Cardiac Drift

What is Cardiac Drift?

Cardiac drift measures how much your heart rate climbs relative to your output (pace or power) over a sustained effort — in other words, how much your aerobic efficiency "drifts" during the workout.

When you hold a steady effort for a long time, your heart rate tends to creep upward to keep delivering the same pace or power. Good Coach captures this by comparing your efficiency — output per heartbeat — in the first and second half of the effort, instead of just comparing raw heart rate. This is the approach sports science calls aerobic decoupling (Pw:HR for power, Pa:HR for pace).

The formula

For each half of the workout we compute an efficiency factor:

EF = average output ÷ average heart rate

Then we compare the two halves:

Cardiac Drift % = (EF first half − EF second half) ÷ EF first half × 100

A positive value means your efficiency dropped — your heart worked harder for the same output (drift). A negative value means you became more efficient in the second half.

Example

90-minute steady ride (with power):

• First half: 220 W at 145 bpm → EF = 220 / 145 = 1.52
• Second half: 220 W at 155 bpm → EF = 220 / 155 = 1.42
• Cardiac Drift = (1.52 − 1.42) / 1.52 × 100 = 6.6%

Your power stayed the same, but it cost more heartbeats in the second half — that is cardiac drift.

How Good Coach calculates Cardiac Drift

The "output" in the formula depends on your sport:

Sport	Output used
Cycling, rowing (with a power meter)	Power (watts)
Running	Grade-adjusted pace — speed corrected for hills
Other endurance sports (swimming, XC skiing…)	Speed / pace
No power or speed data (some indoor sessions)	Heart-rate-only comparison (fallback)

Why divide by output? Comparing heart rate alone assumes you held exactly the same pace the whole time. Dividing output by heart rate keeps the metric accurate even when your pace varied, so it reflects genuine cardiovascular drift rather than just a change of speed.

A few details that make the number trustworthy:

• Warm-up is removed automatically before the calculation, and the effort must be at least ~20 minutes after that.
• The workout is split into two halves by elapsed time, and the averages are time-weighted — so the result is correct even when your device records data unevenly.
• Only steady efforts are measured; workouts with intervals or big surges are excluded (the metric would be meaningless for them).

Hills and grade-adjusted pace (running)

On hilly runs your raw pace swings with the terrain — slow uphill, fast downhill — which would distort a pace-based comparison. For runs, Good Coach converts your speed into grade-adjusted pace: the equivalent flat-ground speed for the same effort, based on the slope of the terrain (from your elevation data) and a standard running energy-cost model.

This means cardiac drift now works on rolling courses, not just flat ones. If a run doesn't have reliable elevation or distance data to work out the grade, the metric is skipped for that run instead of reporting a misleading value.

Why does Cardiac Drift matter?

Cardiac drift reveals important information about your current state:

1. Hydration status

• Low drift (<5%): Well-hydrated
• High drift (>10%): Likely dehydrated
• Blood volume decreases when dehydrated, forcing your heart to beat faster to maintain cardiac output

2. Heat adaptation

• Low drift: Well adapted to current conditions
• High drift: Struggling with heat/humidity
• Body diverts blood to skin for cooling, reducing available volume for muscles

3. Recovery state

• Low drift: Well rested and recovered
• High drift: Fatigued or under-recovered
• Tired body works harder to maintain same effort

4. Aerobic fitness

• Low drift: Better aerobic fitness
• High drift: Lower fitness or overtraining
• Fit athletes maintain stable cardiovascular function longer

Quality ratings explained

Good Coach App categorizes your cardiac drift into three quality levels:

Drift %	Quality	What it Means
<5%	Excellent	Well-hydrated, good conditions, recovered. Everything is optimal!
5-10%	Acceptable	Normal for longer efforts. Slight fatigue or warm conditions.
>10%	Poor	Dehydration, heat stress, fatigue, or illness. Take action!

Examples

Excellent Drift (2.1%):

Morning easy run, cool weather, well-hydrated
First half: 138 bpm → Second half: 141 bpm

Acceptable Drift (7.3%):

Long run on warm day, moderate hydration
First half: 142 bpm → Second half: 152 bpm

Poor Drift (12.8%):

Hot afternoon run, inadequate hydration
First half: 145 bpm → Second half: 164 bpm

Understanding negative drift

Sometimes you'll see negative cardiac drift (HR decreases in second half). This is not an error!

What negative drift means

Negative drift (-2.5%):

First half: 147 bpm → Second half: 143 bpm

Common causes:

• Starting too fast - HR settles as you find sustainable pace
• Cooling conditions - Weather cools during workout
• Excellent pacing - Staying relaxed and controlled throughout
• Very easy recovery run - HR naturally stabilizes after warmup

Interpretation:

• Usually a good sign - shows discipline and control
• Common in recovery runs and easy efforts
• Can indicate good pacing strategy

Which workouts work for Cardiac Drift?

Cardiac drift is calculated for steady-state endurance workouts across all endurance sports.

Sports included

• Running
• Cycling (road, mountain, track)
• Rowing
• Swimming
• Cross-country skiing

Ideal workout characteristics

Duration:

• Minimum: 30 minutes total
• Need: 20+ minutes after warmup removal
• Best: 45-90 minutes

Effort:

• Steady, sustained effort
• No intervals or surges
• Hills are fine for running — pace is grade-adjusted automatically

Examples of Good Workouts:

• Easy 60-minute run at conversational pace
• 90-minute Zone 2 cycling ride
• 45-minute steady rowing session
• Long swim at cruise pace
• 2-hour XC ski at comfortable effort
• Steady run on a rolling, hilly course (pace is grade-adjusted)

Workouts that don't work

Interval Training:

• Track workouts
• Fartlek sessions
• Hill repeats
• Any workout with hard/easy segments

Very Short Workouts:

• <30 minutes total duration

Missing Heart Rate Data:

• Forgot HR monitor
• Device malfunction

Runs Without Elevation Data:

• Outdoor runs missing GPS or elevation data, where the grade can't be calculated (the run is skipped rather than scored on uncorrected pace)

How to use Cardiac Drift

Day-to-day training guidance

Before workout:

• Check previous run's drift
• If drift was high (>10%), focus on hydration today

During workout:

• Can't see drift live (calculated after)
• But notice if HR creeping up unusually fast
• Consider slowing down or increasing fluid intake

After workout:

• Review drift percentage
• Compare to your typical values
• Adjust hydration or recovery if needed

Weekly patterns

Track your normal range:

Week 1: 3.2%, 4.1%, 2.8%, 5.6% → Average ~4%
Week 2: 8.4%, 9.2%, 11.3%, 7.8% → Average ~9%

High drift week = Time to assess:

• Are you hydrating enough?
• Is training load too high?
• Need more recovery?
• Weather unusually hot?

Seasonal trends

Summer to Fall transition:

July (hot):     Average drift 7.2%
August (hot):   Average drift 6.8%
September (cooler): Average drift 4.1%
October (cool): Average drift 3.5%

Pattern: Drift decreases as you adapt to heat or conditions improve

Training block progression:

Week 1 (fresh):    Average drift 3.8%
Week 2:            Average drift 4.2%
Week 3 (tired):    Average drift 6.1%
Week 4 (recovery): Average drift 3.2%

Pattern: Drift increases with fatigue, decreases with recovery

Common questions

Q: What's a "normal" cardiac drift for me?

A: Everyone is different! Your normal depends on:

• Fitness level: Fitter athletes often 2-6%
• Sport: Running typically higher than cycling
• Conditions: Hot weather increases drift
• Duration: Longer efforts show more drift

Action: Track your personal baseline over 4-6 weeks of similar workouts.

Q: Should I worry about high drift during a race?

A: Race drift is often higher and that's expected:

• Higher intensity = more drift
• Longer duration = more drift
• Competition stress = more drift

8-12% drift in a race can be normal even for fit athletes.

Q: My drift is always low (<2%). Is something wrong?

A: No! Consistently low drift is typically excellent:

• You're well-trained
• Good hydration habits
• Running in good conditions
• Excellent aerobic fitness

Only concern if combined with:

• Unusual fatigue
• Declining performance
• Other symptoms

Q: Can I use cardiac drift for training zones?

A: Not directly, but it informs training:

• Low drift = Can potentially handle more volume
• High drift = May need easier week or more recovery
• Increasing trend = Consider deload week

Q: Does cardiac drift work for interval workouts?

A: No. Intervals are excluded because:

• HR varies by design (hard to easy to hard)
• Can't compare first vs. second half
• Would give meaningless numbers

Use cardiac drift for steady efforts only.

Q: How quickly can cardiac drift improve?

Changes within:

• Days: Hydration, recovery, heat adaptation
• Weeks: Training adaptations, fitness improvements
• Months: Major aerobic fitness gains

Example timeline:

Week 1 (dehydrated):     Average 9.2%
Week 2 (hydrated):       Average 5.1% ← Rapid improvement
Month 3 (fitter):        Average 3.8% ← Training effect

Q: Should I compare my drift to other athletes?

A: No! Cardiac drift is highly individual:

• Different fitness levels
• Different training history
• Different physiology
• Different conditions

Only compare to your own baseline.

Practical tips

1. Stay hydrated

Before workout:

• Drink 400-600ml water 2-3 hours before
• Another 200-300ml 15-20 minutes before

During workout:

• 150-250ml every 15-20 minutes for runs >60 min
• More in hot conditions
• Don't wait until thirsty

After workout:

• Check drift - if high, hydrate more next time
• Rehydrate with 1.5× fluid lost (weigh before/after)

2. Monitor environmental conditions

Hot weather strategy:

• Expect 2-4% higher drift
• Start easier than usual
• Increase hydration
• Consider time of day (early morning vs. afternoon)

Track patterns:

• Morning runs: Lower drift
• Afternoon runs: Higher drift
• Indoor treadmill: Different from outdoor

3. Use Cardiac rift for recovery assessment

Weekly check:

• Do same easy run each week (e.g., Monday 45-min recovery)
• Compare drift values
• Rising trend = need more recovery

Example:

Week 1: 3.2%
Week 2: 4.1%
Week 3: 5.8%  ← Warning sign
Week 4: Take easier week
Week 5: 3.5%  ← Back to normal

4. Pre-race preparation

Race week:

• Monitor drift on easy runs
• Target: drift at or below your normal
• If elevated: prioritize hydration and rest

Race day:

• Accept higher drift during race (normal!)
• Use training drift as fitness indicator
• Well-adapted athletes handle race drift better

5. Long-term tracking

Monthly average:

• Calculate average drift for similar workouts
• Watch for trends (improving vs. declining)
• Correlate with performance and training load

Seasonal patterns:

• Note baseline for different seasons
• Summer: higher drift is normal
• Winter: lower drift is typical
• Spring/Fall: transitional periods

Real-world examples

Example 1: Hot weather impact

Athlete: Recreational runner, moderate fitness

Cool morning (15°C):

• First half: 138 bpm
• Second half: 141 bpm
• Drift: 2.2% (Excellent)

Hot afternoon (32°C), same route, same pace:

• First half: 142 bpm
• Second half: 155 bpm
• Drift: 9.2% (Acceptable)

Lesson: Heat significantly impacts drift. This is normal and expected!

Example 2: training block fatigue

Athlete: Competitive runner, high fitness

Fresh (Week 1):

• Easy run: 2.8% drift
• Long run: 4.1% drift

Mid-block (Week 3):

• Easy run: 5.2% drift
• Long run: 7.8% drift

Recovery week (Week 4):

• Easy run: 2.9% drift
• Long run: 3.8% drift

Lesson: Drift increases with fatigue, decreases with recovery. Use this to manage training load!

Example 3: Hydration learning

Athlete: Marathon training, learning hydration strategy

Long run #1 (minimal hydration):

• Drift: 11.4% (Poor)
• Struggled in second half

Long run #2 (planned hydration):

• Drift: 5.8% (Acceptable)
• Felt much better

Long run #3 (optimized hydration):

• Drift: 4.2% (Excellent)
• Strong finish

Lesson: Proper hydration dramatically reduces drift and improves performance.

Quick reference guide

Interpretation cheat sheet

Your Drift	Status	Action
<3%	Optimal	Great! Maintain current approach
3-5%	Excellent	Normal for fit athletes, keep it up
5-7%	Good	Acceptable, monitor trends
7-10%	Borderline	Check hydration and recovery
10-12%	High	Review hydration, heat, fatigue
>12%	Very High	Take action - hydrate, rest, cool down

When to take action

Single high reading:

• Don't panic
• Review conditions (heat, hydration, sleep)
• Monitor next workout

Consistent high readings (3+ in a row):

• Increase hydration
• Add recovery days
• Reduce training intensity
• Check for illness

Steady upward trend:

• Sign of accumulating fatigue
• Plan recovery week
• Assess training load
• Prioritize sleep and nutrition

Summary

Cardiac drift is a valuable daily indicator that tells you:

• How well you're hydrated
• How adapted you are to conditions
• How recovered you are
• How your fitness is trending

Use it to:

• Adjust hydration strategy
• Monitor recovery status
• Guide training intensity
• Track fitness improvements

Remember:

• Lower drift = better (generally)
• Negative drift is usually good
• Compare only to yourself
• Trends matter more than single values
• It's one tool among many - use with other metrics

Key insight: Unlike metrics that change slowly (fitness, VO2max), cardiac drift responds quickly to hydration, heat, and recovery - making it perfect for day-to-day training adjustments!