The Submerged Engine: Physiology, Thresholds, and the Art of Data-Driven Swimming

Swimming is unique among endurance sports in its physiological demands. It is an activity where the athlete is suspended in a buoyant, cooling medium, horizontal in orientation, and forced to perform controlled breath-holding (hypoxic) intervals. These factors create a physiological environment vastly different from running or cycling. Consequently, the metrics used to track and improve performance must also be specialized. It is not enough to simply track speed; one must monitor the engine that generates it.

The transition from “swimming for fitness” to “training for performance” is marked by the shift from accumulating volume to targeting specific physiological zones. This evolution requires tools that can peer inside the body’s metabolic processes while submerged. Devices like the Garmin Swim 2 serve as the bridge between raw effort and physiological insight, enabling a training methodology rooted in the science of thresholds and recovery.

The Optical Challenge: Monitoring the Submerged Pulse

For decades, the chest strap was the undisputed king of heart rate monitoring. It measured the electrical signals (ECG) of the heart directly. However, in the pool, chest straps are cumbersome; they slip during push-offs and can chafe the skin. The industry craved a wrist-based solution, but water presents a formidable optical barrier.

Wrist-based heart rate monitoring uses Photoplethysmography (PPG). This technology shines light (usually green) into the skin and measures the amount reflected back. Blood absorbs green light; therefore, the reflection pulses in rhythm with the heartbeat. On land, this is relatively straightforward. In water, it is a physics nightmare.

Water absorbs and scatters light, potentially diluting the signal before it returns to the sensor. Furthermore, the mechanics of swimming involve forceful, rhythmic arm movements that create “motion artifacts”—noise that can mimic the frequency of a heartbeat. Cold water also causes vasoconstriction, reducing blood flow to the capillaries in the wrist, making the signal fainter.

To overcome this, advanced optical sensors like Garmin’s Elevate technology employ higher-luminosity LEDs and ultra-sensitive photodetectors designed to penetrate deeper and distinguish the weak signal of capillary blood flow from the “noise” of the water and arm movement. The algorithm must aggressively filter out the cadence of the stroke (which often sits around 60-80 strokes per minute, dangerously close to resting or recovery heart rates) to isolate the true cardiac rhythm. This engineering feat allows swimmers to train by heart rate zone—ensuring that an “easy” recovery swim stays aerobic and a “sprint” set truly hits the anaerobic capacity—without the friction of a chest strap.

The Physiology of Thresholds: Decoding Critical Swim Speed

In running, the “Lactate Threshold” is a well-known benchmark—the pace at which lactate begins to accumulate in the blood faster than it can be cleared. In swimming, the equivalent gold standard is Critical Swim Speed (CSS).

CSS is defined as the theoretical speed that a swimmer can maintain continuously without exhaustion. Physiologically, it corresponds closely to the maximal lactate steady state (MLSS). It is the “red line” of your aerobic engine. Swimming slightly below your CSS pace is sustainable for long distances; swimming slightly above it initiates a rapid countdown to fatigue as metabolic byproducts accumulate.

Calculating CSS traditionally required invasive blood lactate testing or complex mathematical plotting of multiple time trials. Modern wearable technology has democratized this metric. By guiding the swimmer through a specific protocol—typically a 400-meter time trial followed by a 200-meter time trial at maximal effort—the Garmin Swim 2 calculates the slope of the distance-time relationship.

$$CSS = \frac{D_1 - D_2}{T_1 - T_2}$$

Where $D$ is distance and $T$ is time. This resulting pace (e.g., 1:35 per 100m) becomes the North Star of training. It allows for the construction of “Threshold Sets”—workouts designed specifically to push that red line higher. Instead of swimming blindly fast, the athlete swims at CSS pace to improve their aerobic ceiling. This data-driven approach shifts the focus from “trying harder” to “training precisely,” targeting the specific metabolic energy systems required for improvement.

The Art of Discipline: Drill Logging and Technique

While physiology is the engine, technique is the transmission. In no other sport is mechanical efficiency as critical as in swimming. Water is approximately 800 times denser than air. A massive engine (high VO2 max) paired with poor hydrodynamics will always lose to a smaller engine with superior technique.

This creates a dilemma for digital tracking: standard algorithms struggle to track drills. Kicking sets (where arms are stationary), one-arm drills, or sculling do not generate the rhythmic wrist acceleration that watches use to count laps. This often leads to swimmers skipping drills to avoid “messing up their data.”

The solution is the Drill Log feature. This is a manual intervention in the digital workflow. It acknowledges the limitations of sensors and empowers the user to input data directly. By allowing swimmers to manually enter the distance of a drill set, the device preserves the integrity of the total training volume. More importantly, it encourages the practice of technique. It validates the time spent on slow, methodical drills as an essential part of the workout, not just “dead time” between intervals. This feature reinforces a crucial training philosophy: technique work is not separate from training; it is the foundation of it.

The Holistic Athlete: Stress, Sleep, and Recovery

The swimmer does not exist only in the pool. Performance is the result of a 24-hour cycle of stress and recovery. The concept of Body Battery or energy monitoring brings the “invisible training” of recovery into the spotlight.

Training imposes a stress load on the body. Improvement occurs not during the workout, but during the recovery from the workout. If a swimmer trains hard but sleeps poorly, or experiences high physiological stress during the workday, their capacity to adapt is compromised. Wearables now monitor Heart Rate Variability (HRV)—the millisecond variations between heartbeats—to estimate the status of the autonomic nervous system.

High HRV indicates a dominance of the parasympathetic (rest and digest) system, signaling readiness to train. Low HRV suggests sympathetic (fight or flight) dominance, indicating stress or fatigue. By quantifying this “readiness,” devices like the Garmin Swim 2 protect the athlete from overtraining. They provide the permission to rest, which is often harder for motivated athletes to accept than the mandate to work. This holistic view integrates the aquatic session into the broader context of life, treating the swimmer as a complete biological system rather than just a machine that produces lap times.

Conclusion: The Quantified Element

The digitization of swimming is about more than just numbers on a screen; it is about revealing the hidden narratives of our physiology. It turns the murky, subjective experience of the water into a transparent, objective reality.

From the optical physics that capture a heartbeat through refracted light to the physiological algorithms that define our endurance thresholds, technology has given us a new language to understand our efforts. It teaches us that speed is a function of efficiency, that endurance is a calculation of metabolic limits, and that recovery is a tangible metric. As we continue to merge with these digital tools, we do not become less human; rather, we gain a sharper, deeper understanding of the incredible biological machinery that propels us through the water.