During physical exertion, the body produces energy through distinct mechanisms that depend on the intensity and duration of the exercise. The key molecule is adenosine triphosphate (ATP), the only fuel capable of triggering a muscle contraction. ATP reserves are depleted within seconds, forcing the body to continuously resynthesize it through three pathways known as energy systems.
Understanding aerobic and anaerobic activities in sports allows for precise targeting of the energy system engaged and directing each session towards a measurable goal.
Energy systems: three pathways for ATP production
The aerobic/anaerobic distinction is based on the presence or absence of oxygen in the ATP production process. The body does not switch from one system to another like a light switch. The three systems operate in parallel, but their respective contributions change according to intensity.
The alactic anaerobic pathway
This pathway mobilizes the phosphocreatine reserves stored directly in the muscle. The resynthesis of ATP is almost instantaneous, without the production of lactate. This system dominates during the first few seconds of maximal effort (sprint start, jump, throw).
Its limitation is its very reduced capacity. The available phosphocreatine depletes quickly, and power drops if the effort continues.
The lactic anaerobic pathway
When intense effort lasts beyond a few seconds, the body breaks down glucose without sufficient oxygen. This process, anaerobic glycolysis, produces ATP faster than the aerobic pathway but generates lactate as a byproduct. The accumulation of lactate and hydrogen ions in the muscle causes a burning sensation and eventually limits muscle contraction.
This system predominates during intense efforts lasting from a few tens of seconds to a few minutes.
The aerobic (oxidative) pathway
For any prolonged effort beyond a few minutes, the body relies on oxidative metabolism. It uses oxygen to break down carbohydrates and lipids in the mitochondria. The energy yield is significantly higher than the other two pathways, which explains why a runner can maintain a moderate effort for hours.
Aerobic threshold and anaerobic threshold: concrete markers for training
The three systems always coexist, but two physiological thresholds allow for the delineation of exploitable intensity zones in training.
The threshold 1 (aerobic threshold) corresponds to the intensity beyond which lactate begins to accumulate measurably in the blood. Below this, the effort relies almost exclusively on the oxidative pathway. This is the fundamental endurance zone, where the body learns to better utilize fats as an energy substrate.
The threshold 2 (anaerobic threshold) marks the point where lactate production exceeds the body’s recycling capacity. Beyond this, fatigue sets in quickly, and the effort can no longer be sustained for long. Working around this threshold develops the ability to tolerate and eliminate lactate, which pushes the performance ceiling higher.
Specialized facilities offer lactate profiling tests, either in the lab or on the field, which provide the speed and heart rate associated with each threshold for a given individual. This data replaces generic formulas and allows for significantly more precise session programming.
Monitoring the effects of aerobic and anaerobic training with a watch
Recent sports watches provide a concrete tool to distinguish the impact of each session on the two systems. The latest Garmin generations (Fēnix 8, Forerunner 970, for example) calculate a distinct Aerobic Training Effect and Anaerobic Training Effect, displayable during the effort and in the post-session summary.
The interest goes beyond a mere gadget. A runner who performs interval sessions will see their Anaerobic Training Effect rise while their Aerobic score stagnates. This imbalance signals that they are neglecting their endurance base. Conversely, a practitioner who only does slow jogging will find their Anaerobic Training Effect nearly zero, which hinders their power progression.
This objective feedback after each training session allows for adjusting the proportion of aerobic and anaerobic sessions throughout the week, rather than relying solely on feelings.
Planning the week: distributing demands to progress
Differentiating between aerobic and anaerobic has value only if this distinction guides planning. Three principles structure an effective training week:
- The majority of the weekly volume is done below the aerobic threshold, in fundamental endurance, to develop oxidative capacity and recovery between intensive sessions.
- One to two sessions target the area around the anaerobic threshold (tempo, threshold, long intervals) to push lactate tolerance and improve sustained power.
- One short and very intense session (sprints, short intervals) targets the alactic and lactic anaerobic pathways, stimulating maximum power and the speed of ATP resynthesis.
A common mistake is spending too much time in an intermediate zone, neither easy enough to regenerate nor hard enough to provoke adaptation. Easy sessions must remain easy, hard sessions must remain hard. This principle of polarization is well documented among endurance athletes and applies to most sports.
A runner, cyclist, or swimmer who controls their intensity zones using heart rate, lactate testing, or data from their watch has a clear framework. Each session has a specific metabolic objective, and progress becomes measurable week after week.
The final trap to avoid: wanting to work on everything at once. Each training cycle benefits from prioritizing one pathway for a few weeks before shifting the focus to the other, rather than diluting the stimulus across all zones in every session.