Arm curls can improve strength if done within the proper structure.
While weight training can, in many circumstances, improve strength, such training isn't constrained to that single goal. Simply doing a particular exercise won't necessarily make you stronger; it's more important how an exercise is done rather than the selection of the exercise itself. For instance, the arm curl can be used toward achieving any number of goals, from improving strength to increasing muscle mass to facilitating muscular endurance. No matter which goal you desire, the form of the exercise is basically the same, but the frequency and volume must be significantly altered depending on which goal you seek.
The arm curl is a basic two-stage exercise that primarily engages the biceps. Secondary engagement for the exercise includes the forearms, wrists, deltoids, trapezius and neck muscles. These are the muscles you can expect to make stronger through curls. The exercise can be performed with a variety of equipment, everything from a barbell with weight plates, a dumbbell, a kettlebell or any other type of weight that you can safely grip with your hand. Begin the exercise with your arms down and fully extended, gripping the weight underhand. Keep your arms pinned to your sides, and while bending at the elbows, raise the weight up toward your chest in an arc. Raise the weight as high as possible, compressing your arms as much as possible. Lower the weight back down.
The form of the curl lends itself to developing natural, functional strength, because it uses a simple isolation motion that follows the elbow's natural range of motion. With the proper set structure, and utilizing weight within a proper range, the curl can develop arm strength. The proper weight depends on your current fitness level and varies by individual. Generally, it's the level of weight you can manage through a series of no more than 12 repetitions, where the resistance taxes your arms but doesn't cause muscle failure. The number of sets and repetitions you do also determine if you'll actually develop strength. A good range is up to six sets of 10 to 12 reps each.
Your Day of Rest
While performing your arm curls using the proper protocols is a necessary component of developing strength, it isn't the only factor. To adequately improve strength over time, you need to follow up your workout days with at least one day of rest, where you don't engage the muscles you used during your curls. The muscle tissue that the curl breaks down needs to be rebuilt before you curl again. If you hit the gym multiple days each week and do curls each time without any rest time, your arm strength will diminish and you'll run the risk of injury to your arm muscles and joints.
While a full, complete curl improves strength under the correct procedure, there is a modification that can also help you improve strength. Doing partial repetitions of the curl in conjunction with sets of complete curls will provide concentration on the biceps for the partial sets, and broad engagement of other muscles during the traditional sets. The partial repetition is similar to a full curl, except the range of motion is much short. The rep will start with your elbows already bent and the weight at about the level of your lower chest. Lift the weight up just a little bit, to just above chest level, then lower it again.