Meta-analysis — 2021 — Repetitions & load · Hypertrophy · Strength
Resistance Training Load Effects on Muscle Hypertrophy and Strength Gain: Systematic Review and Network Meta-analysis
What did the researchers investigate?
Low (more than 15 RM), moderate (9-15 RM) and high (8 RM or fewer) loads trained to failure, across 28 studies with 747 healthy adults.
What did they find?
No differences in muscle growth between loads. Strength improved more with high and moderate loads than with low loads, with a non-significant edge for high over moderate. Untrained people grew more, and trained people benefited from more sessions.
What does this mean for training?
A moderate range like 9-15 reps supports both muscle growth and strength well; very light loads are weaker for strength.
How does FitX AI use this evidence?
FitX AI's default 8-15 working range lines up closely with the moderate-load zone studied here.
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.
Session check. Flagging a heavy squat or hinge with a top of range above 12 and 3+ sets, more than 6 hard sets of those lifts in a session, and three same-movement exercises for one muscle are FitX AI heuristics. Exercise order (early exercises get more reps and progress more for strength) is from reviews logged as candidates, not yet in the library. Fatigue varies by person, so these are pointers, never limits.
Limitations
Only studies with sets to failure were included; network meta-analysis combines indirect comparisons.