14.3 Energy-System Application During Cardio Execution
Key Takeaways
- Phosphagen (~5–10 s), fast glycolysis (~15–30 s), mixed glycolytic/oxidative (~1–3 min), and oxidative (>~3 min) dominate different cardio tasks—all systems are always "on" to some degree.
- Match work-to-rest to the goal: ~1:12–1:20 for 5–10 s phosphagen sprints; much denser rest for steady 20–60 min oxidative work.
- Karvonen target HR = (HRmax − HRrest) × %intensity + HRrest; %HRmax is a different math and a common exam trap.
- HIIT, moderate-intensity continuous training (MICT), and long slow distance (LSD) are tools—choose them from the goal, not from marketing.
- Excess post-exercise oxygen consumption (EPOC) is a modest extra recovery cost, not a magic fat-loss promise.
Energy systems while the client is actually moving
DCO 3.B.6–7 is how they move. This section is which metabolic demand you just created so the next interval, rest, or machine setting is not random. NSCA teaching (the same energy-system table used in NSCA strength-and-conditioning education) treats the pathways as overlapping, with a primary system by duration and intensity:
| Primary emphasis | Typical duration | Example cardio task | Common work:rest (NSCA table) |
|---|---|---|---|
| Phosphagen (ATP–PCr) | ~5–10 seconds | Very short sprints, hill bursts, assault-bike all-outs | 1:12 to 1:20 |
| Fast glycolysis | ~15–30 seconds | 200–300 m repeats, hard rowing pieces | 1:3 to 1:5 |
| Fast glycolysis + oxidative | ~1–3 minutes | 800 m efforts, tough 2-minute bike intervals | 1:3 to 1:4 |
| Oxidative | >~3 minutes, often 20–60+ min | Steady jog, easy spin, long swim | 1:1 to 1:3 if intervalled; continuous if MICT/LSD |
A 10-second treadmill sprint therefore needs about 2–3.5 minutes of recovery if you actually want phosphagen quality on the next rep (10 s × 12 = 120 s; 10 s × 20 = 200 s). A 20–60 minute conversational ride is oxidative by design—do not sprinkle 10-second rests as if that were the same session.
Crossover concept
As intensity rises, carbohydrate supplies a larger share of energy and fat a smaller share (the crossover). That does not mean low-intensity work is the only way to lose fat. Fat oxidation can be relatively high at easy paces, but total calorie burn and weekly volume still drive body-composition change. Do not tell clients that the treadmill's "fat burn" LED is a physiology law, or that they must stay below an unpublished magic heart rate forever.
All three systems contribute in mixed sessions. HIIT uses phosphagen and glycolytic bursts with oxidative recovery between reps. LSD sits on the oxidative end. Your coaching sentence on the floor is: match the clock and the rest to the system you meant to train.
A 40-year-old client has a resting heart rate of 70 bpm. Using 220 − age to estimate HRmax, what is the Karvonen (heart-rate reserve) target at 70% intensity?
Karvonen and %HRmax recap, tied to machine settings
Estimated HRmax on many CPT exams still uses 220 − age unless the item gives a true measured max. Heart-rate reserve (HRR) is HRmax − HRrest. Karvonen target:
Target HR = (HRR × %intensity) + HRrest
%HRmax target is simply HRmax × %intensity—different numbers at the same advertised percent.
Worked example (same client as the quiz): age 40, HRrest 70, estimated HRmax 180, HRR 110.
- 50% HRR → (110 × 0.50) + 70 = 125 bpm
- 60% HRR → (110 × 0.60) + 70 = 136 bpm
- 70% HRR → (110 × 0.70) + 70 = 147 bpm
- 85% HRR → (110 × 0.85) + 70 = 163.5 ≈ 164 bpm
- 70% HRmax → 180 × 0.70 = 126 bpm (easy to confuse with 70% HRR)
Rough ACSM-style intensity bands you will see in NSCA-aligned programming: moderate ~40–59% HRR, vigorous ~60–89% HRR. Translate the target onto the device:
- Treadmill: raise or lower speed, then grade, until HR and RPE sit in the zone without rail-holding that fakes a lower cost.
- Bike/UBE: change watts or resistance and cadence; a seated grind at 40 rpm may hit HR by misery rather than by intended mechanics.
- Rower: change pace/split and stroke rate; slamming damper 10 is not a Karvonen method.
- Outdoor run: use the same HR or talk-test; heat will inflate HR at a given pace—slow the pace, do not ignore the thermometer.
If a beta-blocked client cannot use HR, switch to RPE and the talk-test (Domain 2 FITT) rather than inventing a Karvonen number.
You want repeat 10-second all-out bike sprints that actually emphasize the phosphagen system. Which work-to-rest structure best matches standard NSCA ratios?
HIIT vs MICT vs long slow distance
These are execution patterns, not moral identities.
- HIIT (high-intensity interval training): repeated bouts near vigorous or near-maximal effort with planned rest or easy active recovery. Use it when the goal is time-efficient vigorous stress, sport-like bursts, or VO2-oriented work and the client can tolerate the mechanics (no rail-hanging sprint, no valgus landing). Rest must be long enough that the work interval stays high—that is the 1:12–1:20 vs 1:3–1:5 distinction above, not a random 10/20 Tabata copy for every beginner.
- MICT (moderate-intensity continuous training): steady moderate work, often 20–60 minutes in the ~40–59% HRR band, conversational or just above. Excellent default for general fitness, health, and clients who are not interval-ready.
- LSD (long slow distance): longer, easier oxidative work (still usually minutes to an hour+, not 10-second reps). Use it for aerobic base, recovery days, and clients who enjoy duration more than intensity. "Slow" is relative to that client, not to a social-media easy run.
A beginner who cannot walk 20 minutes should not open with HIIT sprints on a 12% treadmill. An amateur 5K runner may still need MICT or LSD volume rather than daily all-out intervals. Program Execution means you can explain why this bout, this rest, this machine setting.
EPOC is modest, not magic
Excess post-exercise oxygen consumption (EPOC) is the extra oxygen (and energy) cost after exercise as the body restores phosphagen, clears metabolites, cools, and returns toward baseline. Harder and longer sessions tend to produce more EPOC than easy 20-minute walks, and HIIT can produce a noticeable afterburn relative to a very easy session.
What EPOC is not:
- A guarantee that "the real fat burn starts after you leave"
- A replacement for the calories used during the session
- A reason to destroy mechanics so the afterburn poster looks impressive
- A 24-hour metabolic doubling for a typical personal-training bout
Most of the energy cost is in the work you just coached. Body-composition change still depends on weekly activity, nutrition within scope, and adherence. Sell good intervals and honest steady work—not a miracle EPOC story.
Floor script you can reuse: "This 10-second sprint block uses long rests so you can actually repeat quality efforts. This 40-minute bike is the oxidative, moderate piece. Neither one is a magic afterburn machine; both only work if your posture and settings stay honest."
A client asks whether HIIT is worth doing "only for the afterburn." What is the most accurate statement about EPOC for NSCA-CPT execution?
Which pairing best applies energy-system logic during cardio execution?