Continuous High Intensity (CHI) Training
Continuous High Intensity (CHI) training is a cardio method that sustains a near-maximal effort for a set duration, typically 15-20 minutes. In the NSW preparation program, CHI sessions bridge the gap between LSD endurance work and short-burst interval training by developing the ability to maintain a fast pace without rest breaks.
How CHI Training Works
A CHI session prescribes a fixed duration — usually 15 to 20 minutes — at roughly 85-95% of maximum sustainable pace. Unlike intervals, there are no rest periods; unlike LSD, the pace is significantly faster than conversational. CHI runs and CHI swims appear in the training week alongside LSD and interval sessions, each targeting a different energy system.
CHI vs. LSD and Intervals
The three cardio modalities in the NSW program form a spectrum. LSD sits at the low-intensity, long-duration end. Intervals sit at the high-intensity, short-burst end. CHI occupies the middle — sustained intensity for a moderate duration. Together, the three methods develop the full range of cardiovascular capacity: the aerobic base (LSD), lactate threshold tolerance (CHI), and anaerobic power (intervals).
When CHI Appears in the Program
CHI sessions appear from the first week of the 26-week program and remain a consistent element throughout. They typically alternate with LSD and interval days across the training week. As the program progresses, CHI session durations may increase and the expected effort level intensifies in line with the candidate's improving fitness.
CHI pace should feel uncomfortably fast but sustainable for the full duration. If you have to stop, you went too hard — dial back 5% and hold it.
Related Terms
Long Slow Distance (LSD) training is a cardio methodology that emphasizes sustained, moderate-intensity effort over extended durations. In BUD/S preparation, LSD sessions build the aerobic base required for long swims, runs, and the prolonged physical output demanded throughout SEAL selection.
Interval training is a cardio method that alternates between high-intensity work periods and active recovery periods. In the NSW preparation program, interval sessions target running and swimming speed by training the body to sustain faster paces through repeated timed efforts.
The 500-yard swim is the first event of the Physical Screening Test (PST) and measures a candidate's aquatic endurance. Performed using either the combat sidestroke (CSS) or breaststroke, it tests the sustained swimming capacity required for BUD/S water-based evolutions.
The 1.5-mile run is the final event of the Physical Screening Test (PST) and measures a candidate's running speed and cardiovascular endurance. Performed after the swim and calisthenics events, it tests the ability to produce a fast run effort on fatigued legs — a skill directly relevant to BUD/S demands.
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