Decoding the Digital Athlete: How Sports Watches Measure Human Performance

For decades, the main job of watches was simple: to tell time. Today, that same wrist-worn device can monitor heart rhythms, distance that a user traveled, and sleep quality. The evolution that sports watches underwent illustrates a broader shift in the way that athletes understand and quantify performance. 

Currently, sports watches and other forms of wearable technology are at the forefront of development in the global sports and technology industry. As a result, sports watches are one of the few devices that can be comfortably and unobtrusively worn close to the human body for extended periods. Located close to the blood vessels and nerve signals in the wrist, the various sensors on sports watches can easily obtain data, a function that other devices, such as cell phones, aren’t able to perform accurately.

Heart rate is measured by a photoplethysmography (PPG) sensor. A PPG sensor is composed of a green light-emitting diode (LED), which illuminates the skin, and a photodiode, which converts the reflected light into an electric signal. When illuminated by LEDs, blood vessels underneath the wearer’s skin reflect red light and absorb green light. When a pulse passes the wrist, the blood volume in the blood vessel increases, which also increases absorption of green light and reduces reflected light. Between two pulses, the blood volume decreases and therefore absorbs less green light while reflecting more red light. This repeated decrease and increase in electrical signals forms a series of waves that represent the heartbeat. The heart rate is calculated from the time between two heartbeats, typically in beats per minute. When a sports watch measures heart rate, movement can interfere with accuracy. During periods of acceleration, a user’s heart rate may increase sporadically, leading the watch’s PPG sensor to display imprecise readings. Therefore, PPG data is significantly more accurate when the user is stationary.

The heart’s rhythm can reveal more than just the user’s pulse. For example, heart-related data can be analyzed to estimate respiratory rate, a metric that measures how often a person breathes. Respiratory rate can be estimated using PPG-derived heart rate data through a phenomenon known as respiratory sinus arrhythmia. During inhalation, heart rate slightly increases, while during exhalation, it decreases. The rhythmic variations in heart rate create a repeating pattern that corresponds to breathing cycles. By analyzing these fluctuations over time, a sports watch can calculate the number of breaths per minute without requiring a separate respiratory sensor. 

Understanding heart rate is only part of understanding an individual’s health. Sports watches can measure various other health monitoring needs, such as electrocardiogram (ECG), blood pressure, and blood oxygen levels. In order to fulfill these many demands, even a small sports watch requires strong integration and scheduling capacities, along with a sufficient number of sensors, to complete the detection and analysis of multiple health indicators in a short period of time.

Electrical signals from the heart provide especially valuable diagnostic information. In order for a sports watch to record an ECG, a circuit with two electrodes needs to be completed. The circuit begins when part of the sports watch touches the user’s wrist and is completed once the user touches a button or ‘knot’ on the watch with a finger from their opposite hand. The closed electrical circuit moves from finger to heart, heart to wrist, wrist to watch, and back to the finger. This allows the watch to detect and record the heart’s electrical impulse, resulting in an ECG. Unlike basic heart rate monitoring, an ECG provides information about the quality and pattern of each heartbeat, making it easier to detect irregular heartbeats (cardiac arrhythmia). 

In addition to electrical activity, sports watches can also non-invasively monitor the chemical composition of blood. Blood oxygen saturation is measured using reflective pulse oximetry; hemoglobin in the blood carries oxygen and reflects red light, with the intensity of this reflected light varying depending on the level of oxygen present. An LED on the watch emits red light on the wrist, and a photodiode converts the reflected light into an electric signal. The watch then analyzes the intensity of this signal to determine the blood oxygen saturation level, which illustrates how efficiently blood is being carried throughout the body. It is an important metric in determining fitness level and early detection of heart conditions. 

To further expand health monitoring capabilities, some sports watches have a blood pressure measurement function that utilizes both pulse transit time (PTT), the time it takes for a pulse to travel from the heart to the wrist, and blood pressure. High blood pressure speeds up blood flow, shortening the PTT, while low blood pressure lengthens the transit time. Monitoring blood pressure helps to track how hard your heart is pumping blood against your arteries. It is valuable in providing early indicators of hypertension or heart strain and can be used to assess overall cardiovascular health. 

In addition to daily health metrics, sports watches also provide insights into the body’s recovery during rest. Sleep quality monitoring is based on the structure of sleep cycles. A single sleep is made up of four to six sleep cycles, each lasting about 90 minutes. Each cycle has three stages: light sleep, deep sleep, and rapid eye movement sleep. By converting mechanical forces into electrical signals, the watch’s accelerometer detects body movements as well as heart rate changes to obtain the sleep stages and evaluate sleep quality. 

Sports watches typically attract consumers invested in their health and fitness, as watches provide insights available anytime, anywhere. In addition to health monitoring, sports watches play an equally important role in tracking daily physical activity.

A pedometer uses an accelerometer to record the number of steps a user takes. Any type of movement will produce acceleration, allowing the sensor to detect even subtle movements such as quick turns or sudden stops. A three-axis sensor can measure acceleration in three directions, including gravitational acceleration when stationary. The ratio of the acceleration values in the three directions can be used to determine the orientation of the sensor when stationary, which also indicates the orientation of the sports watch. Additionally, a watch’s pedometer considers arm movements to distinguish if users are running or just moving their arms. While the pedometer can tell you how far you have walked, it is unfortunately often inaccurate because the stride length is estimated. Accelerometers rely on movement patterns, not direct measurements of each step, which means that slight differences in running style and terrain can cause distortions in measurement data. 

While pedometers focus on horizontal movement, sports watches also track vertical activity. Altitude is calculated based on changes in air pressure, which decreases as altitude increases. By measuring the difference in air pressure between two points, a sports watch can determine the height difference between them. Modern barometers are now small enough to be integrated into wearable devices and enable real-time elevation tracking. Whether a user is climbing a mountain or walking up stairs, the watch can calculate elevation gain using barometric pressure data. 

As technology advances, people’s expectations for sports watches continue to rise. Sports watches that support exercise and track health have become a glimpse into the future digital lifestyle. Since watches sit directly on the skin, today’s wearables can track multiple health indicators, alert users to changes, and even identify unusual patterns that may signal long-term health risks. Thanks to their comfortable, all-day design, sports watches can transform occasional checkups into real-time health monitoring, warning users of future health problems like diabetes. The data collected becomes a valuable resource for managing and improving health. Current research points to the use of advanced biometrics, which AI will use to deliver real-time coping and stress management strategies. Ideally, these systems will continue to evolve and adapt, growing smarter as technology and our needs change, and bringing us closer to a future where our wristwatch not only tracks our health but helps us improve it every day.