RPE and Heart Rate Drift in Brick Training: Why Is Running Heart Rate 10 bpm Higher After Cycling?

“I usually run at 5:30/km with a heart rate around 150 bpm, so why does my heart rate jump to 162 bpm at the same pace after getting off the bike?” This is the most common question among triathletes. This article thoroughly breaks down where this 10 bpm gap comes from.

1. Actual Data on Brick Heart Rate Drift

Using an athlete weighing 70 kg with a marathon PR of 3:30 as an example:

Scenario	Pace	Heart Rate	RPE
Running 5K alone	5:00/km	152 bpm	6
Running after a short Brick	5:00/km	162 bpm	7
Running after T2 in a 51.5 race	5:00/km	168 bpm	8
Running after T2 in an IM 226	5:00/km	Cannot maintain due to muscle fatigue	9

At the same pace, the heart rate difference can reach 10–16 bpm.

2. Analysis of the Five Main Causes

Cause 1: Dehydration (Greatest Impact)

You keep sweating during the bike leg, and even with fluid intake, body fluids still decrease by 1–2% of body weight. For every 1% of body weight lost as water, heart rate rises by about 7 bpm.

Body Weight Loss %	Expected Heart Rate Increase
1%	5–8 bpm
2%	10–12 bpm
3%	15–18 bpm
4% and above	20+ bpm (already at dehydration warning level)

Countermeasure: Drink 600–750ml of fluid with electrolytes per hour during the bike leg.

Cause 2: Rising Core Body Temperature

It is common for core body temperature to rise from a resting 37°C to 38.5°C during the bike leg. For every 0.5°C rise, heart rate increases by about 5 bpm (the cooling mechanism requires more blood flow to the skin surface).

Countermeasure: Cool down at T2 by pouring cold water over your head, back of the neck, and wrists.

Cause 3: Reduced Cardiac Output

After prolonged cycling, venous return efficiency declines, and stroke volume decreases by about 5–10%. To maintain the same cardiac output, heart rate must rise to compensate.

Countermeasure: For the first 500 meters after T2, deliberately shorten your stride and increase cadence to help the calf muscle pump aid venous return.

Cause 4: Reduced Muscle Efficiency

After cycling, quadriceps glycogen is depleted and neural fatigue sets in. Running at the same pace requires recruiting more muscle fibers and consuming more oxygen. Increased oxygen consumption → higher heart rate.

Countermeasure: Do Brick training for 12 weeks before race day to improve muscle switching efficiency.

Cause 5: Psychological Factors

Anxiety after getting off the bike, rushing to “make up time,” and sympathetic nervous system arousal all raise heart rate. This accounts for about 10–15% of the total drift.

Countermeasure: For the first 1 km after T2, deliberately take deep breaths (inhale for 3 steps, exhale for 3 steps), which can lower heart rate by about 3–5 bpm.

3. RPE Is More Reliable Than Heart Rate

In a Brick state, heart rate is unstable due to multiple influencing factors. It is recommended to pace using RPE (Rating of Perceived Exertion) instead:

RPE	Feeling	Corresponding State
5	Can maintain for a long time	IM marathon pace
6	Slightly challenging	51.5 marathon pace
7	Must focus on breathing	Half-distance triathlon pace
8	Uncomfortable but can maintain for 30 min	51.5 finish sprint
9	Near limit	Can only maintain for 5 minutes

4. Heart Rate Zone Adjustments

If your maximum heart rate is 185 bpm, your target heart rate in a Brick state should be adjusted:

Zone	Running Only	Brick Running
Z1	111–129	121–139
Z2	130–148	140–158
Z3	149–166	159–176
Z4	167–175	177–LT

Note that the upper limit remains unchanged (LT heart rate), but the “target heart rate” at the start needs to be raised by 10 bpm.

5. Race-Day Strategies

Pre-Race Testing

Do one Brick session at 51.5 intensity and record:

• Average heart rate during the bike leg
• Heart rate changes at the 1st, 3rd, 5th, and 10th km after T2
• Heart rate values corresponding to subjective feelings

On Race Day

• Don’t chase pace by watching heart rate: Heart rate will drift due to dehydration and body temperature
• Use RPE and power: Control power on the bike leg, control RPE on the run leg
• Don’t look at your watch for the first 2 km of the run: Settle into a steady rhythm by feel
• Reassess after 5 km: Refer to heart rate once your body state stabilizes

6. Training to Improve Heart Rate Drift

The following training can reduce Brick heart rate drift:

1. Heat acclimation training: Train in hot conditions twice a week (wear long sleeves indoors on the trainer or outdoors on hot days). After 4 weeks, heat tolerance improves and heart rate drift decreases by 3–5 bpm
2. Long Z2 rides: One 3+ hour Z2 ride per week to improve cardiovascular efficiency
3. Core training: Strong core → improved venous return → stable stroke volume
4. Moderate dehydration training: Occasionally deliberately drink less during training to improve dehydration tolerance (use with caution)

Understanding the causes of heart rate drift is the key to correctly interpreting your body’s signals during a race.

Related Reading

• The Science of Cardiac Drift: Why Heart Rate Keeps Rising at the Same Pace
• Heart Rate Drift on the Bike: Strategies for Managing Rising Heart Rate in Long Rides
• In-Race HR Drift: Why Heart Rate Rises in the Latter Half While Pace Stays the Same
• Heart Rate Drift in Road Running: Causes and Strategies for Rising Heart Rate at a Constant Pace