Power vs Heart Rate vs Breathing: What Should You Trust?
Share
For a long time, endurance athletes have relied on external and internal metrics such as power and heart rate to guide training intensity. These tools have become standard, particularly in structured training environments where precision is important.
However, these metrics are often treated as fixed targets, while the body itself is continuously changing.
One of the key challenges in endurance training is that physiological thresholds do not remain constant over the course of a session. As fatigue accumulates, the relationship between effort and output begins to shift. What feels controlled early in a workout may represent a different physiological state later on.
This raises a fundamental question. What metric actually reflects the body’s true state during exercise?
How Intensity Changes Over Time
To understand this, it is useful to look at how different metrics behave during prolonged exercise, particularly around the first ventilatory threshold, which defines the upper boundary of aerobic training.
At this intensity, the goal is to maintain a stable metabolic state. However, as the session progresses, the systems that support this effort begin to respond differently.
External output, cardiovascular response, and breathing do not remain aligned. Instead, they begin to diverge.
This divergence is what creates the difficulty in maintaining consistent intensity.
Power, Pace and the External Load
Power provides a precise measure of external work. It allows athletes to define intensity with a high degree of control and is widely used for structuring training sessions.
However, power does not account for internal changes.
Over the course of prolonged exercise, the power associated with a given physiological state begins to decline. In controlled conditions, power at the first ventilatory threshold has been shown to decrease over time during steady efforts.
This means that maintaining a constant power output does not guarantee that the body remains at the same intensity. As fatigue develops, the same workload becomes more demanding.
In practice, an athlete who starts a session at the correct aerobic intensity may gradually drift above it if they continue to hold the same power or pace.
Heart Rate and Cardiovascular Drift
Heart rate provides a measure of the body’s internal response to exercise. It reflects how hard the cardiovascular system is working to support the effort.
Unlike power, heart rate does respond to internal changes. However, it does not remain stable either.
During prolonged exercise, heart rate tends to increase over time, even when external workload remains constant. This phenomenon, often referred to as cardiovascular drift, becomes more pronounced as fatigue and other stressors accumulate.
At the level of the first ventilatory threshold, heart rate has been shown to increase over the course of a session.
This creates a different kind of problem. If an athlete relies strictly on heart rate zones defined in a rested state, they may begin to reduce their effort unnecessarily as heart rate rises, even if the underlying metabolic demand has not changed.
As a result, heart rate can drift away from the physiological anchor it is meant to represent.
Breathing and Metabolic Stability
Breathing, and more specifically minute ventilation, reflects the body’s metabolic state more directly.
At the first ventilatory threshold, ventilation corresponds to the balance between energy production and carbon dioxide removal. This balance defines the upper boundary of sustainable aerobic work.
What is notable is how stable this signal remains over time.
In the same conditions where power decreases and heart rate increases, ventilation at this threshold shows minimal change.
Although the pattern of breathing may shift, with changes in breathing rate and tidal volume, the total ventilation remains consistent. This indicates that the underlying metabolic demand is being maintained, even as other systems respond to fatigue.
This stability suggests that ventilation remains anchored to the physiological processes that define intensity.
What This Means in Practice
These differences between metrics become important when trying to maintain a consistent training intensity.
If power is held constant, the body may drift above the intended zone. If heart rate is followed strictly, effort may be reduced unnecessarily. In both cases, the link between the target and the underlying physiology begins to break.
Breathing provides a way to maintain that link.
By anchoring intensity to ventilation, it becomes possible to adjust output as the session progresses while keeping the metabolic demand consistent. This allows the athlete to remain within the intended zone, even as fatigue develops.
Final Thoughts
Endurance training is often framed in terms of precision. Athletes look for exact values for power, heart rate, and pace, assuming that these numbers define the correct intensity.
However, the body does not operate on fixed numbers. It adapts continuously throughout a session.
The evidence shows that while power and heart rate provide useful information, they do not remain stable representations of physiological intensity over time. Breathing, and specifically ventilation, maintains a closer connection to the underlying metabolic processes.
This does not replace other metrics, but it changes how they are interpreted.
The goal is not simply to hold a number, but to maintain the intended physiological state.
References: