For anyone looking to add some muscle tissue. Here are some of my  favourite training principles I always used to do in my bodybuilding days. Especially in my off season.

Forced Reps: Having a partner or spotter help you lift the weight while you lower it yourself under control.

Drop Sets: Upon failure, reduce the weight you’re using and continue your set until you reach your desired rep range or failure.

Holds: Upon completing the set, you hold the weight with muscles contracted until failure.

Rest-Pause Set: Upon failure, rest for 10-15 seconds and continue your set until you reach failure again or reach your desired rep range.

All of the above works. As long as you apply them properly and consistently.