Muscle fibre recruitment is simply how many of the fibres within a muscle are being recruited to perform a certain task.
Muscle fibres are recruited based on the need. That is, the lower the demand, the fewer fibres required, the greater the demand, the more fibres required. The body recruits the lower threshold motor units first (slow twitch), followed by the higher threshold motor units (fast twitch) and continues to recruit and fire motor units until you’ve applied enough force to do whatever it is you’re trying to do regarding movement.
Research shows that you eventually need to recruit all of the motor units and muscle fibres within a muscle to stimulate maximum muscle growth. This means we want to achieve 100% fibre recruitment in a muscle each and every set to get the best possible results out of our workouts. To take this even further we want to get 100% fibre recruitment in a muscle each and every rep we perform. So how do we do this? Studies have found if you use a weight that is 80-85% of your 1 Rep Max, the body will recruit more fibres. (Your 1 Rep Max is the maximum amount of weight you can lift for one repetition for a given movement). Above 85% there is no further recruitment and force output is improved via rate coding.
When you use 80-85% of your 1 Rep Max you are pretty much at 100% fibre recruitment from the very first rep.
For most people 80-85% of a persons maximum is roughly 4-8 repetitions. However because everyone is different there is variance in this between individuals and muscle groups but for most this is a good estimate.
When you lift lighter loads under 80% of your maximum you will still achieve 100% fibre recruitment, but you will only achieve 100% on your last few reps of a set as you fatigue. So if you were performing a set of 12 repetitions, only the last 3-4 reps at the very end will be effective. That set would of only had 3-4 effective reps at 100% fibre recruitment. If you had performed that set with a heavier load of say 6 reps, you would have achieved 100% fibre recruitment from reps 1 to 6. That’s 6 effective reps at 100% fibre recruitment. This way you stimulate maximum muscle growth and get the most value out of your time in the gym.