Measurement study — 2006 — Strength · Repetitions & load
Prediction of one repetition maximum strength from multiple repetition maximum testing and anthropometry
What did the researchers investigate?
How the load you can lift falls from 1 to 5, 10 and 20 reps to failure on the bench press and leg press, in 70 adults aged 18-69, and equations to predict 1RM.
What did they find?
Load fell non-linearly as reps increased. Predictions from 5RM data were the most accurate. The authors conclude 1RM can be accurately estimated from multiple-rep testing, but no more than 10 reps should be used in linear equations.
What does this mean for training?
A one-rep max can be estimated without testing it, but estimates from high-rep sets are less precise.
How does FitX AI use this evidence?
FitX AI estimates your 1RM from every rated set to track trends and set targets, never as a tested max. Many working sets have more than 10 reps, so the estimate is treated as a guide, and a heavy set is read with the middle of four formulas.
Where FitX AI's own choices go beyond this paper
Estimated 1RM. Estimates are approximate; they vary by exercise and person.
Careful heavy-set estimate. Taking the median of four formulas is an engineering choice, not a validated method.
Exercise history and targets from it. Pooling sessions within six weeks of the newest, keeping 95% of the estimate after 29-90 days without the exercise and 90% after longer, and recalibrating after 90 days or with only one rated session are FitX AI choices. Strength is lost gradually with time off and by different amounts for different people; the library has no paper that sets these numbers. Sets without an effort rating are read as to failure (the most conservative reading); an effort is never filled in.
Limitations
Two exercises, and equations derived from one sample. FitX AI's estimate uses a simple linear formula, which this study suggests is less precise above 10 reps.