Aerobic vs Anaerobic Training: What They Are and When to Use Each

Two energy systems. Your body runs on both, and the difference between them shapes everything from your power output to your recovery, which is why aerobic training builds your aerobic capacity and the foundation of distance performance while anaerobic training sharpens your ability to tolerate and clear lactate, boosting your top-end speed and power. Different adaptations. Neither is superior. Which one you reach for, and when, is the variable most athletes get wrong.

Aerobic training: the steady foundation

Aerobic training sits below your anaerobic threshold, where oxygen supply meets oxygen demand, your muscles burn a mix of fat and carbohydrate, lactate stays stable, and you could theoretically continue for hours. Low to moderate intensity.

What happens in your body during aerobic work:

• Your heart strengthens. Each beat pumps more blood, because aerobic training improves left ventricle function and ejection volume, and the mechanism is well-established: regular aerobic work increases stroke volume and cardiac output over time.
• Your mitochondria multiply. Muscle cells build more of the powerhouses that convert fuel to energy, and more mitochondria means more aerobic capacity and faster endurance performance.
• Capillary density increases: more small blood vessels grow into your muscles, delivering oxygen more efficiently.
• Fat oxidation improves. Your aerobic system gets better at burning fat as fuel, which spares glycogen for the hard efforts.

The stimulus is sustained and repetitive. Zone 2 runs, long slow cycling, steady swimming, where the pace feels conversational and you are not taxing your anaerobic system.

Anaerobic training: the high-intensity edge

Above your anaerobic threshold, demand exceeds oxygen supply, and your muscles pull energy from stored ATP and creatine phosphate while lactate accumulates faster than your body can clear it. Intensity is high.

One thing to be clear on: "anaerobic" does not mean zero oxygen. Your aerobic system is still firing, it is overwhelmed by the demand, and when people say "anaerobic" they mean intensity above the threshold where lactate starts accumulating in the blood.

What happens in your body during anaerobic work:

• Type 2 muscle fibers activate. Hard efforts recruit the fast-twitch fibers your body uses for power and speed, and Type 2 fibers are preferentially recruited at high intensity, obeying the size principle of motor recruitment.
• Lactate tolerance improves. Muscles get better at handling and clearing lactate, and training at or slightly above anaerobic threshold intensity improves both aerobic capacity and raises your lactate threshold, meaning you can hold higher intensity without accumulation.
• Mitochondria develop in Type 2 fibers. Steady aerobic work builds mitochondrial density in slow-twitch fibers; high-intensity training triggers mitochondrial biogenesis specifically in fast-twitch fibers.
• Power output rises. Your nervous system learns to recruit more muscle fibers, faster. You get stronger at high speeds.
• VO2max climbs faster. Per unit of training time, hard efforts drive greater VO2max gains, and HIIT shows significantly greater improvements in VO2max compared with control groups, with studies showing effect sizes of 1.11 and VO2peak improvements of 0.78.

The catch is recovery. These efforts are stressful and they fatigue the nervous system, which is why you cannot do them every day.

Lactate threshold: the boundary

Your lactate threshold is the intensity where lactate appearance equals removal. Below it, blood lactate stays stable. Above it, it piles up.

The practical marker is your anaerobic threshold (AT), often measured at around 4.0 mmol/L of blood lactate, while your lactate threshold sits lower, typically in the range of 2-3 mmol/L, and between the two sits the sweet spot for threshold training.

Why it matters: elite distance runners show higher running speeds at lactate threshold than less-trained athletes, and lactate threshold correlates more strongly with endurance performance than VO2max alone. Your threshold moves. So does your ceiling.

Training at threshold intensity brings the fastest gains, and a 40-week endurance program improved lactate threshold variables by 15-18%, independent of VO2max changes. You do not max out. Threshold is the intensity where you start accumulating lactate but are still in relative control.

Matching intensity to adaptation

The adaptation you get depends on the intensity you choose. Nothing else.

Below lactate threshold (Zone 1-2): Aerobic base building. Fat oxidation, mitochondrial growth in slow-twitch fibers, recovery, long-term endurance. This is your weekly foundation.

At lactate threshold (Zone 3): Threshold/tempo training. Better lactate clearance, higher sustainable intensity, and the edge-building work of the week, 20-40 minute efforts, hard but not maximal.

Above anaerobic threshold (Zone 4-5): VO2max and anaerobic capacity. Fast-twitch fiber development, lactate tolerance, power, nervous system stress, and the speed work that gets you there, meaning short intervals of 30 seconds to 5 minutes, maximum effort with full recovery.

Training structure: how to use both

Most athletes get the balance backwards. Too much hard work, not enough easy work. No real threshold stimulus anywhere in the week.

Aerobic base: The foundation. Most of your volume goes easy, below lactate threshold, which for a runner might be 70-80% of weekly running volume at conversational pace, and that is what builds capillary density, mitochondrial capacity, and aerobic power without piling up fatigue. Cyclist, same rule. Long steady rides, low heart rate.

A large Danish cohort found 1-2.4 hours of moderate aerobic activity 2-3 times weekly provided the greatest reduction in mortality risk, and higher volumes showed diminishing returns on that specific outcome.

Threshold work: The edge. One session per week, sustained at or slightly above lactate threshold, 20-40 minutes for runners and 30-50 minutes for cyclists, broken into blocks if needed. Hard but controlled. You should be able to speak in short sentences.

VO2max/anaerobic work: The peak. One session per week, done fresh. Intervals of 3-5 minutes at VO2max intensity, or 30 seconds to 2 minutes at anaerobic capacity, with full recovery between efforts, and that is the work that drives the fastest aerobic gains and builds speed. HIIT induces comparable chronic physiological adaptation with less time expenditure than classical endurance programs.

Recovery: At least one full easy/rest day per week. Adaptation happens during recovery, not during training.

A sample week for a runner might look like:

• Monday: Rest or easy run
• Tuesday: Threshold effort (tempo run or sustained intervals)
• Wednesday: Easy run
• Thursday: VO2max intervals (5x3 min hard, 2 min easy, or 8x2 min hard, 90 sec easy)
• Friday: Easy run or rest
• Saturday: Long easy run
• Sunday: Rest

A cyclist runs a similar split, longer steady rides on the weekend days and shorter, sharper intervals midweek.

Aerobic capacity ceiling and combined adaptation

Worth pinning down: HIIT produces adaptations in both cardiovascular and muscular systems, and high-intensity training increases stroke volume, capillary density, and mitochondrial function. It supplements aerobic training. It does not replace it.

Combined aerobic and anaerobic training produces greater improvements than either alone, because the aerobic base gives you the capacity to recover between hard efforts and to sustain the lactate clearance that threshold work demands, while the anaerobic work lifts your ceiling and drives the fastest VO2max gains per hour of training.

Neither system works in isolation. You need both.

When to emphasize each phase

Base phase (off-season): Aerobic volume dominates. Easy running or cycling, long steady efforts, consistency, with most of your training time spent below lactate threshold while you build the foundation for the season.

Build phase: Balance shifts toward threshold and VO2max work as the race approaches. The aerobic base stays. The intensity distribution shifts up.

Peak/race phase: Maintenance only. Threshold and VO2max work shrink, one short hard session per week is enough to hold your edge, and most volume returns to easy pace because the racing uses the hard intensity.

Taper (1-2 weeks pre-race): Mostly easy. A few short, sharp intervals to stay sharp. Nothing that accumulates fatigue.

How Movement Rebels fits

The hard part is knowing which intensity you are at. And whether it matches the plan. Garmin or Apple Health gives you heart rate, but heart rate drifts with fatigue and environmental stress. The coach reads your training load, your recent hard efforts, your sleep and readiness, and your periodization phase, then works out where you should sit on the aerobic-to-anaerobic spectrum for the week.

Flat week or poor sleep, it pulls the hard work back and protects your aerobic base. Recovered and ready, it adds a VO2max session, and it reads whether you did threshold work or easy running and adapts next week from that. The coach sees your current aerobic capacity (via VO2max estimates from Garmin) and your lactate threshold (inferred from pace at given heart rates), and prescribes work that matches your fitness and phase.

Start with two solid weeks of mostly easy running or cycling, at least 80% below lactate threshold. Feel the difference between aerobic and anaerobic paces. Then add threshold work once per week. Power and pace climb. The body adapts to what you ask it to do.