Your muscle will grow when are subjected to an OVERLOAD.

They will not respond to anything less. Muscles will not grow bigger or stronger unless you force them to. Making your muscles contract against a level of resistance they are not used to will cause them to adapt and grow stronger. But once they have adapted sufficiently, this progress will stop. When this happens, the only way to make your muscles continue to grow is by further increasing the amount of OVERLOAD to which you subject them. The primary way of doing this is to add weight to your exercises.

But the best way is the MICRO-LOADING system (adding 1,25 kg).

There are numerous ways to micro-load, you can use the washers - chains - collars - micro plates.