Randomised trial — 2018 — Repetitions & load · Hypertrophy · Strength
Effects of different intensities of resistance training with equated volume load on muscle strength and hypertrophy
What did the researchers investigate?
Training at 20%, 40%, 60% or 80% of 1RM with matched volume load, twice a week for 12 weeks, in 30 men (one limb at 20%, the other at a heavier load).
What did they find?
All intensities increased strength and muscle size, but 20% of 1RM was suboptimal. 80% of 1RM produced more muscle growth than 20%, and strength increased more at 60% and 80%.
What does this mean for training?
There is a lower limit: very light loads are less effective, even when the total work is matched.
How does FitX AI use this evidence?
FitX AI keeps targets inside a working range rather than letting sets drift to very high reps with very light weights.
Where FitX AI's own choices go beyond this paper
8-15 working range. 8-15 is a practical default range, not a research-derived limit. 6-10 for barbell squats and hinges, and RPE 9 as their ceiling, are FitX AI choices to limit the fatigue of the most demanding lifts: no paper sets these numbers. A range the user has set is never changed.
Limitations
30 men over 12 weeks, using a within-person (one limb versus the other) design.