A heart rate (HR) measurement represents the number of times your heart beats per minute. This metric is a common way to gauge the intensity of physical effort, with a higher rate generally reflecting greater exertion. The dilemma of a low reading despite feeling highly exerted during exercise is a common point of confusion for many people who monitor their fitness. Understanding your personal maximum heart rate allows you to determine your target heart rate zones. When your measured heart rate fails to align with your feeling of intense physical effort, it suggests a disconnect that requires investigation.
Is Your Heart Rate Monitor Accurate
The first explanation for a suppressed heart rate reading often lies with the device itself. Most wrist-worn fitness trackers use optical sensors that shine a light into the skin to detect changes in blood volume with each pulse. This method, known as photoplethysmography (PPG), is convenient for everyday use but is susceptible to interference during intense or repetitive movement.
Rapid arm motion, such as during running or rowing, can introduce “motion artifact,” causing the sensor to lose consistent contact with the skin and leading to inaccurate, often artificially low, readings. Wrist-based monitors are generally less accurate than chest-strap monitors, which use electrical sensors to detect the heart’s electrical impulses directly. A chest strap is considered the gold standard for personal tracking and typically shows a much higher correlation with a medical electrocardiogram (EKG). External factors like a poor fit or cold weather constricting blood vessels can also hinder the optical sensor’s ability to accurately detect blood flow.
Training Efficiency and Athletic Adaptation
If your heart rate monitor is confirmed to be accurate, the next likely explanation is a positive physiological change due to consistent training. Endurance training causes the heart muscle to strengthen, a change often referred to as “athlete’s heart.” A stronger heart can pump a larger volume of blood with each beat, a measurement known as stroke volume.
This enhanced efficiency means the heart requires fewer beats per minute to deliver the necessary oxygenated blood to the working muscles at a given level of effort. While the average adult resting heart rate is between 60 and 100 beats per minute, highly trained athletes may have resting rates as low as 40 beats per minute. During exercise, this adaptation translates to a lower heart rate response for the same workload compared to an untrained individual. Your body is simply more efficient, requiring a greater physical stimulus to push your heart rate into the higher training zones.
Medications and Health Conditions
A more complex and serious set of reasons for a persistently low exercise heart rate involves certain medications or underlying health issues.
Medications
A class of prescription drugs called beta-blockers is specifically designed to reduce the heart’s workload by blocking the effects of stress hormones like adrenaline and norepinephrine. These medications, which are commonly prescribed for conditions such as high blood pressure, heart failure, migraines, and anxiety, deliberately suppress your heart rate response.
For individuals taking beta-blockers, the expected maximum heart rate during exercise will be significantly lower, and attempting to reach the standard target heart rate zones can be difficult or impossible. Other cardiac medications, including some calcium channel blockers, can also slow the heart’s electrical conduction and reduce the heart rate during exertion. If you are taking any prescription medication, it is important to discuss its potential effects on your exercise heart rate with your healthcare provider.
Health Conditions
Beyond medication, several health conditions can interfere with the heart’s ability to accelerate during exercise. Problems with the heart’s electrical system, particularly the sinoatrial (SA) node, which is the natural pacemaker, can result in an inappropriately slow heart rate, a condition called chronotropic incompetence.
Other conditions like an underactive thyroid gland, known as hypothyroidism, can also slow the body’s overall metabolism and heart rate response. These internal issues prevent the heart from responding adequately to the body’s increased oxygen demand during physical activity.
When to Consult a Healthcare Professional
While a low heart rate may be a positive sign of cardiovascular fitness, it is important to recognize when it may signal a medical concern. If your low heart rate reading is accompanied by noticeable symptoms, it warrants a medical evaluation.
Warning signs that should prompt a doctor’s visit include:
- Feeling dizzy or lightheadedness
- Experiencing near-fainting episodes during or immediately after exercise
- Disproportionate fatigue
- Shortness of breath
- Chest pain that occurs concurrently with the low heart rate
Your physician may recommend diagnostic tests, such as an electrocardiogram (EKG) or a stress test, to accurately assess your heart’s electrical activity and its performance under exertion. If you are concerned about your maximum exercise heart rate, particularly if you are taking medication, a doctor can help establish a safe and effective new target heart rate based on your individual response.