One-Rep Max Applet
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Drag the yellow strength line around with the mouse.
Up/Down arrow keys change the maximum weight shown.
Left/Right arrow keys change the maximum reps shown.
'a' key toggles alternate formula in which 315x8 = 405 instead of 300x10=400
'r' key toggles some samples from Ron Fedkiw's idea of what the strength curves should look like.
(NOTE: you may have to click on the applet once before it will accept keyboard input)

"One-rep max" refers to the maximum amount of weight a person can lift for one repetition. Often this can be estimated surprisingly accurately from reps-to-fatigue with a smaller amount of weight. The rule of thumb that seems to work for most people is that 10 repetitions can be performed with a weight equal to approximately 75% of a maximal load, and that the number of repetitions decrease in an approximately linear fashion as the weight increases (at least in the range of 1 to 10 reps, which is what the above graph shows-- the linearity assumption tends not to be accurate for more than 10 reps).

So, the formula that works for most people is:

```            1 rep max = (weight lifted) * 36 / (37 - reps)
weight lifted = (1 rep max) * (37 - reps) / 36
reps = 37 - ( 36 * (weight lifted) / (1 rep max) )
```
References: Matt Brzycki, "Strength Testing - Predicting a One-Rep Max from Reps-to-Fatigue"

Download source code for this applet (compressed jar file contains class files and source)