Adding load to the bar is the most tried and true method for applying progressive overload. Sure, there are other methods of progression such as adding repetitions or more sets, but adding load to the bar is one of the most common and effective means of progression, especially for strength oriented goals. However, since the lowest common increment one can make with traditional weight plates is usually a 2.5 lbs plate on each side, you're forced to make 5 lb jumps in load that may not always be practical or safe under all conditions; this is where micro loading with fractional plates come in handy.
In order to progress and get bigger and stronger, we need to overload our muscles. Overload, in the context of resistance training, refers to taking a working set near or all the way to failure. Naturally, as we get stronger, the workouts that we used to get to our current strength levels may no longer be hard enough to be considered overload, so we must make our workouts harder in same way; this is commonly done by adding load to the bar or performing more repetitions with the same load. This is referred to as progressive overload.
Adding load to the bar is the easiest and most effective form of progressive overload. Sure, adding repetitions can work as well, but ultimately adding load is more practical and will be a part of nearly any routine you do. After all, if you were to simply add reps and never add load, you would eventually be performing over 100 reps per set -- not a very practical way to train, nor does it build much strength because strength stimulus is tied to how heavy you train.
Now that we know why adding load is important, let's talk about why you might use fractional plates to micro load. First of all, what are fractional plates? Fractional plates are exactly what they sound like; they are weight pairs that weigh less than the lightest traditional weights of 2.5 lbs. These come in many different variations, but commonly you will see sets of fractional plates that consist of pairs of 1 lb, 3/4 lb and 1/2 lb plates.
While it may not be immediately obvious, there are quite a few benefits of micro loading using fractional plates, especially in specific situations that require you to add load more slowly than the typical five pound increments. Typically, beginners likely won't need fractional plates because they can gain strength very rapidly, often able to even add more than 5 lbs to a lift rather often. However, here are some reasons a lifter may want to use micro loading.
When micro loading, you're not increasing the demand on the lifter by nearly as much as if you were to add traditional 5 lbs. This can help a person who is timid or simply gains strength at a lower rate than average, to continuously progress without fear or anxiety. Often when we add 5 lbs to a bar, especially depending on the movement, it can feel like you've just added a ton. This isn't a problem with micro loading and thus you not only ease into the heavier weights more easily, but this can help you lift with confidence and lower the risk of injury.
Since the majority of dumbbell movements are performed with only one arm at a time, five pound jumps are much more noticeable on a dumbbell than they are a barbell and just a 5 lb increase can result in a loss of many repetitions. Micro loading makes adding weight to a dumbbell movement much more manageable and safe, while allowing you to focus more on load increases rather than always adding repetitions instead, as is common with dumbbell movements.
For example, one common method of progression with dumbbell movements when a lifter does not have fractional plates is to instead add repetitions for a period of time, usually 3-4 reps, finally followed by adding 5lbs of load. With fractional plates, you can more safely and comfortably maintain a given rep range (which can be important for acquiring a certain stimulus) and slowly add weight in as low as 1 lb increments.
As mentioned above, maintaining a relatively low rep range is best for strengthening your muscles. Making your muscles stronger with resistance training can be a great way to prevent injury or even rehabilitate an injured part of your body, as long as you lift with proper form and practice safe lifting. Micro loading is perfect for this, because it allows you to safely focus on load increases, which is the best method of progressive overload in terms of building strength. Without fractional plates, I'd typically be very wary of someone increasing load too often when training an area of their body that was previously injured. However, with fractional plates, the load increases are MUCH safer and the benefits of being able to very slowly and gradually add load are exactly what you might need to rehabilitate a previously injured and weakened area.
There are certain types of lifters that would be much better suited using fractional plates as opposed to trying to make 5 pound jumps in load. This can be for various reasons, but we'll touch on them below.
Lifters who have a lot of experience with resistance training are typically unable to make fast gains in strength. The stronger you become over the years, the harder it is to make increases in load due to diminishing returns. Because of this, making a 5 pound increment is much harder and takes much longer for advanced lifters, so using fractional plates to micro load can be a great way to continue adding load at a reasonable pace.
Since children and the elderly aren't able to make strength gains as fast as someone who's a young adult, micro loading can be a great option to make sure that they can progress consistently and safely. It's very important that both of these age groups pay special attention to proper form and good technique to minimize the chance of injury. Micro loading is much more beneficial for this goal, since it's far less overwhelming when the load increases occurs.
Lifters who have previously had an injury or are at a higher risk for injury should usually micro load instead of using traditional 5 pound increases. This doesn't necessarily mean you should micro load on all exercises, but micro loading on exercises that target an area of your body that is at higher risk of injury can be exactly what you need in order to keep progressing safely. If you have a back injury for example, any exercise that targets the back should be micro loaded if you want to focus on strengthening that area. As mentioned previously, micro loading with fractional plates can be a great way to continue training a weakened or previously injured muscle safely and can be a great form of rehabilitation.