Narrative review — 2021 — Repetitions & load · Strength · Hypertrophy
Loading Recommendations for Muscle Strength, Hypertrophy, and Local Endurance: A Re-Examination of the Repetition Continuum
What did the researchers investigate?
Whether the 'repetition continuum' (heavy loads for strength, moderate loads for muscle growth, light loads for local muscular endurance) is supported by current research.
What did they find?
The authors conclude that current research fails to support some of the continuum's underlying presumptions. They propose a paradigm in which muscular adaptations can be obtained, and in some cases optimised, across a wide spectrum of loading zones.
What does this mean for training?
Strength and muscle growth can both be pursued across a wide range of loads, so no one rep range needs to be treated as required.
How does FitX AI use this evidence?
Backs FitX AI using a broad default working range (8-15 reps) instead of a narrow 'hypertrophy zone'. The 8-15 bounds are a practical product choice; this review does not prescribe them.
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
A narrative review: it argues for a new paradigm from selected evidence, and the abstract reports no pooled estimates. The nuances and implications sit in the full text, which this summary does not cover.