Interpreting Your Garmin/Strava Data: What Your VO2Max Estimate, Training Load, and HRV Actually Mean (And Where AI Gets It Wrong)

Opening Answer

Garmin VO2Max estimates use algorithms based on age, weight, and race times—approximations with 10-15% error margins, not actual lab measurements. Training Load aggregates workout stress into weekly scores that indicate if you're progressing appropriately or overreaching. Heart Rate Variability (HRV) signals nervous system recovery state. These metrics are only useful when you understand their limitations and how they interact. Generic AI responses treat them as standalone numbers; intelligent coaches treat them as interconnected signals about your actual fitness.

The Wearable Data Problem: Metrics Without Context

You wake up. Check your Garmin. See "HRV: 67AU" and think: "Is that good? Should I train hard or take it easy?" Then you check Strava and notice your VO2Max estimate dropped from 52 to 51.5 yesterday. Panic: "Am I losing fitness?"

Your watch gives you numbers. What it doesn't give you is context—what these numbers mean for your training, how they interact with each other, when to trust them, and when to ignore them.

Most athletes—and unfortunately, many generic AI responses—treat metrics as gospel truth:

"Your VO2Max is 52, which is good"
"Do 60g carbs per hour" (ignoring that your individual absorption is 70g)
"HRV below baseline means skip hard training"

But metrics are tools, not truth. A hammer tells you how hard you've hit your thumb. A thermometer tells you your temperature. Neither tells you whether you need medical attention. Context matters.

Consider this: Two athletes have identical metrics one morning:

HRV: 52AU (below their normal 60AU baseline)
Resting HR: 48 bpm (above normal 44 bpm)
Garmin readiness: "Marginal—recommend easy day"

Athlete A had poor sleep due to work stress but good recovery from training. Decision: "My nervous system needs sleep recovery more than training recovery. I'll do an easy run to promote circulation and calm my mind, then prioritize sleep."

Athlete B had good sleep but completed a hard workout yesterday. Decision: "My body needs physical recovery. Complete rest day."

Identical metrics, contradictory appropriate responses. Generic AI says "Marginal—easy day." Both athletes need easy activity, but for different reasons with different follow-up priorities.

This is where understanding metrics—not just reading them—becomes crucial.

VO2Max Estimates: Understanding Approximations

Your Garmin displays "VO2Max: 52 ml/kg/min." Your watch's calculation feels scientific, backed by algorithms and your running data. But here's the reality: this is an estimate, not a measurement.

How Garmin VO2Max Is Calculated

Garmin's algorithm analyzes three inputs:

Your recent race times (pace at which you ran specific distances)
Your heart rate response to those efforts
Your age and weight

From these, Garmin estimates what your VO2Max probably is using a formula derived from exercise physiology literature. The math is legitimate, but the inputs are imperfect.

The Problem: Thousands of variables affect race pace beyond VO2Max. Course terrain. Pacing strategy. Weather. Fueling decisions. Mental toughness that day. A conservative race performance (running slower than fitness allows) produces a lower VO2Max estimate than your actual capacity.

Accuracy Range: Garmin estimates typically fall within ±3-5 ml/kg/min of your actual VO2Max. For someone with VO2Max 50, that means your actual VO2Max is somewhere between 45-55. Useful directionally, imprecise as absolutes.

How to Get Actual VO2Max: Only lab testing (VO2Max treadmill test with metabolic cart) measures true VO2Max. Everything else is estimation.

What VO2Max Actually Measures

VO2Max = the maximum volume of oxygen your body can utilize per minute, expressed relative to body weight. Higher VO2Max correlates with better endurance performance—more oxygen availability means faster sustainable pace.

Key distinction: VO2Max ≠ race performance

Two runners, identical VO2Max (52 ml/kg/min), different performance:

Runner A: 52 VO2Max, but poor running economy (requires more oxygen per pace). 5K time 18:00
Runner B: 52 VO2Max, but excellent running economy (efficient). 5K time 17:15

Runner B is faster despite identical VO2Max. Why? Running economy—how efficiently muscles utilize that oxygen.

This is why isolated VO2Max estimates can mislead. Your 52 VO2Max is meaningless without context: Am I improving? Is my economy improving? How does this fit my overall fitness?

Interpreting VO2Max Changes

Small changes (±2-3 points): Noise. Within normal estimation error. Ignore.

Moderate changes (3-5 points over 4-6 weeks): Real adaptation. You're developing aerobic capacity through BASE phase training or VO2Max-specific work during BUILD phase.

Large drops (5+ points suddenly): Red flag. Not adaptation—something's wrong.

Illness: Always suppresses VO2Max estimates temporarily
Overtraining: Accumulated fatigue manifests as reduced VO2Max performance
Extreme heat: Heat stress reduces measured VO2Max capacity
Poor race execution: Conservative pacing produces low estimate

Large jumps (5+ points suddenly): Likely measurement error or unusually aggressive racing, not real improvement. VO2Max improvements take weeks of specific training.

Useful Interpretation: Track VO2Max trend over months, not days. Ascending trend = improving aerobic fitness. Plateau = current training not developing VO2Max further (might need different stimulus). Decline = overtraining or illness.

Training Load / Training Stress Score (TSS): The Weekly Load Indicator

This is probably the most misunderstood metric: "My Garmin says my training load is 350 this week. Is that good?"

The answer is context-dependent. Is 350 higher than normal for you? Lower? How's your recovery? What training phase are you in?

What TSS Actually Measures

Training Stress Score = (Duration × Intensity Factor²) ÷ 36

Simplified: How much stress did this workout place on your system?

100 TSS = approximately 1 hour at threshold effort (very hard)
50 TSS = approximately 1 hour at easy pace (low stress)
30 TSS = approximately 30 minutes very easy or 10 minutes moderately hard

TSS accounts for both duration and intensity. A 2-hour easy run (100 TSS) and a 45-minute threshold session (90 TSS) stress your system similarly, despite different durations.

Weekly TSS = Sum of all daily TSS scores. Used to assess cumulative weekly stress load.

What Good Weekly TSS Looks Like

This varies dramatically by training phase and individual:

BASE Phase: 200-300 TSS/week

Goal: Build aerobic base with high volume, low intensity
Looks like: 5-6 runs, mostly easy, one moderately hard session
Example: 5 easy runs (40 TSS each) + 1 long run (80 TSS) + 1 tempo session (60 TSS) = 340 TSS (high for BASE, but acceptable)

BUILD Phase: 300-500 TSS/week

Goal: Maintain aerobic base while adding intensity
Looks like: 2 hard sessions + 2-3 easy runs + 1 long run
Example: 2 threshold sessions (90 TSS each) + 1 VO2Max (85 TSS) + 3 easy runs (40 TSS each) + 1 long run (100 TSS) = 515 TSS

PEAK Phase: 400-600 TSS/week

Goal: Race-specific fitness with maintained intensity
Looks like: High intensity maintained, slightly reduced volume
Example: Race simulation + multiple hard efforts + easy recovery = 520 TSS

TAPER: 100-200 TSS/week

Goal: Extreme load reduction with intensity maintenance
Looks like: Mostly easy runs, brief high-intensity touches
Example: 5 easy runs + 2 short hard efforts = 150 TSS

Safe Progression Rule: Increase weekly TSS by maximum 10% per week. Jump from 300 to 400 TSS overnight = injury risk. Progression should be gradual: 300 → 330 → 360 → 390 → 420 across four weeks.

Red Flags: TSS Warnings

TSS Suddenly Spikes 30%+: Warning sign.

Possible causes: Extra hard session you forgot about, attempted ambitious workout, or measurement error
Action: Next week, return to normal TSS. Don't compound high weeks

Chronic TSS Suppression: You're not training hard enough

Week 1: 250 TSS
Week 2: 255 TSS
Week 3: 260 TSS
Problem: TSS barely increasing despite weeks of training. You're not creating progressive stress
Action: Identify the bottleneck. Too much easy volume? Not enough hard sessions? Address root cause

TSS Declining While Performance Improves: Possible good sign, but verify

Scenario: Week 8 (400 TSS) vs. Week 4 (300 TSS), but Week 8 workouts feel easier and you're faster
Meaning: Improved efficiency. Same stimulus produces less stress (adaptation)
Verify: Make sure declining TSS isn't measurement error (forgot to wear watch, short workouts)

Heart Rate Variability (HRV): The Recovery Signal

HRV is where most athletes (and AI coaches) go wrong. It's useful, but frequently misinterpreted.

What HRV Actually Measures

HRV = variability in time between heartbeats. Your heart doesn't beat perfectly regularly (50-60-50-61-59 bpm), it varies. The amount of variation indicates which nervous system state is dominant.

High HRV (big beat-to-beat variation):

Parasympathetic nervous system dominant (rest, recovery, adaptation state)
Indicates: Good recovery, ready for hard training
Heart rate example: Beats separated by 1.0, 1.1, 0.95, 1.05 seconds (high variation)

Low HRV (small beat-to-beat variation):

Sympathetic nervous system dominant (stress response, fighting mode)
Indicates: Stressed, fatigued, needs recovery
Heart rate example: Beats separated by 0.95, 0.96, 0.96, 0.95 seconds (low variation)

The Nervous System Connection: The parasympathetic nervous system (vagus nerve) naturally varies your heart rate. Strong parasympathetic tone = high variation. Stressed/fatigued state suppresses parasympathetic tone = low variation.

How to Interpret Your HRV

Establish a baseline (2-3 weeks of data):

Measure HRV consistently (morning, before getting out of bed, most reliable)
Your "normal HRV" becomes reference point
Example baseline: average 58 AU, range 50-65 AU

Interpret changes relative to baseline:

HRV vs. Baseline	Meaning	Action
Within ±5 AU	Normal daily variation	Train as planned
5-10 AU below	Mild fatigue or life stress	Consider easier workout
10-15 AU below	Significant fatigue accumulation	Reduce intensity or take easy day
15%+ below	Major fatigue or illness	Rest day recommended
Above normal by 5+ AU	Excellent recovery	Can handle hard session

Important caveat: HRV influenced by many factors beyond training:

Sleep quality (poor sleep = lower HRV)
Alcohol (suppresses HRV for 24+ hours)
Caffeine (can increase or decrease depending on sensitivity)
Stress (work problems = lower HRV)
Time of measurement (HRV varies throughout day)
Illness (infection drops HRV 24-48 hours before symptoms)

This is why context matters. Low HRV could mean:

"I need training recovery" (skip hard workout)
"I need sleep recovery" (do easy run but prioritize sleep)
"I'm getting sick" (rest day + sleep + fluids)
"I had three coffees this morning" (false alarm—retest tomorrow)

Generic AI can't distinguish. It just says "HRV low—rest day." You need context.

When HRV Is Useful, When It's Not

Useful:

Detecting overtraining patterns (consistent HRV decline over weeks)
Predicting illness (HRV drops 24-48 hours before symptoms appear)
Validating that recovery is working (HRV should improve with reduced training)
Timing peak performance (HRV should spike days before important races)

Not Useful (often misinterpreted):

Daily binary decisions ("HRV is 48, so I must skip hard training")—too noisy for single-day decisions
Comparing between athletes (your HRV 60 isn't comparable to training partner's 60—genetics vary)
Expecting linear improvements (HRV bounces around; weekly or monthly trends matter more)
Using HRV alone without other recovery data

Best Practice: Track HRV alongside other metrics (resting HR, perceived recovery, sleep quality). Look for patterns. If HRV low AND resting HR elevated AND sleep poor, that's a signal. HRV low but everything else normal? Likely noise.

Resting Heart Rate (RHR): The Aerobic Fitness Proxy

RHR = heart rate immediately upon waking, before getting out of bed.

RHR improves with aerobic training (your cardiovascular system becomes more efficient). RHR elevation signals fatigue or illness.

Establishing Your RHR Baseline

Measure for 2 weeks, same time each morning:

Lie in bed for 1-2 minutes
Take pulse for 60 seconds
Record daily values
Calculate average = baseline RHR
Note normal range (probably ±3-4 bpm from average)

Example baseline: Average 46 bpm, normal range 44-48 bpm

Interpreting RHR Changes

Gradual decrease over weeks/months: Good sign

Indicates improving aerobic fitness
Your heart pumps more efficiently, needs fewer beats per minute at rest
Normal improvement: 1-2 bpm decrease per 4-6 weeks of aerobic training

Sudden elevation 3-5 bpm: Red flag

Indicates: Accumulated fatigue, illness, or insufficient recovery
Action: Reduce training intensity, monitor other metrics (HRV, temperature)
If elevated 24+ hours: Could be coming illness—might show symptoms within 24-48 hours

Sustained elevation 24+ hours: Alarm signal

Strong indicator of overtraining or active illness
Action: Rest day minimum, monitor symptoms
Consider medical consultation if elevated >48 hours with other symptoms

RHR Paradox: Sometimes RHR elevation is temporary and unrelated to training (stress, caffeine, poor sleep). That's why tracking RHR trends matters more than individual days.

Max Heart Rate (Max HR): Why Your Formula Is Probably Wrong

You've probably heard: "Max HR = 220 minus your age"

This formula is convenient, popular, and wrong for 60% of people. Research shows individual max HR varies 10-30 bpm from formula estimates depending on genetics and training.

Problems with Formula-Based Max HR

Formula: 220 - Age

40-year-old gets predicted max HR of 180 bpm
Actual max HR might be 165-195 bpm
30 bpm error range completely throws off zone training

Why Inaccurate:

Doesn't account for genetics (some people naturally lower/higher max HR)
Doesn't account for training (endurance athletes often have lower max HR due to efficiency)
Assumes linear decline with age (not always true)

How to Test Actual Max HR

Progressive 6-Minute Test (safest):

Warm up 10 minutes easy
Run/cycle at moderate intensity
At minute 4, increase to hard effort
At minute 5-6, go all-out maximum effort
Record highest HR achieved
Rest, note this as potential max HR
Repeat weekly for 3-4 weeks
Highest value achieved = your max HR

Why repeat? Max HR is stressful to test. First test might not yield true max due to nervous system holding back. After 3-4 tests, your true max should emerge.

Example: Week 1 test gets 178 bpm. Week 2 gets 180 bpm. Week 3 gets 183 bpm. Week 4 gets 183 bpm again (consistent). Your max HR is approximately 183 bpm.

Why This Matters

All your training zones depend on max HR:

Zone 2 = 65-80% max HR
Zone 4 (threshold) = 90-95% max HR

10 bpm error in max HR means every zone is off by 1-2 bpm, cumulating into training misalignment across weeks.

Accurate max HR = accurate zone training = proper adaptation stress

Pace/Power Zones: The Foundation of Intensity

Your watch tells you what zones you trained in. But are those zones accurate?

Running Zones (Heart Rate Based)

Zone	Intensity	%Max HR	Purpose	Feel
Zone 1	Recovery	50-65%	Very easy aerobic	Conversational, could talk paragraphs
Zone 2	Aerobic Base	65-80%	Easy aerobic, fat adaptation	Conversational, controlled breathing
Zone 3	Tempo	80-90%	Sustained moderate effort	Controlled, could speak short sentences
Zone 4	Threshold	90-95%	Lactate threshold	Hard, could speak 1-2 words
Zone 5	VO2Max	95-99%	Maximal aerobic capacity	Very hard, cannot sustain long

Critical Emphasis: Zone 1-2 should comprise 80% of training. Yet many athletes train mostly in Zone 3-4 (too hard for recovery, too easy for real intensity). This is why zone clarity matters—prevents "junk miles" of moderate intensity that stress you without providing adaptation.

Cycling Zones (Power Based)

Zone	%FTP	Purpose	Watts (example, 300W FTP)
Zone 1	<55%	Recovery	<165W
Zone 2	56-75%	Sweet Spot/Aerobic	168-225W
Zone 3	76-90%	Tempo	228-270W
Zone 4	91-105%	Threshold	273-315W
Zone 5	106-120%	VO2Max	318-360W
Zone 6	121-150%	Anaerobic	363-450W
Zone 7	>150%	Neuromuscular Power	>450W

FTP (Functional Threshold Power) must be tested every 6-8 weeks. Testing method: 20-minute all-out effort, multiply by 0.95.

Zones only work if FTP is accurate. 20W error in FTP means your entire Zone 3-5 is off target.

The Interconnection: How Metrics Work Together

This is where most athletes (and AI) fail: treating metrics as separate. Smart coaches treat them as interconnected signals.

Real-World Example: Reading the Full Signal

Monday Morning:

HRV: 48 (down from baseline 60, significant drop)
RHR: 50 (up from baseline 44, elevated)
Sleep: 6.5 hours (below normal 8)
Resting temperature: 98.1°F (normal, 98.6°F)
Garmin readiness: "Marginal—consider easy day"
Your feeling: "Tired, head feels heavy"

Generic AI Response: "HRV low, RHR elevated—skip hard training, do easy day"

Intelligent Response Considers:

Is this illness onset? (Elevated temp would suggest yes; normal temp suggests no)
Is this overtraining? (Yes if last week's TSS was 30%+ above normal; no if normal)
Is this life stress? (Check: Did you have stressful work/personal events?)
Is this sleep deprivation? (Yes—6.5 hours is below normal)

Likely Diagnosis: Sleep deprivation + psychological stress (not training-induced fatigue)

Appropriate Response:

Do easy 45-minute run to promote circulation and mental relief
Prioritize sleep tonight (8-9 hours)
Skip hard training today (nervous system needs recovery from both fronts)
Monitor tomorrow morning—if HRV rebounds and RHR normalizes with good sleep, issue was sleep/stress, not overtraining

Dangerous Response: Skip all training thinking you're overtrained, when actually you need gentle movement + sleep.

The metrics told the story. Generic AI told you to rest. Understanding metrics told you what to actually do.

What AI Gets Right and What It Gets Wrong About Metrics

Where AI Excels

Pattern detection: AI can identify overtraining signals weeks before you notice
Data aggregation: Correlating HRV drops with illness patterns across months of data
Threshold setting: "Your normal HRV range is 52-65; drops below 50 warrant attention"
Progression tracking: "Your VO2Max has improved 4% over 8 weeks—aerobic training is working"

Where AI Falls Short

Context missing: "Your HRV is low—rest" ignores whether you had poor sleep (manageable) or genuine overtraining (dangerous)
Individual variation ignored: "Average HRV is 60" doesn't account for your genetics (some people naturally 45, others 75)
Causation assumed: "HRV low therefore skip hard training" ignores that HRV is marker, not cause
Oversimplified rules: "60g carbs per hour" works for 70% of athletes; ignores your individual 75g/hour absorption

The smarter AI coaches (like CADENCE) add context:

Why is HRV low? (Overtraining vs. sleep vs. stress)
What does this mean for training? (Different adjustments for each cause)
How do metrics interact? (HRV + RHR + sleep + stress tell full story)

FAQ: Metric Interpretation

Q: Should I make daily training decisions based on HRV?

A: No. HRV is too variable for daily binary decisions. Instead:

Track weekly averages (less noise)
Look for trends over 2-3 weeks
Use HRV as one of several recovery signals
Make big decisions (reduce training block, take rest week) based on multiple metrics confirming pattern

Q: My VO2Max estimate decreased—am I losing fitness?

A: Probably not. VO2Max estimates fluctuate ±3-5 points from real changes. Drop could be:

Measurement noise
Conservative pacing in recent race
Measurement error (forgot watch during workout)
Actual fitness decline (unlikely unless TSS/training dropped significantly)

Assess: Is your actual performance (pace times) improving or declining? That's more reliable than estimates.

Q: What's a good Training Load for my sport?

A: Depends on:

Your sport (cycling generally higher TSS than running same duration due to power measurements)
Your training phase (BASE 200-300, BUILD 300-500, PEAK 400-600)
Your experience level (beginners handle lower TSS than advanced)
Your life stress (high stress = same TSS feels harder)

Track your own baseline. "Good" is individual.

Q: Can I use metrics to predict race performance?

A: Approximately. Good indicators:

VO2Max improving over months = speed potential improving
RHR declining = aerobic fitness improving
TSS appropriately distributed = training stimulus correct
HRV recovering into peak week = nervous system fresh for race

But predict outcome? No. Fitness doesn't guarantee execution. Race day pacing, nutrition, mental toughness matter enormously.

Q: How often should I test or retest metrics?

Max HR: Every 6-8 weeks (if using HR zones)
FTP (power): Every 6-8 weeks (critical—directly affects power zones)
VO2Max: Track trend over months, not weeks
HRV: Daily measurement, but analyze weekly averages
RHR: Daily measurement, track baseline trend

Practical Framework: Using Metrics Intelligently

Daily (upon waking):

Measure RHR, HRV, note sleep quality
Identify any unusual signals (elevated RHR, low HRV, poor sleep)

Weekly (Sunday evening):

Calculate average HRV, average RHR for week
Check if trends are improving, stable, or declining
Note any spikes or drops that need investigation

Monthly (end of month):

Review TSS progression (are you increasing appropriately?)
Check VO2Max trend (improving, stable, declining?)
Assess whether metrics align with how training felt
If misalignment, investigate source (watch accuracy? lifestyle changes? illness?)

Every 6-8 Weeks:

Retest max HR or FTP (update zone targets if changed)
Full metrics review: Are zones still appropriate? Training load sustainable?
Adjust training targets based on updated baselines

The CADENCE Advantage: Metrics That Adjust Your Training

CADENCE doesn't just report metrics—it uses them to adjust your training:

HRV low? We detect it and extend recovery day before next hard session
RHR elevated? We reduce intensity for the week, monitor for illness
TSS trending too high? We back off intensity to prevent injury
VO2Max improving? We intensify subsequent training to continue stimulating improvement
Multiple metrics declining? We trigger full recovery week, prioritize nervous system restoration

This is intelligent metric use: not just telling you your numbers, but translating them into training adjustments that protect you and optimize results.

Are you guessing at what your metrics mean? CADENCE tracks your Garmin/Strava data and uses it to adjust your training weekly—not just reporting numbers, but translating them into personalized workout adjustments. See how context-aware metric interpretation transforms your training.

[Understand Your Training Data →]

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Garmin VO2Max estimates use algorithms based on age, weight, and race times—approximations with 10-15% error margins, not actual lab measurements. Training L...