Wathen experimented to find formula constants that worked best for different exercise types using lots of lifting logs.
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The Wathen 1RM prediction formula is:
1RM = Weight / (1 - (Reps/Constant))
Where: Weight = Load lifted (lbs or kg) Reps = Number of reps completed Constant = A value between 35-42 depending on exercise
This formula has some similarities to the Berger formula discussed earlier, as it uses an exercise-specific constant in the denominator.
Don Wathen derived this equation by analyzing training log data across a wide variety of exercises and rep ranges from recreational and elite lifters.
Through trial-and-error curve fitting, Wathen found that using a constant value between 35-42 provided the best 1RM prediction accuracy for most exercises.
Specifically, he recommended using: Constant = 40 for upper body pressing exercises like bench Constant = 35 for lower body exercises like squats Constant = 42 for pulling exercises like rows and deadlifts
So while the formula itself is relatively simple, the key innovation was determining these optimal constant values for different exercise categories based on the empirical training data.
This accounted for the different strength curves and leverages involved in pushing, pulling, and lower body movements.
Wathen claimed this exercise-specific constant approach improved 1RM prediction accuracy compared to using generalized formulas that didn't differentiate by movement pattern.
The simplicity of the Wathen formula also made it easy to implement for quickly estimating 1RMs across a wide variety of exercises in practice.
It highlights another technique of using empirically-derived constants, rather than regression coefficients, to customize 1RM prediction for different lifts.