The peak torque of the first X number of repetitions (usually 5) divided by the peak torque of the last X number of repetitions (usually 5) multiplied by 100 to give a percentage.

In reality the line of least squares fit is used for the first 5 reps. This is divided by the line of least squares fit for the last 5 reps X by 100.

Any figure under 100 means the subject fatigued (the lower the figure the more the fatigue e.g. a fatigue ratio of 65% means the subject tired by 35% during the test).

A figure over 100 means the test went wrong!