🧬 biology

Work Rate Adjustments During High-Intensity Interval Training in the Heat With Active and Passive Recovery

Maintaining target perceived exertion during high-intensity interval training in the heat resulted in similar thermal and cardiovascular strain between active and passive recovery, though active recovery necessitated greater work rate reductions and led to comparable decreases in maximal aerobic capacity regardless of recovery mode.

Original authors: Hillary A. Yoder, Anne M. Mulholland, Hayley V. MacDonald, Jonathan E. Wingo

Published 2026-10-26

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CC BY 4.0

Original authors: Hillary A. Yoder, Anne M. Mulholland, Hayley V. MacDonald, Jonathan E. Wingo

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Exercise in the heat is a battle between the body's need to move and its need to cool down. When we work hard, our muscles generate heat, and our bodies must shed that excess warmth to keep our internal temperature from rising to dangerous levels. One way the body cools itself is by pumping more blood to the skin, which helps release heat into the air. However, this creates a conflict: the same blood needed to cool the skin is also needed to fuel the working muscles. To manage this, athletes and coaches often use interval training, a method where short bursts of intense effort are followed by periods of rest. During these rest periods, the choice is often between doing nothing (passive recovery) or continuing to move at a much slower pace (active recovery). While active recovery is often thought to help clear waste products from muscles faster, it also keeps the body generating heat, which could be risky in a hot environment. The question remains: in the heat, does keeping the body moving during rest help maintain performance, or does it simply overheat the system and force the athlete to slow down?

Researchers at New Mexico State University and the University of Alabama set out to answer this by putting eight healthy adults through a rigorous test. They asked participants to ride stationary bicycles in a room heated to 35 degrees Celsius, a temperature that simulates a very hot summer day. The exercise consisted of a warm-up followed by five rounds of high-intensity intervals. Each round involved four minutes of hard pedaling, followed by three minutes of recovery. The participants were given a specific target for how hard the exercise should feel, known as the rating of perceived exertion, rather than a fixed speed or power setting. This meant they had to adjust their own pedaling speed to keep the effort feeling the same throughout the session. Half the time, the recovery involved sitting still on the bike; the other half, they continued to pedal slowly. The researchers measured core body temperature, heart rate, and how much power the participants could produce at the end of the session.

The study found that the body's thermal response was surprisingly similar regardless of how the participants recovered. Whether they sat still or kept pedaling during the rest periods, their core body temperature rose by nearly the same amount over the course of the workout. By the end of the session, the average core temperature had climbed to about 38.3 degrees Celsius. This suggests that the total heat generated by the exercise itself was the dominant factor, and the small amount of extra heat produced by the slow pedaling during active recovery did not significantly change the overall thermal strain on the body. However, the cardiovascular system told a different story. During the active recovery periods, the participants' heart rates remained significantly higher than when they sat still. On average, the heart beat 24 times per minute faster during the active recovery intervals compared to the passive ones.

Because the heart was working harder to support the active recovery, the participants had to make a significant adjustment to their main exercise intervals to keep the effort feeling the same. In the sessions with active recovery, the participants had to reduce their pedaling power by an average of 21 watts by the final round compared to the first. In contrast, those who rested passively barely had to change their power output at all. This indicates that the extra strain on the heart and circulation during active recovery made it difficult to sustain the same level of work intensity without feeling like they were pushing too hard. Despite these differences in how the heart and muscles responded, the overall drop in maximum aerobic capacity—the body's ability to use oxygen for energy—was the same for both groups. After the full workout, everyone's maximum capacity had decreased by roughly 11.5 percent, a temporary decline caused by the heat and the duration of the exercise, but this drop was not worse for the group that chose active recovery.

The results suggest that while active recovery does not make the body hotter in a meaningful way compared to passive recovery, it does place a heavier load on the heart. For an athlete trying to maintain a high work rate during intense intervals in the heat, sitting still during the break allows the heart rate to drop more effectively, which in turn allows them to push harder during the next work interval. If the goal is to maximize the total amount of energy burned during the entire session, active recovery might still be useful because it keeps the body moving and burning calories even during the rest periods. However, if the primary goal is to maintain the highest possible speed or power output during the intense parts of the workout, passive recovery appears to be the more effective strategy in hot conditions. The study confirms that in the heat, the body's ability to cool itself is limited, and the choice of recovery method changes how the heart copes, which ultimately dictates how much work a person can sustain.

Technical Summary: Work Rate Adjustments During High-Intensity Interval Training in the Heat with Active and Passive Recovery

Problem Statement
High-intensity interval training (HIIT) is an effective method for improving health and fitness, yet recommendations regarding active versus passive recovery are primarily derived from studies conducted in temperate environments. In hot ambient temperatures, exercise limitations may alter the physiological responses to recovery modes. While active recovery is often recommended for longer recovery intervals (>3–4 min) to facilitate lactate clearance, it theoretically sustains a higher rate of metabolic heat production ( $\dot{M} - \dot{W}$ ) compared to passive recovery. It remains unclear whether active recovery during HIIT with longer work bouts (≥4 min) in the heat results in greater thermal and cardiovascular strain, necessitates larger work rate adjustments to maintain a target intensity, or causes a more significant reduction in maximal aerobic capacity ( $\dot{V}O_2max$ ) compared to passive recovery.

Methodology
The study employed a repeated-measures crossover design involving eight recreationally active to competitive endurance adults (2 women, 6 men). Participants completed five trials in an environmental chamber maintained at 35.1 °C and 40.2% relative humidity:

Control Trial: A graded exercise test (GXT) in a temperate environment (22.6 °C) to determine baseline $\dot{V}O_2max$ and maximal heart rate ( $HR_{max}$ ).
Experimental Trials: Four trials in the heat consisting of an 8-minute warm-up at a Rating of Perceived Exertion (RPE) of 12, followed by HIIT protocols:
- 15ACT / 15PASS: One round of HIIT (4 min work at RPE 17, 3 min recovery) followed immediately by a GXT.
- 43ACT / 43PASS: Five rounds of HIIT (4 min work at RPE 17, 3 min recovery) followed immediately by a GXT.
- Recovery Conditions: Active recovery (ACT) required cycling at RPE 12 (or 30 W if RPE 12 was unattainable); Passive recovery (PASS) involved sitting on the ergometer.

Participants adjusted their work rate self-paced to maintain the prescribed RPE. Physiological measures included rectal temperature ( $T_{re}$ ), mean skin temperature ( $\bar{T}_{sk}$ ), mean body temperature ( $\bar{T}_b$ ), heart rate (HR), oxygen uptake ( $\dot{V}O_2$ ), blood lactate, and plasma volume changes. $\dot{V}O_2max$ was measured immediately following the final recovery bout of each trial.

Key Results

Thermal Strain: Rectal temperature ( $T_{re}$ ) increased significantly over time in both 43-minute trials but was not significantly different between active and passive recovery conditions at the end of the exercise session (43ACT: 38.3 °C vs. 43PASS: 38.2 °C). Mean body temperature ( $\bar{T}_b$ ) was also comparable between conditions.
Cardiovascular Strain: Heart rate during work intervals was comparable between conditions. However, during recovery intervals, average HR was 24 b·min⁻¹ higher in the active recovery condition compared to passive recovery. Over the course of the five intervals, HR during recovery increased significantly in both conditions, but the magnitude of increase was more pronounced in the active recovery condition.
Work Rate Adjustments: To maintain the target RPE of 17 during work intervals, participants in the 43ACT trial reduced their power output significantly more than those in the 43PASS trial. The percent change in work rate from the first to the fifth interval was –14% (±24%) for 43ACT versus –1% (±16%) for 43PASS ( $p=0.04$ ). On average, the final work interval power output was 21 W lower in the active recovery condition.
Maximal Aerobic Capacity ( $\dot{V}O_2max$ ): $\dot{V}O_2max$ was significantly lower following the 43-minute trials compared to the 15-minute trials (a reduction of 0.4 L·min⁻¹ or 11.5%). However, there was no significant difference in the magnitude of this reduction between active and passive recovery modes.
Energy Expenditure: Total energy expenditure over the 43 minutes was significantly higher in the 43ACT trial (450 ± 161 kcal) compared to the 43PASS trial (384 ± 142 kcal).

Significance and Conclusions
The study concludes that maintaining a target RPE during HIIT in the heat results in comparable thermal strain regardless of whether recovery is active or passive. However, active recovery imposes a significantly greater cardiovascular strain, evidenced by higher heart rates during recovery intervals. To compensate for this strain and maintain the prescribed RPE, participants utilizing active recovery were forced to reduce their work rate more substantially than those using passive recovery.

The authors note that while passive recovery allowed for the maintenance of higher work rates during the work intervals, the lack of metabolic activity during passive recovery periods may offset the benefits regarding total training stimulus. Conversely, active recovery provided a higher total energy expenditure without exacerbating thermal strain, though it required greater work rate reductions. The findings suggest that in hot environments, passive recovery may be preferable for maintaining high-intensity work rates and minimizing cardiovascular drift, whereas active recovery may be beneficial if the primary goal is maximizing total energy expenditure, provided the reduction in work rate is acceptable. The study also highlights the challenges of using RPE alone for exercise prescription in the heat, as it can lead to dissociation between perceived intensity and physiological markers like heart rate, particularly during active recovery.

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