How Fluid Loss and Core Temperature Drive Cardiovascular Drift During Steady-State Running

When an endurance athlete crosses the 20-minute mark of a steady run, their heart rate often begins to climb even if their pace remains perfectly flat. At a constant speed, the working muscles demand a fixed volume of oxygenated blood per minute, and the heart initially settles into a stable rhythm to deliver it. But as the clock ticks forward, a divergence begins. By the end of an hour, a monitor that initially read 145 beats per minute might register 160, despite no change in elevation or effort. This decoupling of internal workload from external output is known as cardiovascular drift.[1][4]

The phenomenon routinely frustrates athletes who train strictly by heart rate zones, prompting them to slow down to force their pulse back into a target window. However, that upward drift is rarely a sign of sudden metabolic fatigue. Instead, it is primarily a mechanical consequence of thermoregulation. As core body temperature rises during prolonged exercise, the autonomic nervous system dilates blood vessels near the skin's surface. This shunts a portion of the circulating blood outward to dissipate heat through sweat, fundamentally altering the fluid dynamics returning to the heart.[1][4]

The mathematics of cardiac output dictate the physiological response. Cardiac output—the total volume of blood pumped per minute—is the product of heart rate multiplied by stroke volume, which is the amount of blood ejected with each single beat. When blood is diverted to the skin and fluid is lost through sweat, the total volume of plasma circulating in the central vessels drops. Research indicates that plasma volume can fall by 10 to 15 percent during a long bout of moderate exercise. With less blood returning to the chambers, the heart's stroke volume inevitably decreases.[4]

To maintain the same total cardiac output and keep the legs supplied with oxygen, the heart has only one available compensation mechanism: it must beat faster. The progressive decline in stroke volume is directly mirrored by a progressive increase in heart rate. The heart is not working less efficiently; it is actively adjusting its rhythm to balance the equation in real time. This is the benign, thermoregulatory half of cardiovascular drift, and it occurs in virtually all athletes exercising in warm or neutral environments.[1][4]

The exact mechanisms underlying the phenomenon remain a subject of active research. A 2021 review published in the journal Life Sciences noted that while the traditional model points to skin blood flow displacing central volume, alternative theories exist. In a landmark 2001 paper, physiologists Edward Coyle and José González-Alonso challenged the sequence of events. "We propose that cardiovascular drift, characterized by a progressive decline in stroke volume after 10–20 min of exercise, is primarily due to increased heart rate rather than a progressive increase in cutaneous blood flow as body temperature rises," they wrote.[1][2]

Regardless of the exact sequence, dehydration acts as a severe amplifier to the baseline drift. Net fluid loss reduces total blood volume on top of the plasma redistribution from sweating, impairing the body's ability to dissipate heat. When athletes lose more than roughly 2 percent of their body mass to sweat, cardiovascular drift accelerates noticeably. Coyle and González-Alonso demonstrated that preventing dehydration through fluid ingestion results in remarkably stable cardiovascular responses, effectively halting the drift in core temperature and stroke volume over a 120-minute test.[3][4]

The implications for pacing and race execution are substantial. During constant-speed exercise, the progressive increase in heart rate causes the perceived intensity to drift upward. Conversely, when an athlete clamps their intensity by maintaining a strict heart rate target, they are forced to progressively reduce their running speed or cycling power to compensate for the drift. Experimental studies have shown that this heart-rate-clamped approach produces a lower external workload and oxygen uptake than constant-speed exercise performed at the same initial physiological intensity.[1][3]

This creates a tension in endurance training. Slowing down to obey a heart rate monitor during the late stages of a long run can inadvertently under-stimulate the aerobic system. If breathing remains relaxed and the perceived exertion is stable, the rising heart rate is simply compensating for fluid loss. A runner who strictly caps their heart rate at 150 beats per minute might find themselves walking the final miles of a marathon training run, missing the mechanical and muscular conditioning the session was designed to provide.[4][5]

However, the drift is not infinite, and it cannot be ignored entirely. If stroke volume continues to drop over several hours, the heart eventually approaches its upper limit, transitioning from benign compensation to genuine physiological strain. At a certain point, the elevated heart rate actively increases cardiac strain, elevates core body temperature further, and shifts the body's fuel preference toward burning carbohydrates rather than fat.[4]

Recognizing the threshold between thermoregulatory drift and actual fatigue requires looking beyond the pulse. Minute ventilation—the volume of air breathed per minute—remains tightly coupled to metabolic demand and does not get fooled by plasma volume loss. When a runner's breathing rate suddenly deepens or their perceived effort spikes at a constant pace, the drift has crossed into metabolic territory, often driven by glycogen depletion. Until that point, a climbing heart rate is simply the sound of the body keeping itself cool.[4]