Cardiovascular Strain vs. Muscular Strain

WHOOP's Strain metric specifically measures the cardiovascular load on the body. In other words,  Strain is directly influenced by elevating the heart rate. 


Cardiovascular vs. Muscular Endurance

Cardiovascular endurance is the ability of your heart and lungs to fuel your body with oxygen (aerobic = with oxygen).

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


Training the cardiovascular system increases your heart rate for longer periods of time which in turn increases your Strain more rapidly when compared to weight training. Due to the different inherent natures of various training modalities, they affect our heart rate in different ways.

While training the cardiovascular system, your Strain threshold may be higher (e.g., 14-18) when compared to weight 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). It is important to note "bands" that different activities occupy on the overall Strain scale.


If Strain is low, how can I judge strength-based training?

Although weight training might produce a low Strain score because your heart is not elevated for long durations, it impacts on your body is taken into account during recovery. 

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.

Many WHOOP members have wondered how their data will reflect a strenuous strength session, so here’s the breakdown:

  • Increasing weight and adding new strength exercises to your routine will likely cause soreness and muscle fatigue, otherwise known as delayed onset muscle soreness (DOMS). This microtrauma from the eccentric lengthening of the muscle fibers typically causes a reduction of HRV the following morning, especially if your body is not used to this kind of stressor.
  • Fatigued muscles often result in a higher Strain on the following day because your body is working harder to recover as it responds to disturbances in your homeostatic state.
  • When training appropriately (without overtraining), your HRV should be within the normal or high range (green or yellow recovery). This indicates the training load you put on your body the previous day was appropriate to your body's ability to recover.
  • People who strength train tend to have lower baseline stress hormones with a greater diurnal variation of cortisol, meaning that they have significantly higher levels of cortisol in the morning than in the evening, which allows their body to fluctuate between higher arousal during the day and deeper recovery at night.
  • Over time with strength training, monitor your HRV for increasing trends.

In order to analyze true muscular strain following a workout, you’d likely need additional tools such as an EMG or blood test.


Tips for Using Strain and Strain Coach for Strength Training

Using Strain Coach during your workouts can help keep you in the appropriate training zones to aim for depending on the type of workout you are completing.


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

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

  • 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

Anaerobic zone: 80 to 90% of your maximum heart rate. Training in this zone works best when going hard for short periods of time, then resting for equal or longer periods of time. 

Anaerobic training has the following benefits:

  • Build muscle
  • Burn fat 
  • Increase in metabolism
  • Increase HGH 
  • Strengthen bones and improve joint function
  • Improve your immune system
  • Increase your lactic acid threshold and endurance
  • Remove toxins through perspiration and exercise-induced lymphatic drainage
  • Increase your fast-twitch muscle fibers (strength, speed, and power)


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