1. What is Intensity and volume
In the context of muscle hypertrophy, intensity refers to how close to muscular failure each exercise and set is performed. Meanwhile, volume refers to the total amount of sets and reps you perform. Intensity and volume generally have an inverse relationship: the harder your train the less you can train. This is because training close to failure fatigues your central nervous system (CNS), and attempting to train before adequately recovered while limit your ability to train hard.
2. What is required for hypertrophy
Muscle growth, in essence, is an adaptation by your body in response to muscles being challenged. Lifting a heavy load close to failure signals to your body that your muscles need to get bigger and stronger in order to survive. Based on this, intensity is more important than volume for muscle growth. When you lift the same weight for the same sets and reps, your body adapts and it no longer stimulates new growth. For that, progressive overload has to be applied, which means adding more weight, sets, and/or reps over time. After a certain point, it becomes inefficient to keep adding more sets and reps to reach failure. Any set that needs to be taken past 15-20 reps to reach failure begins to challenge your cardiovascular system more than the target muscle. Cardio also has its place, but when training with muscle growth in mind, 2-3 sets taken to failure within 6-10 reps for most exercises seems to be the sweet spot.
3. CNS and recovery ability
As mentioned before, training close to failure will fatigue your CNS, especially big compound movements. A few sets of curls to failure may leave your biceps sore for a day or two, but heavy squats or weighted dips will leave you with full body fatigue for up to several days. Attempting to train again with this systemic fatigue will limit your ability to push as hard as your full potential if you were well rested. Generally, the CNS will take around 48 hours to recover fully. You can still train different body parts within a shorter time gap, as seen with a push/pull/legs split or an upper body/lower body split, but adding rest days every 1-3 days is vital to allow proper recovery. There is also a genetic component to this, as your genetics may give you faster or slower recovery. Therefore, it is also important to try different training splits for 6-8 weeks each to see where you make the greatest progress.
4. How to use this information to program your workouts
If training primarily for muscle growth, intensity must be prioritized. Frequency should be as often as you possibly can without impeding recovery. As mentioned, recovery ability can vary based on factors such as genetics; the best way to gauge it is just to simply experiment with different protocols and see which one allows you to consistently get stronger from week to week, assuming nutrition and sleep are on point. If you’re getting stronger consistently, meaning you can add weight and/or reps to given exercise week to week and month to month, that means you’re training enough to trigger adaptive responses and also getting enough rest for your body to actually adapt to the stimulus. I would suggest starting with 3 days per week of full-body workouts as it’s a good middle ground, and increase or decrease frequency from there based on how your body adapts. You also don’t want to be constantly switching programs. Stick with a program for at least 6-8 weeks before altering or switching it.