Question: If Zone 2 is supposed to be ‘easy,’ why does my heart rate monitor say otherwise halfway through a long ride?
I’ve always been told that Zone 2 means an easy, conversational pace and roughly 70–80% of max heart rate. That makes sense early in a ride.
But on long endurance rides, I often see my heart rate slowly creep above that range—even though I still feel (RPE=Rate of Perceived Exertion) comfortable, breathing is controlled, and I could talk in full sentences. Power stays steady, effort feels easy, yet heart rate keeps climbing.
Is this normal? Why does heart rate rise over time if I’m not actually working harder—and does that mean I’ve left Zone 2?
1. What Zone 2 Represents Physiologically
Zone boundaries are metabolic, not cardiovascular or neuromuscular absolutes.
Remember, the Zones of training, correspond physiologic utilization of different fuel substrates (Carbs, Fat, Lactate) and the metabolic consequences of using that fuel. Zone 2 training corresponds to sustained aerobic work below the first lactate/ventilatory threshold (LT1/VT1). Energy production is dominated by mitochondrial oxidative metabolism, lactate remains low and stable, ventilation is controlled, and work is sustainable for long durations.
2. Heart Rate Drift: Why HR Rises Without Increased Intensity
During prolonged steady exercise, heart rate commonly rises despite constant power and stable RPE. This phenomenon, known as cardiovascular drift, is primarily driven by thermoregulation and fluid shifts. Rising core temperature increases skin blood flow and sweating, reducing plasma volume and stroke volume. Heart rate increases to maintain cardiac output. Importantly, this does not reflect increased metabolic intensity.
3. Why Perceived Exertion Can Remain Stable
Perceived exertion reflects central motor command (brain as the master regulator), breathing strain, and global metabolic stress. During Zone 2 work, oxygen delivery remains adequate and ventilation remains controlled. As a result, RPE can stay constant even while heart rate increases to compensate for heat and circulatory demands.
Question: Ok, but even though I’m not feeling gassed, why do my legs start to feel sore/tired/strained, even though my RPE is at Zone 2?
4. Peripheral Fatigue and Muscle Strain During Zone 2
Local muscle strain during prolonged aerobic exercise reflects peripheral fatigue rather than a shift out of Zone 2. Active muscle fibers experience declining contractile efficiency due to partial glycogen depletion, metabolite accumulation, ionic shifts, and impaired calcium handling. To maintain force, the nervous system increases neural drive (message from the brain) and rotates activity among low-threshold motor units (motor unit substitution), increasing afferent feedback (message to the brain) from the muscle. This is perceived as leg strain despite unchanged metabolic demand.
5. Electromyographic (EMG) Evidence Supporting This Model
Surface EMG (electrodes that measure the electric activity of muscles) studies during sustained submaximal exercise demonstrate increasing EMG amplitude (greater neural drive) and decreasing median frequency (slower muscle fiber conduction velocity) over time. These changes occur without recruitment of higher-threshold motor units and without rises in lactate or ventilation, providing objective support for peripheral fatigue as the source of muscle strain.
6. Why Cycling Magnifies Local Muscle Fatigue
Cycling involves repetitive contractions at fixed joint angles with limited variability in muscle recruitment. This concentrates load on a narrow group of fibers, accelerating local fatigue. In contrast, running and rowing distribute work across more muscle groups and movement patterns, leading to more global fatigue before localized muscle strain dominates.
7. Heat, Climate, and Training in Warm Environments
Hot and humid conditions amplify cardiovascular drift and peripheral fatigue. Heat increases sweating and plasma volume (the amount of fluid in your circulation) loss, accelerates glycogen utilization, and impairs neuromuscular efficiency. With heat acclimation (typically 7–14 days), plasma volume expands, sweat sodium concentration decreases, and cardiovascular stability improves, reducing heart rate drift at a given workload.
8. Sweat Rate, Sodium Loss, and Fluid Balance
Endurance athletes typically exhibit sweat rates between 0.5 and 2.5 liters per hour, with sodium concentrations ranging from approximately 400 to 1,200 mg per liter. Inadequate fluid or sodium replacement* accelerates plasma volume loss, exacerbates heart rate drift, and contributes to neuromuscular fatigue. Dehydration exceeding ~2% of body mass is consistently associated with performance decline.
9. Cadence as a Tool to Manage Peripheral Fatigue
Cadence (how fast you pedal-RPM) directly affects force per contraction. Lower cadences increase torque (rotation Force applied to pedals; Torque = Force x Distance) demands and accelerate muscle fatigue, while slightly higher cadences reduce per-stroke force at the cost of modest cardiovascular demand. Elite cyclists often self-select cadences in the 80–95 rpm range during prolonged Zone 2 work and use small cadence variations to redistribute load and delay localized fatigue**.
10. Heart Rate Variability (HRV) and Zone Interpretation
HRV reflects autonomic nervous system (the division of your nervous system not under voluntary control; sympathetic “flight/flight/flee” vs. parasympathetic “relax/recover” sytems) balance and recovery status. Lower-than-baseline HRV is associated with increased sympathetic tone and reduced cardiovascular efficiency. On such days, heart rate may rise earlier and drift more at the same workload. HRV should be used as a readiness and expectation-setting tool rather than as a means to redefine metabolic training zones.***
11. Practical Integration for Zone 2 Training
Effective Zone 2 execution integrates multiple signals: sustainable power or pace, stable RPE, controlled breathing, contextual heart rate interpretation, hydration and sodium management, cadence selection, and environmental conditions. Heart rate should inform, not dictate, decision-making—particularly in long or hot sessions.
Key Teaching Takeaways
- Heart rate drift is a compensatory response, not proof of intensity escalation.
- Muscle strain reflects peripheral fatigue and increased neural drive, not loss of aerobic metabolism.
- Heat and dehydration magnify both cardiovascular drift and neuromuscular inefficiency.
- Cadence manipulation is a powerful lever for managing fatigue without changing zones.
- HRV provides context for readiness but does not redefine metabolic thresholds.
