Cardiovascular Strain vs. Muscular Strain


Last Updated: 09/23/2021 


WHOOP's Strain metric specifically measures cardiovascular load on the body. This means that Strain is directly influenced by Heart Rate (HR) elevation. 


Cardiovascular vs. Muscular Endurance

Cardiovascular endurance: the ability of your heart and lungs to fuel your body with oxygen. 

(aerobic = with oxygen) 

Muscular endurance: the ability of your muscles to perform repetitive contractions without fatigue.  (anaerobic = without oxygen)

Cardiovascular endurance, or training, increases your heart rate for longer periods of time; therefore, cardiovascular training increases your Strain more rapidly than muscle training, or anaerobic exercise, such as weight training. 

NOTE: Different training modules affect HR differently. 

While training your cardiovascular system, your Strain threshold may be higher (e.g.: 14-18) compared to muscle training, which predominantly utilizes anaerobic pathways. As a result, your cardiovascular load will most likely be lower resulting in a lower Strain score (e.g.: 5-10).


If Strain is low, how do I measure levels of strength-based training?

Although weight training produces lower Strain initially (due to your heart rate not being elevated for long durations), the impact it produces on your body is accounted for during your recovery. 

WHOOP measures Strain through cardiovascular output and the duration of various HR zones. Therefore, if you're doing a strength-based workout with minimal repetitions and separated by length rest periods (ex: Olympic weightlifting), you will have a lower Strain score. 

WHOOP measures Strain based on cardiovascular output and time spent in various heart rate zones. Therefore, if you’re doing a strength-based workout with minimal reps and separated by lengthy periods of rest (such as Olympic weightlifting), you will have a lower Strain score.


How does a strenuous strength workout affect my data?

  • Have you ever completed a particularly 'intensive' workout or done a strenuous activity outside of your normal routine, and then wake up the next morning, extra sore and barely able to move/get out of bed? This type of 'micro trauma', or lengthening your muscle fibers through increasing weight (during weight training) and/or adding new strength exercises typically causes HRV (Heart Rate Variability) reduction the morning after (especially if your body isn't accustomed to this type of stressor), resulting in delayed onset muscle soreness (DOMS). 
  • Fatigued muscles often result in a higher Strain on the following day because your body is working harder to recover from the 'disturbances' in your homeostatic (default or regular) state. 
  • When training appropriately, your HRV should be within the normal or high range (green or yellow Recovery), indicating the training load your body endured was appropriate to its natural recovery level. 
  • Users who strength train tend to have lower baseline stress hormones and have higher levels of cortisol (the primary stress hormone that increases sugar (glucose) in the bloodstream and increases availability of substances that repair tissues) in the morning than the evening. This allows the body to fluctuate between higher arousal during the day, and experience deeper recovery at night.
  • Users who strength train should monitor their HRV for increasing trends (over time). 

NOTE: To accurately measure muscular strain following a workout, you should consider additional resources such as an EMG (electromyography), a diagnostic procedure that evaluates the health condition of muscles and the nerve cells that control them, or a blood test.


Tips for Using 'Strain' and 'Strain Coach' during Strength Training

Using 'Strain Coach' during your workouts can help you stay in appropriate training zones, depending on the type of workout you are doing. See below screenshot for training zone, exertion level, and fitness goal comparison: 


Recovery zone: 60-70%  of your maximum heart rate. In this zone, you’ll develop basic endurance and aerobic capacity, while burning fat. 

Aerobic zone: 70-80% of your maximum heart rate. Operating in this zone will develop your cardiovascular system and optimize your cardio health, respiratory system, and pulmonary health (lungs).

NOTE: Aerobic means "needs oxygen." Therefore, aerobic exercise conditions the cardiovascular system, and increases the breathing and rate during activity. 

Benefits of Aerobic training: 

  • The heart pumps blood more efficiently (more blood pumped per contraction)
  • More oxygen delivered to muscles and cells
  • Increased perfusion of tissues and organs with blood
  • More oxygen into the bloodstream
  • More carbon dioxide out of the blood
  • More effective respirations
  • More efficient gas exchange in the lungs

To learn more about the Aerobic Heart Rate Zone, visit our Locker post: What is the Aerobic Heart Rate Zone and How Do You Target It? 

Anaerobic zone: 80-90%  of your maximum heart rate. Training in this zone works best when going hard for short periods, followed by resting for equal and/or longer periods. 

NOTE: Anaerobic means "without oxygen." Therefore, anaerobic exercise tends to be more intense, but shorter in duration than aerobic exercise. 

Benefits of anaerobic training: 

  • Build muscle
  • Burn fat 
  • Increase in metabolism
  • Increase HGH (human growth hormones) 
  • Strengthen bones and improve joint function
  • Improve your immune system
  • Increase your lactic acid threshold and endurance
  • Remove toxins through perspiration (sweating) and exercise-induced lymphatic drainage, which increases lymphatic system flow and can help prevent infections and diseases 
  • Increase your fast-twitch muscle fibers (strength, speed, and power)

To learn more about the anaerobic heart rate zone, visit our Locker post: Target the Anaerobic Heart Rate Zone and its Benefits 

Other Related Resources: A Simple Guide to Training with Heart Rate Zones 

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