Why heart rate rises during a steady effort
The same effort starts to feel different
You settle into a steady run or ride. Half an hour later your pace or power is much the same, but your heart rate has climbed. The change can help you understand how the demands of a session develop: a pace or wattage that felt comfortable early on may require a greater cardiovascular response later.
This is often called heart-rate drift. It is useful context for long runs, endurance rides and pacing in the heat, but it is not a stand-alone fitness test. The interactive example starts at 130 bpm and rises to 150 bpm while the running pace stays at 5:00 per kilometre or cycling power averages 200 W in each half. Both traces are illustrative.
Compare the ratios
Divide the steady output by each half’s average heart rate. The lower second ratio shows the change in this example.
The same effort starts to feel different
You settle into a steady run or ride. Half an hour later your pace or power is much the same, but your heart rate has climbed. The change can help you understand how the demands of a session develop: a pace or wattage that felt comfortable early on may require a greater cardiovascular response later.
This is often called heart-rate drift. It is useful context for long runs, endurance rides and pacing in the heat, but it is not a stand-alone fitness test. The interactive example starts at 130 bpm and rises to 150 bpm while the running pace stays at 5:00 per kilometre or cycling power averages 200 W in each half. Both traces are illustrative.
What is happening inside the body?
Your muscles need a continuous supply of oxygen. Blood carries that oxygen, and cardiac output is the volume of blood the heart pumps each minute. It depends on two things: heart rate, or beats per minute, and stroke volume, the amount pumped with each beat. Cardiac output = heart rate × stroke volume.
During prolonged steady exercise, heart rate can gradually rise while stroke volume falls. If less blood is pumped with each beat, a faster heart rate can help maintain blood flow. For a simple illustration, 140 beats per minute at 100 mL per beat and 150 beats per minute at about 93 mL per beat both give roughly 14 litres per minute. Real responses are more complicated, and these figures are not the measurements behind the chart.
This pattern is called cardiovascular drift. A watch measures heart rate; it does not tell you your stroke volume or cardiac output. A rising trace can be consistent with cardiovascular drift, but heart rate alone cannot establish the mechanism in your particular session.
Why heat changes the demands on circulation
Working muscles produce heat. To release it, the body increases blood flow near the skin and produces sweat; evaporation carries heat away. Your circulation is supporting both exercise and cooling, and hot or humid conditions can make heat loss more difficult.
As exercise continues in the heat, changes in body temperature, blood distribution and the time available for the heart to fill can contribute to cardiovascular drift. The mechanisms interact: it is too simple to say every extra beat is caused by blood being diverted to the skin, or that the same mechanism dominates every session.
In one small study, seven active men completed running and cycling trials at 35°C. Between 15 and 45 minutes, heart rate rose and stroke volume fell in both activities. This demonstrates a response under those conditions, not a prediction of the number of beats you should see.
Where sweating and hydration fit in
Sweat removes water as well as helping the body cool. If fluid losses are substantial and not replaced, blood volume and the amount of blood returning to the heart can fall. That can reduce cardiac filling and stroke volume, increasing cardiovascular strain during a long, hot effort.
In a study of eight people cycling for two hours in the heat, the dehydrated trial produced lower blood volume, cardiac filling, stroke volume and cardiac output than the hydrated comparison. That was a small experiment with a substantial fluid deficit; it does not mean that every heart-rate rise is dehydration or that a watch can tell you how much to drink.
Drift can also occur without dehydration. Conditions, intensity and individual responses matter. A lower drift after changing a drink or cooling setup is an observation worth investigating, not proof that one factor caused the whole difference.
What the two halves actually compare
The chart asks a narrower question than the physiology: how much output was recorded for a given average heart rate in each half? For cycling, divide each half’s average power by its average heart rate. In this example, about 200 W divided by 135 bpm is 1.48 W/bpm; about 200 W divided by 145 bpm is 1.38 W/bpm.
The second ratio is roughly 7% lower than the first. That does not mean you became 7% less fit, used 7% more energy or lost 7% of your stroke volume. It describes the change in this chosen output-to-heart-rate ratio. The visualisation calculates from the unrounded samples, so working from the rounded numbers on screen gives a slightly different answer.
For running, the example converts 5:00 per kilometre to 12 km/h before dividing by heart rate. Speed and pace run in opposite directions: a faster pace has fewer minutes per kilometre but more kilometres per hour. Comparing speed/heart rate keeps a falling ratio in the same direction as the cycling example.
Why your platform may show a different percentage
Different tools use different inputs. TrainingPeaks describes its cycling efficiency factor using Normalized Power and its running version using normalized graded pace. Our chart uses simple average watts or constant speed and average heart rate; it is a teaching example, not a reproduction of that platform’s calculation.
Where you start and end the selection matters too. Including the warm-up, a long stop, a surge or an easier final section can change a half-to-half ratio. Compare the same interval, pause treatment and metric definition before deciding that two numbers disagree.
Is a higher heart rate always a problem?
No. Heart rate responds to more than your cardiovascular system’s response to steady work. A climb, headwind, change in running surface, faster pace, power surges or a hot room can change the demands of a session. Fatigue, sleep, illness, medication and other individual factors may also affect the response.
Holding a heart-rate target constant reverses the picture: if heart rate tends to rise as the session continues, you may need to reduce power or slow your pace to keep the same displayed heart rate. That can be a sensible way to manage the day’s effort, but the change does not identify its cause by itself.
Some coaching tools use a percentage as a heuristic for aerobic endurance. Treat any cut-off as a starting point for a consistent comparison, not a universal pass or fail. The duration, intensity, environment and whether the measurement is reliable all affect the result.
Check the data before interpreting it
Choose a continuous, reasonably steady section after you have warmed up. Inspect the power or pace trace as well as heart rate. For running, a flat route or consistent treadmill conditions make pace easier to compare; hills, wind and stops can make constant pace a poor proxy for constant effort.
Look for obvious recording problems: a sudden implausible step in heart rate, a flatline, missing samples or a pace spike around poor GPS reception. Wrist optical sensors and chest straps measure differently, and accuracy can vary by device and activity. If a surprising result matters, check the fit and compare with a reliable recording rather than treating the graph as unquestionable.
Use drift to ask a better question
Compare similar steady sessions over time: the same sport, interval length, power or pace, sensor and as far as practical temperature, terrain and cooling. Note anything that changed, including how the effort felt. One hot, poorly cooled ride compared with a cool ride cannot tell you whether your endurance improved.
Then ask what is useful for the next session. Did heart rate rise while power and conditions stayed steady? Was the later effort harder than intended? Would a different pace, a better-cooled indoor setup or a more comparable route make the next observation easier to interpret? The value is in the pattern and its context, not chasing a single percentage.