During intense training, when your muscles have a large amount of testosterone, it is necessary to achieve an adequate their reaction to it. In other words, it is necessary to increase the sensitivity of androgen receptors in muscle fibers. This is accomplished by applying a non-traumatic stress to the muscles.
So, your biceps are fully "inflated", the blood and an increase in the number of reps from 21 to 30, for example, should fully tire them. This is not a problem, since you're not going to put a new world record. However, if you feel that you cannot bend the arms, try and rest for about 10 minutes but do not with passive recreation. IN this case is better to stretch your hamstrings to keep the "pumped" muscles.
Now your goal is to lift average weights slowly for 12 times. Then try to do the curls with dumbbells sitting. Make a set with your right hand, rest a little and do a set with the left one. Watch for the precision of your motions, because this movement should be smooth. You should be able to stop it at the top point of the amplitude for two seconds.
There is a significant difference between lifting of heavy weight and conscious contraction of muscles with an average weight. In the first case the weight moves through inertia. This leads to the injury of the muscle fibers, especially when you just drop the weight down because you can’t control its movement. In the moment, you load your nervous system along with the muscles. This is exactly what you need, because it sends a strong signal the muscles to increase their sensitivity to testosterone. Obviously, you need to cut the muscles as much as you can. Do not stop exercise because you have performed 12 reps or your biceps is hurting. Think of your muscles’ androgen receptors, because additional time spent under stress, increase their sensitivity.