What is HIIT – It’s a vigorous (or maximum) activity of short duration interspersed by rest or low-intensity activity between the bouts (sets/rounds), where intensity, volume, and rest time are adjusted according to the training objective.

What is vigorous-intensity – This means the exercise intensity, which may be increased on bike or treadmill easy pushing a button. Running or cycling on the road the intensity can be increased by running/pedalling faster or up a hill. How to measure intensity? A heart monitor is an easy way to do it, but need to be seen as an investment as they can be expensive. Another way is how you perceive the intensity – in other words: Listen to your body! You need improvement, and to achieve your goals and not to die or feel unwell during or after exercising.

What is short duration – The duration of HIIT exercise often varies from 15 sec to 4 min of exercising. The different time of working (the “on” phase) depends on your goals and your physical level too. But, there is a basic rule here. The higher the intensity the shorter the time you have to perform the exercise.

How to rest – The rest can be active or passive. If the rest is active, must be low intensity (for example run 15 sec/walk 15 sec). Passive rest means …. just stop and rest. The rest period, again, depends on your goals. It is possible to work 15 sec (at your maximum) and rest 15 sec only or rest 45-60 sec. The graph below shows my last HIIT exercise in the park. I ran using different intensities and durations (“on” phase) and rest periods to show you how my heart worked during this activity.

To start I did a short warm-up (quadrant 1 in the graph), followed by 3 sets of 4 min running at moderate intensity and resting for 4 mi (quadrants 1 to 6 in the graph).

After that, I did 2 sets (rounds) of 4 min running 15sec (as fast as I could) and resting 15-sec passive quadrant 7 and 9).

The red line shows my heart rate. Basically, my heart worked at the same intensity during 4 min/4 min and 15sec/15 sec – reaching 170-180 bpm. The blue line shows the speed I ran. My speed during 4 min running ranged from 12 to 14 km/h. But the speed during the 15sec/15sec ranged from 19 km/h to 20 km/h. Usain Bolt runs at an average of 37 km/h…. I am pretty slow.

Summary: the intensity that I could change here was just the speed. I was running in the same place. My heart worked almost at the same level for the 2 different HIIT exercises.

The energy used by the muscles during these activities are very different. Always ask your trainer about how to train, instead of just seeing someone on social media and following his/her routine just because it looks nice.

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