superior per muscle' is a reasoned reading, not a direct measurement of total muscle grown per hour. The extra arm circumference from added isolation is one small result with a crude measure.
