What role does training resistance play in muscle building?
The goal of strength training is to fatigue the target muscle used in that exercise as much as possible within a time span of around 60 - 120 seconds. The more the muscle is fatigued in this time, the stronger the anabolic response to this training stimulus will be (provided your protein dosage and timing are right).
The first time you move the resistance, we can assume that you still have 100% of your available muscle strength (provided you're rested). With every repetition, or rather every second, part of your strength fades away due to fatigue. Let's assume you're doing your strength training with a resistance of around 90% of your maximum possible resistance - you'll then be able to keep moving the resistance until your strength drops below this 90%.
Are you really fatigued at that point?
Not really. You still have almost 90% of your possible strength available. You simply can't move the resistance any more because it's too heavy. In this case, you'd have to perform countless sets of the same exercise to fatigue the muscle. This costs you unnecessary time and puts extra strain on your central nervous system.
Now let's assume you choose a resistance for your strength training that's around 60% of your possible resistance. You'll then be able to keep moving the resistance until your muscle strength for the exercise falls below this 60%.
So which resistance is the right one?
The resistance should be chosen so that you can no longer move it a single millimetre after 60 seconds at the earliest and 120 seconds at the latest.
You can also choose the resistance so that you can move it for around 60 seconds. As soon as you can no longer move the resistance, it makes sense to reduce it once. That way, you can keep moving the lighter resistance and fatigue your muscle even further, until the strength you can produce is lower than the weight of that resistance.
Here's a graphic illustration of this.
So make sure you choose a resistance that you can move for around 60 - 120 seconds. If you can only move the resistance for around 60 seconds, you have the option of pausing briefly (max. 3 seconds), reducing the resistance and continuing the exercise straight away.
