What is strain? Daily strain score vs your target, explained
In fitness tracking, strain means the total physical load your body took on in a day, scored from heart rate, workout intensity and duration, and movement. A higher strain score means more load. Whoop scores strain from 0 to 21, and other apps use 0 to 100. In medicine, a strain is something else: a stretched or torn muscle or tendon.
This article covers the fitness meaning: what a strain score measures, how the main devices calculate it, and how to match today's strain to how recovered you are.
What does strain mean?
Strain has two common meanings. In medicine, MedlinePlus defines a strain as a stretched or torn muscle or tendon, while a sprain is a stretched or torn ligament. In fitness tracking, strain is a daily score for how much cardiovascular and muscular load you accumulated. The two meanings are unrelated.
| Term | Meaning | Where you see it |
|---|---|---|
| Muscle strain | Stretched or torn muscle or tendon (an injury) | Doctor's notes, sports medicine |
| Sprain | Stretched or torn ligament (an injury) | Doctor's notes, sports medicine |
| Strain score | Daily physical load from heart rate and activity | Whoop, recovery and training apps |
| Training load | Load summed over days or weeks | Apple Watch, Garmin, coaching apps |
Caption: MedlinePlus for the medical terms. Device documentation for the fitness terms.
If you have pain, swelling or bruising after an injury, that is a medical question, and a doctor or physical therapist can assess it. The rest of this article is about the strain score.
How is a strain score calculated?
A strain score adds up how hard your body worked over the day. Most scores weight time at higher heart rate more than time at lower heart rate, so 20 minutes near your maximum counts more than 20 minutes of easy walking. Some also add an estimate of muscular load from strength work and a share of general movement such as steps.
Sports science has measured load this way for decades. Foster et al. (2001) validated session RPE, which multiplies a workout's rated effort by its duration, against a heart-rate-based method across steady cycling, intervals and basketball. Consumer strain scores automate the heart-rate side of that idea and run it all day.
Common building blocks:
| Building block | What it captures | Used by |
|---|---|---|
| Time in heart rate zones | Cardiovascular effort, weighted by intensity | Whoop Strain, most strain scores |
| Effort rating times duration | Perceived intensity of each workout | Apple Training Load (effort ratings), session RPE in research |
| Excess post-exercise oxygen consumption (EPOC) | Physiological cost of each workout | Garmin acute load |
| Active energy and steps | General daily movement | Some strain scores, including Svanu |
| Muscular load estimate | Strength and bodyweight work | Whoop (activity type, duration, logged sets) |
Caption: each company's documentation and Foster et al. (2001).
How does Whoop strain work?
Whoop Strain is a 0 to 21 score that combines a cardiovascular part, based on time spent in elevated heart rate zones, and a muscular part for strength training. Whoop says the scale was inspired by the Borg rating of perceived exertion and is non-linear: building from 10 to 11 takes less effort than building from 20 to 21.
Whoop groups the scale into four bands:
| Whoop Strain | Band |
|---|---|
| 0–9 | Light |
| 10–13 | Moderate |
| 14–17 | High |
| 18–21 | All out |
Caption: Whoop "How does WHOOP Strain work", as of October 2026.
Two details matter in practice. First, Whoop says strain is personal: two people doing the same workout can get different scores based on their fitness and response. Second, Whoop says emotional stress, illness and a busy travel day can all raise strain, because they raise heart rate. A strain score measures load on the body, not only exercise.
What is a good strain score?
A good strain score is one that matches your recovery and your goal for the day. A hard session on a well-recovered day is a good day. The same hard session on a day with low HRV, high resting heart rate and short sleep adds load your body may not absorb well. That is why apps pair strain with a target band.
Think about strain at three time scales:
- Today: did today's strain fit today's recovery? A target band answers this.
- This week: is the 7-day total close to what you usually do, or far above it? Large jumps matter. Gabbett (2016) cited unpublished observations in professional rugby league players where injury risk rose when weekly load increased by 15% or more, and suggested keeping weekly increases below 10%. Treat that as a rule of thumb, not a tested threshold.
- This season: are you building fitness slowly? A higher long-term load, reached gradually, was associated with fewer injuries in Gabbett's review.
For the second and third questions, read our guide to the acute:chronic workload ratio.
How much strain does a healthy week need?
For health, the US Physical Activity Guidelines recommend 150 to 300 minutes of moderate or 75 to 150 minutes of vigorous aerobic activity a week, plus muscle strengthening on 2 days. A strain score helps you see whether your days add up to that. It does not replace the guideline.
Why can strain be high on a rest day?
Strain can be high on a day without a workout because heart-rate-based scores count anything that raises heart rate. Long walks, physical work, heat, illness, stress and travel all add load. A high strain on a rest day is a reason to look at sleep and stress, not a training failure.
| Rest-day cause | Why strain rises |
|---|---|
| Many steps or physical work | Long time at moderately raised heart rate |
| Heat | Heart rate rises to help cool the body |
| Psychological stress | Heart rate rises with arousal |
| Start of an illness | Resting and daytime heart rate can rise |
If a high resting or daytime heart rate comes with fever, chest pain, palpitations or shortness of breath, contact a doctor.
How does strain relate to recovery?
Strain and recovery are two halves of one loop. Strain measures the load you put on your body today. Recovery measures how ready your body is for load, from overnight HRV, resting heart rate and sleep. High strain tends to lower the next morning's recovery, and good recovery supports a higher strain target.
Our recovery score guide explains the recovery side. When high strain and low recovery persist for weeks, read overtraining symptoms for the signs to watch.
How Svanu calculates strain
Svanu scores strain from 0 to 100 using three inputs from Apple Health: active energy (45%), intensity-weighted workout load (35%) and steps (20%). Each day also gets a target strain band, set from today's recovery and your own recent strain history (personal percentiles), so the target moves with you rather than with a population average.
Daily strain also feeds training load: an acute (7-day) and chronic (42-day) exponentially weighted load that shows how this week compares with your longer history. Strain against target is one part of the daily check-in and day score.
Because strain reads Apple Health, it works with Apple Watch and with other devices that sync activity to Apple Health. The core app is free. See how strain and recovery fit together on the Svanu home page.
Frequently asked questions
What does strain mean on a fitness tracker?
On a fitness tracker, strain is a score for how much physical load you took on in a day. It is usually built from time at elevated heart rate, workout intensity and duration, and general movement. A higher number means more load. It is different from a muscle strain, which is a stretched or torn muscle or tendon.
What is a good daily strain score?
There is no single good number. The right strain for today depends on your fitness, your training goal and how recovered you are. On a low recovery day, a lower strain is usually the better choice. On a high recovery day, a hard session fits. A personal target band is more useful than a fixed number.
How is Whoop strain calculated?
Whoop says Strain combines a cardiovascular part, based on time spent in elevated heart rate zones, and a muscular part for strength work. The scale runs from 0 to 21 and is non-linear, so moving from 10 to 11 takes less effort than moving from 20 to 21. Whoop does not publish the formula.
Does stress count as strain?
On many devices, yes. Strain scores built on heart rate respond to anything that raises heart rate, including emotional stress, illness and travel, not only exercise. Whoop states that these non-exercise stressors can raise Strain. A strain score is a measure of load on the body, not only of workouts.
What is the difference between strain and training load?
Strain usually describes one day. Training load looks across days or weeks, for example the last 7 days compared with the last 4 to 6 weeks. Daily strain is the building block, and training load is the trend you get when you add those days up over time.
Sources
- MedlinePlus: Sprains and Strains
- WHOOP: How does WHOOP Strain work?
- Foster C et al. A new approach to monitoring exercise training. J Strength Cond Res, 2001
- Apple Support: Track your training load on Apple Watch
- Garmin fenix 8 Owner's Manual: Acute Load
- Gabbett TJ. The training-injury prevention paradox: should athletes be training smarter and harder? Br J Sports Med, 2016
- US HHS: Physical Activity Guidelines for Americans, 2nd edition (2018)
This article is for general information and is not medical advice. Svanu is not a medical device.