Heart Rate

What Is Heart Rate?

Heart rate is the number of times the heart beats each minute.

It is expressed as:

beats per minute (bpm)

For example:

Heart Rate = 150 bpm

means the heart is completing approximately 150 beats each minute.

At rest, the body's oxygen and energy requirements are relatively low, so the heart does not need to pump as frequently.

During exercise, working muscles require more oxygen and nutrients and produce more metabolic by-products.

The cardiovascular system responds by increasing blood flow.

One of the primary ways it does this is by increasing heart rate.

As exercise becomes harder, heart rate generally rises.

But the relationship is more complicated than:

More effort means a higher heart rate.

Understanding those complications is what makes heart-rate data useful.

Why Does Heart Rate Increase During Exercise?

Working muscles require energy.

For sustained endurance exercise, much of that energy is produced through aerobic metabolism.

Aerobic metabolism requires oxygen.

Oxygen enters through the lungs, is transported in the blood, and is delivered to working muscles by the cardiovascular system.

The amount of blood the heart pumps each minute is called cardiac output.

Cardiac output depends on two things together: how many times the heart beats each minute, and how much blood it pumps with each beat.

As exercise intensity increases, the body needs greater blood flow.

Heart rate rises as part of the cardiovascular response required to meet that demand.

Heart Rate Is an Internal Load Metric

One of the most useful distinctions in endurance training is between external workload and internal load.

External workload describes what the athlete is producing.

Examples include:

Cycling: power

Running: pace, speed, or running power

Swimming: pace or speed

Internal load describes how the athlete's body responds.

Heart rate is one of the most accessible measures of that response.

Consider a cyclist riding at:

200 W

The power tells us the mechanical workload.

Now add:

Heart Rate = 135 bpm

We know considerably more.

If that same cyclist later requires:

200 W → 150 bpm

to produce the same workload, something about the physiological response has changed.

That difference is often more informative than either number alone.

Heart Rate vs. Power

Power and heart rate should not be viewed as competing metrics.

They measure different things.

Power is what the cyclist produces.

Heart rate is how the cardiovascular system responds.

Suppose a cyclist begins an interval at 350 W.

Power can increase from 150 W to 350 W almost immediately.

Heart rate cannot.

It may take tens of seconds or longer for heart rate to rise toward the level associated with the new workload.

This difference is known as heart-rate lag.

For short intervals, power can therefore be much more useful for controlling intensity.

But heart rate provides information that power cannot.

It tells us how the athlete is responding internally to that workload.

Heart Rate vs. Running Pace

The same principle applies to running.

Suppose a runner increases pace from:

6:00 min/km

to:

4:30 min/km

The pace change occurs immediately.

Heart rate rises more gradually.

Similarly, running uphill at 5:30 min/km may require substantially greater physiological effort than running 5:30 min/km on flat terrain.

Heart rate helps reveal that difference.

This is why pace and heart rate work well together.

Pace describes performance.

Heart rate provides physiological context.

Heart Rate in Swimming

Heart rate can also provide useful information for swimmers, although measurement is more difficult.

Water can interfere with some optical heart-rate sensors, wrist movement affects signal quality, and watches may behave differently depending on stroke and device design.

Chest or specialized swimming sensors can provide better data in some situations.

Swimming also introduces another complication: many pool sessions consist of repeated intervals separated by recovery.

For example:

10 × 200 m with 30 seconds rest

Heart rate rises during each repetition and falls during recovery.

An average heart rate across the entire session may therefore combine two very different physiological states.

This means swimming heart-rate data often needs to be interpreted in the context of the workout structure rather than simply averaged across the entire activity.

Heart Rate Responds More Slowly Than Workload

Heart-rate lag is one of the most important concepts when interpreting exercise data.

Imagine a cyclist riding:

150 W → 150 bpm

and suddenly increasing to:

350 W

The power meter detects the change almost immediately.

Heart rate may initially remain close to its previous value.

Over the following seconds and minutes, it rises.

Now imagine the athlete suddenly reduces power.

Power falls immediately.

Heart rate remains elevated for some time before gradually declining.

Therefore:

This becomes especially important during highly variable exercise.

Why Heart Rate Changes at the Same Workload

One of the strengths of heart rate is also one of its limitations.

Heart rate responds to more than exercise intensity.

Consider an athlete who normally produces:

200 W at 135 bpm

On another day:

200 W at 145 bpm

And another:

200 W at 155 bpm

The external workload is identical.

The physiological response is not.

Possible reasons include:

• heat
• dehydration
• accumulated fatigue
• poor sleep
• illness
• stress
• altitude
• caffeine
• recovery status
• prolonged exercise
• environmental conditions

This means heart rate should not be interpreted as a perfect measurement of workload.

It is a measurement of response to workload within a physiological and environmental context.

Maximum Heart Rate

Maximum heart rate, often abbreviated as HRmax, represents the highest heart rate an athlete can reach during maximal effort.

For example:

Maximum Heart Rate = 190 bpm

Maximum heart rate is influenced strongly by age and individual physiology.

Two athletes of the same age can have substantially different maximum heart rates.

A higher maximum heart rate does not necessarily mean that one athlete is fitter.

Likewise, a lower maximum heart rate does not necessarily indicate poor cardiovascular fitness.

Maximum heart rate is primarily useful as an individual reference point.

Estimated vs. Measured Maximum Heart Rate

Formulas such as:

220 − age

are commonly used to estimate maximum heart rate.

For a 40-year-old athlete:

220 − 40 = 180 bpm

But this is a population-level approximation.

The athlete's actual maximum might be:

165 bpm

or:

195 bpm

Individual variation can be substantial.

For this reason, a maximum heart rate observed from credible maximal exercise data is generally more informative for an individual athlete than a generic age-based formula.

The distinction is important:

Lactate Threshold Heart Rate

Another important heart-rate metric is Lactate Threshold Heart Rate, commonly abbreviated as LTHR. LTHR attempts to represent heart rate associated with sustained exercise around an athlete's lactate or threshold intensity.

This can be more useful for training-zone prescription than maximum heart rate alone.

An athlete might have:

Maximum HR = 190 bpm

and:

Cycling LTHR = 170 bpm

Training zones can then be constructed relative to threshold rather than simply as percentages of maximum heart rate.

Threshold heart rate can also differ between sports.

A cyclist's threshold heart rate should not automatically be assumed to be identical to their running or swimming threshold heart rate.

Heart-Rate Zones

Heart-rate zones divide exercise intensity into ranges.

A simple system might progress from:

Different training systems use different numbers of zones and different boundaries.

Zones may be based on:

• maximum heart rate
• heart-rate reserve
• lactate threshold heart rate
• physiological testing

This means two platforms can show different zones for the same athlete even when both have the same heart-rate data.

The important question is not simply:

“What zone am I in?”

It is:

“How were these zones derived?”

Resting Heart Rate

Resting heart rate is the heart rate measured when the body is at rest.

It is usually much lower than exercise heart rate.

Endurance training can reduce resting heart rate because cardiovascular adaptations allow the heart to pump more blood with each beat.

However, a single resting-heart-rate measurement should not be overinterpreted.

The more useful information is often the athlete's personal baseline and how today's value compares with it.

An unusual increase can sometimes accompany fatigue, stress, illness, poor sleep, or other physiological changes.

But resting heart rate alone cannot identify the cause.

Heart Rate and Fitness

A common assumption is:

A lower heart rate means a fitter athlete.

This is only partly useful.

If the external workload is controlled, changes in heart rate can provide information about adaptation.

Under comparable conditions, this may indicate that the athlete can now produce the same workload with lower cardiovascular demand.

But comparing heart rate without controlling workload is much less informative.

A heart rate of 140 bpm could represent:

• an easy ride
• a moderate run
• a hard swim
• recovery from an interval
• prolonged exercise with cardiovascular drift

Heart rate needs context.

Heart Rate and Cardiovascular Drift

During prolonged steady exercise, heart rate often gradually increases even when external workload remains approximately constant.

For example:

Early:

200 W → 135 bpm

Later:

200 W → 145 bpm

Later still:

200 W → 150 bpm

This phenomenon is commonly referred to as cardiovascular drift.

It can be influenced by rising body temperature, changes in blood volume, hydration status, fatigue, and the duration of exercise.

The cyclist is still producing approximately 200 W, but the cardiovascular cost of producing it has increased. This relationship is one reason heart rate is particularly useful during longer endurance sessions.

Heart Rate and Aerobic Decoupling

Heart rate can be combined with workload to examine aerobic decoupling.

The idea is to compare the relationship between external output and heart rate during different portions of a sustained effort.

For cycling, that means comparing how much power the rider produces for each unit of cardiovascular effort.

For running, a similar relationship can be examined using speed or another appropriate workload measure. If an athlete maintains similar output while heart rate rises substantially, efficiency between external workload and cardiovascular response has changed.

This may indicate that the athlete is having increasing difficulty sustaining the effort.

However, decoupling analysis requires appropriate data.

Warm-ups, cool-downs, stops, coasting, intervals, large terrain changes, and highly variable workload can distort the calculation.

The relationship should therefore be evaluated over a suitable sustained section rather than blindly across every recorded activity.

Heart Rate and VO₂ Max

Heart rate also plays an important role in many VO₂ max estimation systems.

Laboratory VO₂ max testing directly measures respiratory gases.

A watch or training platform usually cannot do that.

Instead, it may infer aerobic fitness from relationships between:

• heart rate
• power
• running speed
• exercise intensity
• athlete characteristics
• sustained effort

For example, if an athlete can produce greater power at a similar heart rate after a period of training, that relationship may provide evidence of improved aerobic fitness.

But heart rate alone cannot directly measure VO₂ max.

The distinction between measurement and estimation is important.

Average Heart Rate

Average heart rate is one of the most commonly displayed activity metrics.

For example:

Average Heart Rate = 148 bpm

It is easy to understand but can sometimes be misleading.

Consider two one-hour workouts.

Workout A

The athlete remains close to:

148 bpm for most of the hour

Workout B

The athlete repeatedly alternates between:

110 bpm recovery

and:

180 bpm hard efforts

Both sessions might produce a similar average heart rate.

Their physiological structure is completely different.

Average heart rate therefore provides a useful summary, but it should not replace analysis of the activity itself.

Time in Heart-Rate Zones

Instead of looking only at average heart rate, athletes often examine how much time was spent in each zone.

For example:

Zone 1: 10 minutes

Zone 2: 55 minutes

Zone 3: 20 minutes

Zone 4: 12 minutes

Zone 5: 3 minutes

This provides more information about the intensity distribution of the session.

Over weeks and months, zone distribution can also help describe the athlete's overall training pattern.

But its usefulness depends heavily on whether the underlying zone boundaries are appropriate.

Incorrect maximum or threshold heart rate can make the entire distribution misleading.

Measuring Heart Rate

Heart rate during exercise is commonly measured using either:

electrical measurement

or:

optical measurement

Chest straps generally detect electrical activity associated with each heartbeat.

Optical sensors use light to detect changes in blood volume beneath the skin, a technique known as photoplethysmography.

Both approaches can provide useful data, but their behavior differs.

Chest Strap vs. Optical Heart Rate

Chest straps are widely used when accurate exercise heart rate is important.

They typically respond quickly to changes and perform well during high-intensity exercise.

Wrist-based optical sensors offer greater convenience because they are built into many sports watches.

However, optical measurement can be affected by:

• movement
• watch fit
• skin contact
• temperature
• vibration
• swimming
• cadence-related artifacts
• sensor placement

For steady endurance exercise, optical data may be perfectly adequate for many athletes.

For rapid intervals or situations where precise heart-rate timing matters, measurement quality becomes more important.

Cadence Lock

One problem sometimes encountered with optical heart-rate sensors is cadence lock.

The device may mistakenly interpret repetitive movement associated with running cadence as the pulse signal.

For example:

Actual heart rate:

145 bpm

Running cadence:

172 steps/min

The watch might incorrectly report a heart rate close to:

172 bpm

This can produce apparently plausible data that is physiologically incorrect.

Automated analysis should therefore consider signal quality rather than assuming every recorded heart-rate value is valid.

Missing and Invalid Heart-Rate Data

Activity files can contain:

• missing samples
• zero values
• sudden spikes
• sudden drops
• sensor disconnections
• implausible readings
• repeated values
• delayed sensor acquisition

These errors matter.

Suppose a heart-rate sensor briefly reports:

245 bpm

That should not automatically become the athlete's maximum heart rate.

Similarly, missing heart-rate samples should not necessarily be treated as genuine zero-heart-rate observations.

A robust analysis system needs to distinguish between:

recorded data

and:

credible physiological data.

Why Data Quality Matters

Heart rate is often used downstream to calculate other metrics.

It may influence:

• maximum heart rate
• threshold heart rate
• heart-rate zones
• VO₂ max estimates
• aerobic decoupling
• Load
• recovery analysis
• intensity classification

An incorrect heart-rate value can therefore affect much more than the activity in which it occurred.

Good physiological analysis should validate the underlying signal before using it to derive additional metrics.

Sport-Specific Heart Rate

Heart rate should not always be treated identically across cycling, running, and swimming.

The same athlete may have different relationships between heart rate and performance in each sport. Differences can result from:

• body position
• muscle mass involved
• movement mechanics
• cooling
• breathing patterns
• sport-specific training
• measurement technology

For this reason, sport-specific threshold values and zones can sometimes provide better training guidance than assuming one heart-rate profile applies universally.

Environmental Conditions Matter

Heart rate is sensitive to the environment.

Heat

When body temperature rises, the cardiovascular system must support both exercise and heat dissipation. Heart rate may therefore be higher at the same external workload.

Altitude

Reduced oxygen availability can alter cardiovascular response and exercise capacity.

Dehydration

Changes in plasma volume and thermoregulation can contribute to increasing cardiovascular strain.

This is why comparing:

200 W at 140 bpm

on a cool day with:

200 W at 150 bpm

during extreme heat does not automatically indicate a loss of fitness.

Context matters.

Heart Rate and Fatigue

Heart-rate behavior can change with fatigue, but the relationship is not always simple.

In some situations, heart rate may be elevated relative to workload.

In others, particularly during substantial fatigue, an athlete may struggle to raise heart rate despite attempting a hard effort.

Therefore, rules such as:

“Higher heart rate means more fatigue”

are too simplistic.

Heart rate should be interpreted alongside workload, perceived effort, recent training, environment, and the athlete's normal response.

What Is a Good Heart Rate?

There is no universal exercise heart rate that is “good.”

A heart rate of 160 bpm might be:

• easy for one athlete
• threshold intensity for another
• near maximum for someone else

Age alone does not solve this problem.

Neither does comparing heart-rate values with friends or professional athletes.

The useful reference is the athlete's own physiology.

Instead of asking:

“Is 160 bpm good?”

ask:

“What does 160 bpm represent for this athlete, in this sport, at this workload?”

Why Trends Matter More Than Isolated Readings

A single heart-rate observation contains limited information.

Patterns become much more useful.

Imagine a cyclist performing comparable endurance rides over several months:

January: 200 W → 148 bpm

March: 200 W → 143 bpm

May: 200 W → 138 bpm

If conditions and measurement quality are sufficiently comparable, this trend may provide meaningful evidence of adaptation.

The same principle applies to running pace and swimming workload.

Longitudinal relationships are usually more informative than isolated numbers.

How Should Athletes Use Heart Rate?

Heart rate becomes most useful when connected to a specific question.

For example:

How hard is my body working at this workload?

Am I staying within the intended aerobic intensity?

Is my heart rate unusually high for my normal endurance power or pace?

Is cardiovascular strain increasing during a sustained effort?

Has my heart-rate response changed as my fitness has improved?

How much time am I spending in each training zone?

Does today's physiological response differ meaningfully from my historical baseline?

These questions use heart rate as physiological information rather than simply another number on the activity summary.

The Most Important Relationship

Heart rate becomes particularly powerful when combined with external workload.

For cycling:

Power ↔ Heart Rate

For running:

Pace / Speed / Power ↔ Heart Rate

For swimming:

Pace ↔ Heart Rate

External workload tells us:

What did the athlete do?

Heart rate helps tell us:

What did it cost the athlete physiologically?

Over time, changes in that relationship can reveal information that neither signal provides independently.

The Most Important Thing to Remember

Heart rate is not a direct measurement of performance.

It is not power.

It is not pace.

It is not VO₂ max.

And a higher or lower heart rate is not automatically good or bad.

Heart rate is a measurement of the cardiovascular system's response within a much larger physiological process.

Its meaning depends on:

workload + duration + athlete physiology + environment + fatigue + measurement quality + sport Without that context, heart rate is simply a number.

With that context, it becomes one of the most useful physiological signals available to endurance athletes.

Conclusion

Heart rate provides a window into how the body responds to exercise.

While cycling power, running pace, and swimming pace describe external performance, heart rate helps describe the internal cardiovascular demand required to produce that performance.

Its greatest value therefore comes not from looking at heart rate in isolation, but from understanding its relationship with workload.

The same power at a lower heart rate may indicate something very different from the same power at a substantially higher heart rate. A rising heart rate during constant workload can reveal increasing cardiovascular strain. Changes in heart-rate response across weeks and months can provide insight into adaptation.

At the same time, heart rate is affected by heat, hydration, fatigue, stress, altitude, sensor quality, exercise duration, and many other factors.

The most useful question is therefore not:

“What was my heart rate?”

It is:

“What workload was I producing, how did my cardiovascular system respond to it, and how does that response compare with what is normal for me?”

That is where heart rate becomes more than a number—and becomes a meaningful measure of physiological response to training.

KEY TAKEAWAY

• Heart rate is an internal measure: it describes the body's response, not the work being produced.
• Estimated maximum heart rate is not the same as measured or observed maximum heart rate.
• The heart rate recorded at a particular second does not necessarily represent the workload occurring at exactly that second.
• Heart rate becomes far more informative when it is interpreted alongside the workload that produced it.