Performance Lab

Understand your key endurance performance anchors — and learn how to measure them manually.

Measure what matters.

Threshold Pace, FTP, CSS, heart-rate threshold, sweat rate, and sweat sodium concentration give you practical reference points for setting training intensity and building race-day pacing, fueling, hydration, and electrolyte strategies.

Some Performance Lab measurements can be performed easily in the field, while others require specialized testing or equipment.

The most useful result is a repeatable result measured with a consistent protocol and interpreted in the context in which it was collected.

EnduroPace 1.5 Pro

Advanced Performance History & Connected Sources

EnduroPace Pro expands Performance Lab history with source, method, measurement date, freshness, confidence, trend context, and race-plan usage.

Supported Apple Health observations remain read-only until you explicitly choose to adopt them as EnduroPace measurements. Adopted measurements preserve their original provenance.

Source-aware planning. When multiple performance measurements are available, EnduroPace can explain which value is currently used for planning and why. Important changes that can affect saved Race Day Plans are presented for review before being applied.

Run Threshold Pace Bike FTP Swim CSS Sweat Rate Sweat Sodium (Optional) Threshold Heart Rate

Run

Threshold Pace

Threshold Pace is an estimate of the fastest running pace you can sustain for a prolonged hard effort without rapidly accumulating fatigue.

It is useful as a reference point for threshold workouts, race planning, and pace-based training zones.

Manual 30-Minute Field Test

Choose a flat, uninterrupted route, track, or treadmill and perform the test when reasonably rested.
Warm up for approximately 15–20 minutes with easy running.
Include a few short strides or faster efforts near the end of the warm-up.
Start a 30-minute time trial.
Run at the hardest pace you believe you can sustain evenly for the entire 30 minutes.
Avoid sprinting the first few minutes. The effort should feel hard but controlled.
Ideally, you should still be able to maintain or slightly increase your pace during the final 5–10 minutes.
Record your average pace for the full 30-minute effort.
Use that average pace as your estimated Threshold Pace.

Manual Calculation

Threshold Pace ≈ average pace for the full 30-minute time trial

Example

If your average pace during the 30-minute test is:

7:40 per mile

Your estimated Run Threshold Pace is approximately:

7:40 / mile

Repeatability matters. Use similar terrain and conditions when retesting. Wind, hills, heat, fatigue, poor pacing, and accumulated training stress can materially affect the result.

Bike

Functional Threshold Power (FTP)

Functional Threshold Power is a practical estimate of the highest cycling power you can sustain for a prolonged steady hard effort.

FTP is measured in watts and is commonly used to build cycling power zones and race-intensity targets.

Manual 20-Minute FTP Field Test

Use a power meter or smart trainer capable of recording reliable average power.
Warm up for approximately 20 minutes at an easy-to-moderate effort.
Include several short cadence pickups or harder efforts during the warm-up.
Optionally perform a hard 5-minute effort before the test, followed by approximately 10 minutes of easy recovery.
Begin the 20-minute test.
Ride at the highest power you believe you can sustain evenly for the full 20 minutes.
Avoid starting dramatically harder than you can sustain.
Record your average power for the entire 20-minute effort.
Multiply the 20-minute average power by 0.95.

Manual Calculation

FTP ≈ 20-minute average power × 0.95

Example

20-minute average power:

250 W

Calculation:

250 × 0.95 = 237.5 W

Estimated FTP:

≈ 238 W

The 95% method is an estimate. Different protocols, indoor versus outdoor testing, cooling, fatigue, athlete physiology, and pacing can produce different FTP estimates. Use the same protocol when comparing results over time.

Swim

Critical Swim Speed (CSS)

Critical Swim Speed is an estimate of your sustainable threshold swim speed.

CSS is normally expressed as pace per 100 meters or pace per 100 yards and can be used as an anchor for threshold-oriented swim training.

Manual 400 / 200 CSS Test

Warm up thoroughly with easy swimming, drills, and several short faster efforts.
Swim a 400-meter or 400-yard time trial at the fastest even effort you can sustain.
Record the exact 400 time.
Recover with approximately 5–10 minutes of easy swimming.
Swim a 200-meter or 200-yard time trial at a very hard effort.
Record the exact 200 time.
Convert both times to seconds.
Subtract the 200 time from the 400 time.
Divide the result by 2.
The result is your CSS pace in seconds per 100 of the same pool unit.

Manual Calculation

CSS pace / 100 = (400 time in seconds − 200 time in seconds) ÷ 2

Example

400 time:
6:40 = 400 seconds

200 time:
3:10 = 190 seconds

Calculation:
(400 − 190) ÷ 2 = 105 seconds

CSS:
1:45 per 100

Keep units consistent. A test performed in a yard pool produces CSS per 100 yards. A test performed in a meter pool produces CSS per 100 meters. Do not mix the two.

Hydration

Sweat Rate Test

Sweat rate estimates how much fluid you lose per hour under a specific combination of sport, intensity, temperature, humidity, clothing, and environment.

It can be one of the most useful inputs for building an individualized race-day hydration strategy.

Manual Sweat Test

Use the bathroom before the test if needed.
Weigh yourself with minimal dry clothing immediately before exercising.
Record your starting body weight.
Exercise for a known duration. Approximately 60 minutes makes the calculation especially simple.
Exercise at an intensity reasonably similar to the training or racing conditions you want to evaluate.
Track exactly how much fluid you consume during the session.
If you urinate during the test, measure or reasonably estimate that volume.
After exercise, towel off surface sweat.
Weigh yourself again using the same clothing conditions as before the test.
Calculate your total sweat loss.
Divide total sweat loss by exercise duration in hours.

Metric Calculation

Sweat loss (L) ≈ pre-weight (kg) − post-weight (kg) + fluid consumed (L) − urine (L)

Sweat rate (L/hour) = sweat loss ÷ exercise duration in hours

Example

Pre-exercise weight:
70.0 kg

Post-exercise weight:
69.4 kg

Fluid consumed:
0.50 L

Urine:
0 L

Duration:
1 hour

Calculation:
70.0 − 69.4 + 0.50 = 1.10 L

Sweat rate:
1.10 L/hour

Do not treat one sweat test as universal. Repeat the test in different seasons, temperatures, humidity levels, sports, and intensities.

Your sweat rate during a hot outdoor run may be very different from your sweat rate during an indoor bike session.

Sweat rate is not automatically your drinking target. Hydration strategy also depends on event duration, thirst, gastrointestinal tolerance, access to fluids, intensity, and environmental conditions.

Optional Precision Input

Sweat Sodium Concentration

A sweat sodium test is nice to have, not required.

Most athletes do not need to measure sweat sodium concentration to use EnduroPace. You can build a Race Day sodium plan without this value. EnduroPace keeps the concentration unknown rather than estimating it from sweat rate.

I don't know my sweat sodium

That is a normal EnduroPace workflow. Build your planned sodium schedule from products, training experience, coaching or nutrition guidance, and individual tolerance.

Estimated sodium loss and replacement percentage remain unavailable because EnduroPace does not invent a concentration.

I have a measured or reported value

Enter the value reported by a reputable sports-science laboratory, professional sweat-testing service, validated patch or sensor, or another reliable source.

Adding a reliable value is an optional precision upgrade. EnduroPace can combine it with measured sweat rate to estimate sodium loss.

Sweat sodium concentration estimates how much sodium is present in each liter of sweat. EnduroPace expresses this value in mg/L.

This is different from sweat rate. Your Sweat Rate Test estimates how much fluid you lose per hour, but it does not determine how much sodium is present in that sweat.

Two separate measurements

Sweat Rate = fluid lost per hour

Sweat Sodium Concentration = sodium in each liter of sweat

When both are known:

Estimated Sodium Loss (mg/hour) = Sweat Rate (L/hour) × Sweat Sodium Concentration (mg/L)

Example

Sweat rate: 1.2 L/hour

Sweat sodium concentration: 900 mg/L

Estimated sodium loss: 1.2 × 900 = 1,080 mg/hour

Estimated sodium loss is not automatically a sodium intake prescription. EnduroPace keeps estimated loss and planned intake separate so you can build and practice an individualized Race Day strategy.

Advanced: How sweat sodium is measured

Sweat sodium concentration is harder to measure than sweat rate. Collecting the sample is only one part of the process; the sample also has to be analyzed for sodium. Collection location, contamination, evaporation, sample handling, and analysis method can affect the result.

Whole-Body Washdown

Whole-body sweat collection is a reference research method used to estimate electrolyte concentration across sweat lost from the entire body.

It can provide a strong whole-body estimate, but it requires specialized laboratory procedures and is not practical as a normal home test.

Exercise Sweat Patch

An absorbent patch collects sweat from a specific skin location during exercise, and the sample is analyzed for sodium concentration.

Sodium concentration varies by body location. A raw regional value should not automatically be treated as whole-body concentration unless the method supports that interpretation or provides a normalized estimate.

Pilocarpine Sweat Test

Some professional services use pilocarpine iontophoresis to stimulate sweat from a small skin area while the athlete is at rest, then analyze the sample for sodium.

This can provide a practical and repeatable estimate, although it should not be assumed to reproduce every exercise intensity or race condition perfectly.

Wearable Sweat Sensors

Some consumer sensors analyze local sweat during exercise and estimate fluid and electrolyte losses. Measurements and whole-body extrapolation are device-specific.

Treat the result as the estimate produced by that validated system rather than assuming it is identical to laboratory whole-body testing.

Can I measure sweat sodium at home?

A useful Sweat Rate Test can be performed at home with a reliable scale, a known exercise duration, and accurate measurement of fluid consumed. Sweat sodium concentration is more difficult because the sample must also be analyzed for sodium.

Recommended approach

If you want a quantitative sweat sodium value for EnduroPace, use a result from a reputable sports-science laboratory, professional sweat-testing service, validated patch or sensor, or another reliable measurement system when possible.

Regional vs. whole-body values

Sweat sodium concentration is not identical everywhere on the body. A forearm, chest, back, thigh, or other regional sample may differ from the body's average concentration.

Regional measurements can be related to whole-body concentration, but the relationship depends on the collection site and testing method.

Do not enter an arbitrary patch value as a whole-body value unless the testing method supports that interpretation. Whenever possible, use the value or whole-body-equivalent estimate supplied by the validated testing method.

mmol/L to mg/L

Some reports express sodium concentration in millimoles per liter rather than milligrams per liter.

Conversion

Sodium mg/L = Sodium mmol/L × 22.99

Example

50 mmol/L × 22.99 = 1,149.5 mg/L ≈ 1,150 mg/L

Do not estimate sweat sodium from sweat rate alone. Two athletes can lose similar amounts of fluid per hour while having substantially different sweat sodium concentrations.

Run / Heart Rate

Lactate-Threshold Heart Rate (LTHR)

If you wear a reliable heart-rate monitor during the same 30-minute running threshold test, you can also estimate your running threshold heart rate.

Manual LTHR Test

Complete the same continuous 30-minute running time trial described above.
Record heart rate throughout the test.
Ignore the first 10 minutes for the heart-rate calculation.
Calculate your average heart rate during the final 20 minutes.
Use that number as your estimated running LTHR.

Manual Calculation

Running LTHR ≈ average heart rate during the final 20 minutes of the 30-minute test

Example

Average heart rate during the final 20 minutes:

168 bpm

Estimated running LTHR:

≈ 168 bpm

Wrist optical heart-rate sensors can be affected by fit, temperature, cadence, and arm movement. A chest strap often provides cleaner data for hard field tests.

Testing Quality

How to Get Better Data

Do	Avoid
Retest using the same protocol.	Comparing results from different protocols as if they are identical.
Test when reasonably rested.	Testing immediately after a very hard training block or poor recovery.
Use consistent equipment and units.	Mixing meters and yards or different power sources without context.
Record heat, humidity, terrain, intensity, and other relevant conditions.	Assuming environmental conditions do not affect performance or sweat measurements.
Track trends across several tests.	Overreacting to one unusually good or bad result.
For sweat sodium, record the collection method and body location when available.	Assuming all regional sweat-sodium measurements are directly interchangeable.