1986 by Martin Cohen.
As part of its elitist tradition, the U.S. Marine Corps demands elite physical fitness. While this manual is not an official DoD document, Cohen acknowledges input from an extensive list of Marine officers and training facilities.
Why "3X"? Cohen prescribes a fitness strategy, various aspects of which revolve around the number 3:
3 program phases: preparatory, conditioning, maintenance
3 workouts/week, one hour per session, at minimum. (That's a Corps regulation for all personnel, BTW.)
3 tests for fitness: pullups for upper torso, situps for lower torso, and running for cardiovascular/respiratory
Exercises cycled in sets of 3 during each workout
3 accessories to effective exercise: proper breathing, correct form, and appropriate pacing.
What constitutes "acceptable" fitness? This is perhaps the most complex part of the program. At the time of publication, the Marine Corps differentiated by gender and age, and assigned point values to various exercises. The Marine must achieve a certain minimal performance in each of three different exercises (yielding a point value), plus additional performance to reach the minimum total point score. Minimal performance in all three exercises would be insufficient to meet the total point minimum. For a male aged 27-39, this means 3 pullups, 35 situps in 2 minutes, and running 3 miles in 29 minutes, plus an additional 26 points to make up the 110-point minimum total. The examinee might make up the 26 points beyond minimum with situps at 1 point each, pullups at 5 points each, or lower 3-mile running time (scored non-linearly). Meeting the minimum point total qualifies the examinee's fitness as "third class"; higher scores would qualify the Marine's fitness as second- or first-class (175 pt and 225 pt, respectively, for the 27-yr old male example).
Body fat composition. Finally, Cohen points out the demonstrated weakness of simple height/weight measurement for determining ideal weight. Muscle is more dense than fat, so it's no great trick to find one unfit person and one athletic person with the same height/weight ratios, and "prove" they both would do well to lose weight. The athlete would be losing lean body mass. To avoid such a mistake, the Marine Corps specifies a maximum body fat composition rather than mandating a maximum weight by height: 18% or less for men, 26% or less for women. Calculating fat composition for men entails measurement at the neck and waist, yielding a % correlative on a simple table. For women, it entails no less than five measurements (thigh, bicep, forearm, neck, and abdomen). Points are accumulated through a number of correlation tables, and a correction constant subtracted. | 
