When untrained men perform dumbbell rows with one arm and biceps curls with the other, the arm that performs a specific exercise gets stronger at that exercise, but not necessarily at the other...
Mechanism
Synthesis from 1 study
When you train one arm with rows and the other with curls, your brain learns exactly how to turn on the right muscles in the right way for each movement — so your row-trained arm gets stronger at rows, and your curl-trained arm gets stronger at curls, even if both arms grow a little...
Most probable mechanism
When you train one arm with dumbbell rows and the other with biceps curls, each arm learns to use its muscles in a very specific way — the row-trained arm gets better at activating the right muscles in the right order for pulling the weight back, and the curl-trained arm gets better at squeezing the biceps to lift the weight up. This happens because practicing the exact movement over and over makes the brain and nerves more efficient at firing the exact muscles needed for that motion, which makes you stronger at that specific exercise — even if both arms get bigger in similar ways, only the trained arm gets better at doing its specific movement (10.1519/JSC.0000000000003234).
Repeated practice of a specific movement pattern enhances the precision of motor cortex activation and corticospinal drive to the muscles involved in that motion, increasing the efficiency of motor unit recruitment during that task (10.1519/JSC.0000000000003234).
Movement-specific motor unit recruitment patterns are reinforced through repeated concentric failure under load, leading to improved synchronization and firing rate of motor units exclusively in the muscles activated during the trained movement (10.1519/JSC.0000000000003234).
The neural adaptation to movement-specific training occurs independently of muscle hypertrophy, as strength gains are disproportionately greater in the trained movement despite similar or lesser muscle growth compared to alternative exercises (10.1519/JSC.0000000000003234).
Less supported by current evidence, but not ruled out
Biceps curls place all the load directly on the elbow flexors, while dumbbell rows spread the load across multiple muscles like the back and shoulders, which can limit how hard the biceps are forced to work — so even if both arms grow a bit, only the arm doing curls gets the full force stimulus needed to maximize biceps strength (10.1519/JSC.0000000000003234).
Single-joint exercises like biceps curls apply mechanical load exclusively to the target muscle group without limitation from weaker synergists, enabling maximal tension development in the elbow flexors (10.1519/JSC.0000000000003234).
Multijoint exercises like dumbbell rows require coordination of multiple muscle groups, where weaker components (e.g., grip, scapular stabilizers) can limit the load or range of motion achievable by the target muscle, reducing mechanical stress on the elbow flexors (10.1519/JSC.0000000000003234).
Reduced mechanical tension in multijoint exercises leads to lower activation of mechanosensitive signaling pathways (e.g., mTOR) in the target muscle, resulting in less hypertrophic stimulus despite similar training volume (10.1519/JSC.0000000000003234).
Evidence from Studies
Supporting (1)
Community contributions welcome
Single-Joint Exercise Results in Higher Hypertrophy of Elbow Flexors Than Multijoint Exercise
Contradicting (0)
Community contributions welcome
Gold Standard Evidence Needed
According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.