From Fellrnr.com, Running tips

This scale is a result of many runs and races where mental toughness was the greatest challenge. Scales of perceived exertion or perceived effort can be useful, but these do not apply well when the limiting factor is not how intensely you are exercising. This scale of is based on how much perceived suffering is experienced. On a 100 mile race, it is possible to have a low perceived effort, but undergo immense suffering.

1 Ivan Castro

I used the units of 'Ivan' after my one of my running heroes Ivan Castro. On many runs I have reached the point where I seriously consider quitting. At these times I often think of Ivan, his determination and spirit. Ivan is not superhuman; he is an ordinary man who refuses to quit.

2 Usage

The indications are suggested guidelines, and a workout meeting one criteria would use that level, e.g. nearly crying would define a run as 7 on the Ivan scale.

3 The Ivan Scale

Movement continues while Motivation overcomes suffering.

Ivan Indications 1 Absence of any suffering 2 Easy workout: Slight discomfort

Tiredness 3 Normal workout: Minor discomfort

Minor tiredness 4 Hard workout: Noticeable but familiar pain.

Some tiredness.

Some fatigue. 5 Very hard workout or race effort: Distracting pain.

Significant fatigue. 6 Extreme workout or hard race effort: Desire to cry.

Doubts turning to fear.

Strong feelings of inadequacy.

Pain hard to ignore.

Overwhelming exhaustion. 7 Bad workout or tough race: Nearly crying.

Depression.

Mild fear.

Significant pain without expectation of lasting physical damage.

Some bad dreams after the event. 8 Ugly race: Crying alone.

Depression.

Moderate fear.

Intense pain with reasonable possibility of lasting physical damage.

Nightmares for a time after the event. 9 Unusual Circumstances: Crying openly.

Deep depression.

Fear.

Very intense pain with an expectation of lasting physical damage.

Persistent nightmares after the event. 10 Special Circumstances: Extreme pain and clear evidence of lasting physical damage.

Flashbacks and persistent nightmares after the event.

4 XKCD

From http://xkcd.com/883/ (Mouse over text "If it were a two or above I wouldn't be able to answer because it would mean a pause in the screaming.")

5 Versions of the Ivan Scale