When performing biceps curls with relaxed form (cheating), the biceps muscle experiences higher mechanical tension when stretched to longer lengths than when performing the same movement with strict...

Over eight weeks, untrained young adults who used momentum while doing bicep curls and triceps pushdowns gained about the same amount of muscle size and arm circumference as those who used strict form, even though they lifted more total weight.

Switching up your shoulder position during bicep curls makes your muscles fire more, even if you're lifting the same total weight.