Cardiovascular Drift Deep Dive: The Physiological Chain Reaction of Rising Heart Rate and Declining Stroke Volume at Constant Power, and Practical Countermeasures

1. Introduction and Frontier Research Background

In the field of cycling sports science, the widespread adoption of power meters has made “constant output” achievable, but the body’s internal responses have never been “constant.” When you ride steadily at 250W on rolling asphalt, your heart rate may quietly climb from 148bpm at the 30-minute mark to 162bpm at the 120-minute mark—this heart rate “drift” of 3 to 8 beats per hour is the phenomenon known in exercise physiology as “Cardiovascular Drift (CV Drift).”

As early as the 1960s, pioneering researchers such as Saltin and Stenberg observed that during prolonged exercise, even when the workload remained unchanged, heart rate gradually increased over time, accompanied by a decline in Stroke Volume (SV). This finding upended the then-simplistic notion that “heart rate equals exercise intensity” and opened a new scientific window for endurance training and race pacing strategies in the years that followed.

Entering the 21st century, teams led by Coyle and González-Alonso further utilized thermodilution methods and ultrasound cardiac imaging to precisely quantify the causal relationship between rising Core Temperature and increased Cutaneous Blood Flow. They found that when core temperature rose from 37.0°C to 39.5°C, blood flow to the cutaneous vascular bed could surge from 0.5 liters per minute to 6–8 liters per minute—equivalent to “shunting” more than 20% of cardiac output to the body surface for heat dissipation. This “appropriation” of blood directly led to a reduction in central Venous Return, which in turn decreased left ventricular End-Diastolic Volume, ultimately manifesting as a significant decline in stroke volume.

In recent years, the understanding of cardiovascular drift in exercise physiology has expanded from a purely “heat stress” perspective to a more complex multifactorial interaction. The latest research indicates that, in addition to thermoregulation, exercise-induced Plasma Volume loss, sustained activation of the sympathetic nervous system, and Hemoconcentration caused by intracellular fluid shifts all play critical roles in the development of cardiovascular drift. Particularly in Taiwan’s high-humidity summer environment, where sweat evaporation efficiency is poor and core temperature rises more sharply, the magnitude of cardiovascular drift tends to be more pronounced than the values reported in temperate-climate literature.

Notably, research after 2020 has begun using “continuous blood pressure waveform analysis” and “portable Impedance Cardiography” to capture the dynamic process of cardiovascular drift in real-world outdoor cycling scenarios. These studies found that during 60 minutes of continuous exercise at 60% VO2max intensity, average stroke volume decreased by approximately 8% to 12%, while the compensatory rise in heart rate was approximately 4 to 8 beats per hour. These figures are not merely academic numbers from the laboratory; they are the physiological reality that every cyclist participating in long-distance challenges may personally experience on the sustained climbs of the East Route to Wuling, or during the 300-kilometer Tour of East Taiwan (Hua-Dong).

Therefore, understanding the deep mechanisms of cardiovascular drift is not just an accumulation of sports science knowledge; it is a critical cornerstone for actually improving athletic performance and reducing race risk. This article will draw on a solid physiological foundation, combine the latest scientific evidence with Taiwan’s local race scenarios, provide readers with a complete analysis of the ins and outs of cardiovascular drift, and offer concrete, executable quantitative countermeasures.

2. Core Mechanisms of Exercise Physiology and Biomechanics

2.1 The Chain Reaction of Heat Stress and Cutaneous Vasodilation

The starting point of cardiovascular drift is the metabolic heat generated by muscle contraction. In cycling, the human body’s mechanical efficiency is only about 20% to 25%, meaning that for every 1000 joules of chemical energy consumed, only 200 to 250 joules are converted into propulsive power, with the remaining 750 to 800 joules released as heat. Riding at 250W generates approximately 675 to 720 kilocalories of heat per hour—enough to bring one liter of water to a boil within 30 minutes. If this heat cannot be effectively dissipated, core temperature will rise sharply at a rate of 1.5°C to 2.0°C per hour.

To relieve heat stress, the thermoregulatory center in the hypothalamus triggers cutaneous Vasodilation via cholinergic fibers of the sympathetic nervous system. This redirects a large volume of blood originally destined for deep tissues (such as the kidneys, viscera, and non-active muscles) to the Arteriovenous Anastomoses near the skin surface, facilitating heat dissipation through radiation, convection, and evaporation. Research shows that under intense heat stress, skin blood flow can reach 6 to 8 liters per minute, accounting for 20% to 30% of total cardiac output.

This “great migration” of blood directly impacts the volume distribution of the circulatory system. Because the cutaneous vascular bed has a large volume and extremely high Compliance, a substantial amount of blood pools peripherally, causing Effective Circulating Volume to decline significantly. Central Venous Pressure may drop from 4 to 6 mmHg at rest to 1 to 2 mmHg. According to the Frank-Starling law of the heart, the reduction in venous return directly decreases left ventricular end-diastolic filling volume, shortening the Initial Fiber Length of myocardial fibers, thereby reducing contractile force and ultimately manifesting as a decrease in stroke volume.

2.2 The Role of Plasma Volume Loss and Blood Viscosity

In addition to the blood redistribution caused by vasodilation, the loss of water and electrolytes through heavy sweating is a major driver that exacerbates cardiovascular drift. In Taiwan’s summer heat and high humidity (relative humidity >70%), sweat loss can reach 1.2 to 2.0 liters per hour. Without timely replacement, plasma volume can decrease by 8% to 12% within 60 to 90 minutes.

The reduction in plasma volume causes the concentration of red blood cells in the blood to rise relatively, increasing Blood Viscosity. According to Poiseuille’s law, the flow rate of fluid through a tube is proportional to the fourth power of the tube radius and inversely proportional to fluid viscosity. Increased blood viscosity means greater resistance to blood flow, forcing the heart to contract at a higher frequency to maintain the same Cardiac Output (Q = HR × SV). When stroke volume decreases due to reduced Preload, heart rate becomes the only variable that can rapidly compensate—this is the direct reason for the continuous rise in heart rate.

Notably, the latest research has found that plasma volume loss is not simply “water loss”; it is also accompanied by the loss of colloidal molecules such as Albumin. Albumin is the primary protein maintaining intravascular Colloid Osmotic Pressure. A decline in its concentration makes it easier for fluid to leak from the vasculature into the interstitial space, further exacerbating the deficiency in effective circulating volume. This explains why simply replenishing water (rather than solutions containing electrolytes and carbohydrates) is often ineffective in reversing cardiovascular drift.

2.3 Sustained Sympathetic Nervous System Activation and Heart Rate Compensation

During exercise, sympathetic nervous system activity progressively increases over time. This arises partly from the direct stimulation of cardiovascular centers by rising core temperature, and partly from the accumulation of muscle metabolites (such as hydrogen ions, potassium ions, and adenosine) triggering group III/IV afferent nerve reflexes. Increased sympathetic activity raises the spontaneous firing rate of the Sinoatrial Node and also enhances myocardial contractility (Inotropy).

However, the positive inotropic effect of the sympathetic nervous system provides only limited compensation when faced with a decline in stroke volume. According to the Fick principle, VO2max = Cardiac Output × Arteriovenous Oxygen Difference. When stroke volume decreases by 10%, heart rate must increase by approximately 10% to maintain the same oxygen delivery. This means that under constant power output, cardiovascular drift actually reflects the heart’s reluctant compromise of “trading volume for frequency.”

2.4 The Interaction Between Biomechanics and Cardiovascular Drift

From a biomechanical perspective, riding posture also exerts a subtle influence on cardiovascular drift. Research shows that maintaining a low-drag Time Trial Position for extended periods increases hip flexion angle, compressing the abdomen and inferior vena cava, which raises resistance to venous return and further exacerbates the decline in stroke volume. Conversely, periodically transitioning to an upright climbing posture not only changes muscle recruitment patterns but also temporarily relieves abdominal compression, promoting venous return.

Additionally, the choice of Cadence affects cardiovascular load. Studies indicate that at the same power output, grinding a large gear at 60rpm, compared to spinning at 95rpm, generates higher Muscle Tension, which impedes blood flow within the muscles and increases peripheral vascular resistance. This forces the heart to generate higher blood pressure to maintain muscle blood flow, indirectly accelerating the onset of cardiovascular drift. Therefore, moderately increasing cadence (90–100rpm) during long-distance riding is not only a means of muscle fatigue management but also a protective strategy for the cardiovascular system.

2.5 Quantitative Model and Numerical Derivation

Synthesizing the mechanisms above, we can construct a simplified mathematical model of cardiovascular drift. Let the initial cardiac output be Q₀ = HR₀ × SV₀. After t minutes of continuous exercise, the proportional decline in stroke volume due to heat stress and plasma loss can be expressed as:

SV(t) = SV₀ × (1 - α × ΔT_core - β × PV_loss)

where α is the sensitivity coefficient of stroke volume to changes in core temperature (approximately 0.02 to 0.03 /°C), ΔT_core is the rise in core temperature (°C), β is the sensitivity coefficient of stroke volume to plasma volume loss (approximately 0.5 to 0.8 /L), and PV_loss is the cumulative plasma volume lost (liters).

Take a 70 kg male cyclist as an example: riding at 250W for 2 hours in an environment of 30°C and 75% humidity, core temperature may rise by 1.8°C, and plasma volume loss is approximately 0.6 liters. Substituting into the model:

SV(t) = SV₀ × (1 - 0.025 × 1.8 - 0.6 × 0.6) = SV₀ × (1 - 0.045 - 0.36) = SV₀ × 0.595

This means stroke volume could decline by as much as 40%. To maintain cardiac output, heart rate would need to rise from an initial 148bpm to approximately 248bpm (assuming cardiac output remains unchanged)—clearly exceeding physiological limits. In reality, the body partially compensates by reducing cardiac output and increasing the arteriovenous oxygen difference, but this also means that exercise intensity (in terms of oxygen uptake) has actually exceeded the value displayed on the power meter, and exercise efficiency has declined significantly.

3. Key Parameter Measurements and Comparative Analysis

To more concretely illustrate the impact of cardiovascular drift, the following compiles key research data from international exercise physiology journals over the past five years, presented in Markdown table format for comparative analysis.

3.1 Comparison of Cardiovascular Drift Under Different Environmental Temperatures

Source: Adapted from Coyle (2004), González-Alonso et al. (2008), and recent outdoor field test data.

The table clearly shows that in Taiwan’s typical summer hot and humid environment (such as the East Rift Valley in July or the western coastal highway), the magnitude of cardiovascular drift is 1.5 to 2.0 times that of a hot and dry environment. This means that at the same power output, Taiwanese cyclists experience a far higher heart rate load than athletes in temperate regions. Without proper countermeasures, this will lead to premature fatigue and a significant drop in performance.

3.2 Cardiovascular Drift at Different Exercise Intensities

Source: Compiled from Wingo et al. (2012) and recent meta-analysis data.

It is worth noting that at low intensity (40% VO2max), the magnitude of cardiovascular drift is relatively mild, and the rise in heart rate primarily stems from thermoregulatory demands. However, when intensity rises above 75%, sympathetic nervous system activity increases substantially, and the accumulation of muscle metabolites amplifies the magnitude of cardiovascular drift sharply. This also explains why, on the sustained climbs of the East Route to Wuling (where intensity often exceeds 75% VO2max), cyclists frequently see their heart rates “peg out” without realizing it, while power output cannot be maintained.

3.3 Effects of Hydration and Cooling Interventions on Cardiovascular Drift

Source: Adapted from combined analyses by Bergeron et al. (2013) and Cheuvront et al. (2010).

This table provides highly practical reference value. Simply supplementing with plain water helps slow the rise in plasma osmolality, but because it lacks electrolytes and energy, its effect on maintaining plasma volume is limited. In contrast, the combined strategy of “electrolyte + carbohydrate solution” paired with “forearm cold-water cooling” can suppress the magnitude of cardiovascular drift to less than one-third of the no-supplementation group. For long-distance challenges in Taiwan’s summer, this is the key to success or failure.

4. Periodized Training Plan or Equipment Setup and Adjustment Guide

Having understood the mechanisms of cardiovascular drift, the focus now shifts to how to “tame” this physiological phenomenon through training and race strategies. Below is an 8-week periodized training plan, complete with specific intensity zones and session arrangements, to help cyclists effectively enhance their cardiovascular system’s “anti-drift” capacity.

4.1 Training Intensity Zone Definitions (Based on FTP)

4.2 Eight-Week Periodized Training Plan

Weeks 1–2: Base Adaptation Phase (4 sessions per week)

The goal of this phase is to build the aerobic base, increase plasma volume, and enhance cardiac end-diastolic volume. Research shows that two consecutive weeks of high-volume, low-intensity training can increase plasma volume by 10% to 15%, forming the first line of defense against cardiovascular drift.

• Session A (Tuesday): Zone 2 endurance ride, 90 minutes, maintaining 70% FTP, cadence 90–95rpm. The key is that heart rate must not exceed the upper limit of Zone 2 throughout. If cardiovascular drift causes heart rate to exceed the target, reduce power until heart rate returns to the target zone.
• Session B (Thursday): Zone 2 endurance ride, 120 minutes. Maintain 75% FTP for the first 60 minutes, then reduce to 65% FTP for the final 60 minutes, simulating a race scenario of “decreasing power, stable heart rate.”
• Session C (Saturday): Long-distance aerobic ride, 180 minutes. Choose a flat or gently rolling route (e.g., North Coast, western coastal highway). Maintain Zone 2 for the first 120 minutes; during the final 60 minutes, if heart rate drifts more than 5%, practice the “reduce power to protect heart rate” strategy.
• Session D (Sunday): Recovery ride, 45 minutes, Zone 1.

Weeks 3–4: Heat Acclimation Induction Phase (4–5 sessions per week)

This phase introduces heat stress stimuli to induce Heat Acclimation. It is recommended to train during the hottest part of the day (1–3 PM) and reduce fan use, allowing the body to naturally accumulate heat stress.

• Session A (Tuesday): Zone 2 ride in hot conditions, 90 minutes, maintaining 70% FTP. Record heart rate and perceived thermal sensation every 15 minutes. The goal is to have core temperature reach above 38.5°C in the latter half of the session.
• Session B (Thursday): Interval training (hot environment), 75 minutes total. After a 20-minute warm-up, perform 6 × 5-minute Zone 3 tempo efforts with 2-minute Zone 1 recoveries. This session aims to improve cardiovascular tolerance under high heat.
• Session C (Saturday): Long-distance heat acclimation ride, 150 minutes. Maintain Zone 2 for the first 90 minutes; during the final 60 minutes, practice “slightly reduce power, stabilize heart rate,” observe the magnitude of heart rate drift, and practice cooling techniques (e.g., pouring water on the neck and forearms).
• Session D (Sunday): Recovery ride, 45 minutes, Zone 1.

Weeks 5–6: Intensity Progression Phase (4–5 sessions per week)

Building on the heat acclimation base, this phase adds threshold and anaerobic stimuli to enhance the cardiovascular system’s compensatory capacity at high intensities.

• Session A (Tuesday): FTP intervals, 90 minutes total. After a 20-minute warm-up, perform 4 × 8-minute Zone 4 efforts (95–100% FTP) with 4-minute Zone 1 recoveries. This session significantly improves stroke volume and cardiac pumping efficiency.
• Session B (Thursday): Tempo ride (hot environment), 90 minutes, maintaining 85% FTP. The focus is on observing heart rate changes at the 30, 60, and 90-minute marks, practicing breathing and pedaling rhythm adjustments when heart rate drift occurs.
• Session C (Saturday): Long mixed-intensity ride, 180 minutes. Choose a route with alternating climbs and flats (e.g., Yangmingshan Fengzhongjian, Guanyinshan). First 60 minutes Zone 2, then 60 minutes with 3 × 15-minute Zone 3 climbs, and final 60 minutes Zone 2 return. Practice hydration and cooling strategies throughout.
• Session D (Sunday): Recovery ride, 60 minutes, Zone 1.

Weeks 7–8: Race Simulation and Taper Phase

• Session A (Tuesday): Simulated race pace ride, 120 minutes. First 60 minutes Zone 2, final 60 minutes Zone 3–4, simulating the intensity increase in the latter part of a race. Strictly execute the hydration and cooling plan throughout.
• Session B (Thursday): Taper session, 60 minutes Zone 1–2, ensuring full recovery.
• Session C (Saturday): Final long ride before race day, 90 minutes Zone 2, including 3 × 1-minute Zone 5 short sprints to awaken the neuromuscular system.
• Session D (Sunday): Complete rest or only a 30-minute very light recovery ride.

4.3 Equipment Setup Recommendations

At the equipment level, the following points should be noted to reduce cardiovascular load:

1. Aero position and abdominal compression: When maintaining a low-drag position for extended periods, stand up to pedal or transition to an upright posture for 10–15 seconds every 15–20 minutes to relieve abdominal compression and promote venous return.
2. Bottle and nutrition configuration: Use a dual bottle cage setup—one bottle with electrolyte solution (6–8% carbohydrate), the other with plain water. In hot weather, consider using insulated bottles to keep drinking water at 10–15°C, which helps lower core temperature.
3. Computer display settings: Display power and heart rate data side by side on the bike computer, and set a heart rate drift alarm (e.g., alert when heart rate exceeds a preset value after the 60-minute mark). This helps cyclists detect cardiovascular drift in real time and make power adjustments.

5. Race Nutrition, Environmental Adaptation, and Race-Day Strategies

5.1 Quantitative Hydration Prescription

The primary strategy against cardiovascular drift is maintaining plasma volume stability. Below is a quantifiable hydration plan suitable for Taiwan’s summer long-distance rides (4+ hours).

Pre-race (24 hours prior):

• Consume 50–60ml of fluid per kilogram of body weight, with moderate sodium (3–4 grams of table salt per liter of water), to ensure adequate pre-race hydration.
• Drink 500–600ml of electrolyte beverage 2 hours before the start, and another 200–300ml 15 minutes before the start.

During the race (every 15–20 minutes):

• Consume 600–1000ml of electrolyte-carbohydrate solution (6–8% concentration) per hour. For a 70 kg cyclist, this means 60–90 grams of carbohydrates and 500–800mg of sodium per hour.
• If the temperature exceeds 30°C, increase to 1000–1200ml per hour, and add 1 electrolyte capsule (containing 200–300mg sodium) every 30 minutes.

Post-race (within 2 hours):

• Replenish 1.5 liters of fluid for every 1 kg of body weight lost, paired with 20–30 grams of protein and 60–90 grams of carbohydrates to promote recovery.

5.2 Scientific Basis for Cooling Strategies

In addition to hydration, external Cooling is the most direct and effective tool against cardiovascular drift. Research shows that immersing the forearms and neck in 10–15°C cold water can significantly reduce the demand for cutaneous vasodilation, allowing more blood to return to the central circulatory system.

Practical Cooling Operation Guide:

• Neck cooling: The neck has abundant superficial vessels (carotid and subclavian arteries). Placing a cold, wet towel or a Cooling Collar on both sides of the neck for 5–10 minutes effectively lowers the temperature of blood flowing to the brain while stimulating cold receptors to reduce thermal discomfort.
• Forearm cooling: The radial and ulnar arteries in the forearm lie close to the surface. Immersing the forearms in cold water or using ice sleeves rapidly cools venous blood returning to the core. Research shows that forearm cooling can lower core temperature by 0.3–0.5°C, equivalent to reducing the magnitude of cardiovascular drift by 30% to 40%.
• Pouring water over the head: On climbing sections (such as the Kunyang to Wuling segment of the East Route to Wuling), support crew can pour bottled water directly over the head and back of the neck. The latent heat of vaporization of water (each liter of evaporated water removes 580 kilocalories of heat) effectively dissipates large amounts of body heat.

5.3 Race-Day Strategies for Classic Taiwanese Events

East Route to Wuling (Dayuling to Wuling, elevation 2565m to 3275m):
This section is approximately 10 km long with an average gradient of 8% to 12%, constituting a high-intensity climb. Due to the altitude gain, ambient temperature drops approximately 6°C per 1000 meters, but on summer afternoons, temperatures at the lower elevation (Dayuling) can still exceed 25°C. Key strategies are as follows:

• First 3 km (Dayuling to Bilu Tunnel): Maintain a pace at 85% FTP, keeping heart rate at the upper limit of Zone 3. Ensure 150–200ml of electrolyte solution is consumed every 15 minutes during this phase.
• Bilu Tunnel to Luoshao (approximately 5 km): The gradient eases slightly, allowing power to increase to 90% FTP, but closely monitor for abnormal heart rate drift. If heart rate rises more than 5bpm within 10 minutes, immediately reduce power by 5–10%.
• Luoshao to Ci’en (approximately 4 km): This section is steep (10–15%). It is recommended to alternate between seated low-cadence pedaling (70–75rpm) and standing out of the saddle to reduce the obstruction of venous return caused by sustained muscle tension.
• Ci’en to Wuling (approximately 3 km): The final push. Temperatures may drop to 10–15°C, but by this point the body has accumulated significant fatigue and heat stress. Before the final kilometer, pour cold electrolyte solution liberally over the head and neck, and consume a caffeinated gel (containing 100mg caffeine) to enhance alertness.

One-Day Taipei to Kaohsiung / Twin Towers (Taipei to Kaohsiung / Pingtung, 380–520 km):
For this type of flat, long-distance challenge, wind resistance and heat stress are the primary enemies. Key strategic points are as follows:

• Maintain Zone 2 power throughout (60–70% FTP), with heart rate targeted to stay within the upper limit of Zone 2. If heart rate rises due to headwinds or fatigue, prioritize reducing power rather than forcing the pace.
• Every 30 minutes, perform a “quick aid station stop” of no more than 5 minutes. The replenishment should include 300–400ml of electrolyte beverage, 1 banana or half an energy bar, and 2–3 salt tablets.
• On headwind sections, adopt a “pace line” strategy, alternating with other riders to take the wind, thereby reducing individual power output. When drafting, the following rider can reduce power by 30% to 40%, which greatly helps reduce heat generation and delay cardiovascular drift.
• During nighttime riding, lower temperatures help slow cardiovascular drift, but electrolyte replenishment should still be emphasized, as sodium loss after prolonged exercise can lead to muscle cramps and blood pressure instability.

6. Common Operational Misconceptions and Scientific Myth-Busting

Myth 1: “As long as I replenish water, I can completely prevent cardiovascular drift”

This is the most common misconception. As mentioned earlier, the causes of cardiovascular drift include multiple factors such as heat stress-induced cutaneous vasodilation, sympathetic activation, and blood redistribution. Simply replenishing water only addresses the single component of “plasma volume loss” and has limited effect on the reduction of venous return caused by cutaneous vasodilation and sympathetic activation. Research shows that even with perfect hydration, stroke volume still declines by approximately 8% to 10% after 60 minutes of exercise at 65% VO2max in a 32°C environment. Therefore, hydration is a necessary condition but not a sufficient one; it must be combined with cooling strategies to effectively suppress cardiovascular drift.

Myth 2: “A rising heart rate means training is working well, so I should keep maintaining power”

This is an extremely dangerous misconception. Under constant power, a rising heart rate does not indicate enhanced cardiorespiratory function; rather, it indicates that the cardiovascular system is under additional stress. Ignoring heart rate drift and continuing to maintain power can lead to uncontrolled core temperature, a drop in blood pressure, and even heat stroke or heat exhaustion. The correct approach is: when heart rate rises more than 5% above the baseline value, proactively reduce power by 5–10% to bring heart rate back into the target zone. This is not a “step backward” but rather a scientific pacing strategy.

Myth 3: “Training in an air-conditioned room can simulate race conditions, so heat acclimation is unnecessary”

This completely overlooks the importance of Heat Acclimation. Heat acclimation refers to the physiological adaptations that occur after 7–14 consecutive days of exposure to a hot environment, including increased plasma volume, higher sweat rates, lower sweat salt concentration, and improved skin blood flow efficiency. These adaptations significantly reduce the magnitude of cardiovascular drift. If training is done only in an air-conditioned room, the body will be unable to cope with sudden heat stress on race day when temperatures exceed 30°C, leading to a significant drop in performance. It is recommended to begin heat acclimation training 2–3 weeks before the race, with at least 3 sessions per week of 60–90 minutes of endurance riding in environments above 28°C.

Myth 4: “Standing up to climb makes your heart beat faster, so you should stay seated the entire time”

This myth requires more nuanced examination. Indeed, when standing to pedal, because body weight participates in the pedaling action, instantaneous power output is higher and heart rate rises briefly. However, maintaining a seated position for extended periods, especially on steep climbs, causes excessive hip flexion, compressing the abdomen and inferior vena cava, obstructing venous return, and paradoxically exacerbating cardiovascular drift. The correct strategy is: on steep sections (>8% gradient), stand up to pedal for 15–20 seconds every 3–5 minutes, using body weight to assist the pedal stroke while extending the hip joint to promote venous return. When standing, slightly reduce the gear ratio to maintain the same or slightly lower power, keeping heart rate fluctuations within an acceptable range.

Myth 5: “Drinking sports drinks during exercise increases gastrointestinal burden, so plain water is better”

This myth overlooks the importance of electrolytes and carbohydrates in sports drinks for maintaining plasma osmolality and energy supply. Research shows that isotonic sports drinks with a 6–8% carbohydrate concentration have a gastric emptying rate comparable to plain water, while simultaneously providing sodium and glucose to help maintain plasma volume and delay fatigue. In contrast, excessive intake of plain water can dilute blood sodium concentration and trigger Hyponatremia, particularly in prolonged, heavy-sweating conditions. The correct approach is: consume 600–1000ml of a 6–8% carbohydrate-electrolyte beverage per hour, paired with solid foods (such as energy bars, bananas) to provide a more complete energy source.

7. Expert FAQ

Q1: When cardiovascular drift occurs, how do I determine whether it is a “normal physiological response” or a “danger warning sign”?

In-depth answer: This is the key question distinguishing “scientific pacing” from “health risk.” In normal physiological cardiovascular drift, heart rate rises slowly at a rate of 3–8bpm per hour, accompanied by a synchronized increase in perceived exertion (RPE), but without symptoms such as dizziness, nausea, difficulty breathing, or chest tightness. In contrast, if heart rate rises sharply by more than 15–20bpm within a short period (10–15 minutes), or is accompanied by a drop in blood pressure (feeling dizzy, blacking out), cold and clammy skin with an extremely high core temperature (>40°C), or confusion, these may be precursors to heat exhaustion or heat stroke. You must immediately stop exercising, move to a shaded area, actively cool down (cold water immersion or copious pouring of water), and seek medical assistance. In Taiwan’s summer races, there have been multiple heat injury incidents caused by ignoring danger warning signs. Cyclists must remain highly vigilant.

Q2: I am taking blood pressure medication (such as beta-blockers). Will the cardiovascular drift phenomenon be different?