Why should you rather train intervals than walk or run long distances at a lower intensity? How many 15-second intervals do you need to have a good effect on endurance? And can it have as good an effect as 4x4 interval? It's the way your heart works
Your heart's pumping capacity limits your endurance
A study we conducted to document the effects of endurance training was based on physiological theory and previous research findings. The endurance of healthy people is limited by our heart function and the heart's pumping capacity of blood. It has previously been shown that large muscle groups that are working can receive 2-3 times as much oxygen-rich blood and produce 2-3 times more energy than the heart's pumping capacity indicates. The heart is therefore unable to deliver enough blood. Our heart rate is innate and cannot be changed, therefore we must work to increase the size of the pumping capacity. In a normal person engaged in hard work, the heart pumps approx. 0.15-0.20 liters of blood in each stroke, in comparison, the heart of a top athlete pumps twice as much.
How to change the size of your heart?
Research in medicine has shown that changes in the size of the heart occur through two mechanisms, "shear stress" and mechanical influence of the greatest possible blood flow in the heart. "Shear stress" is the friction of blood flow along the heart walls during pumping. Both mechanisms give signals that the size of the heart should be increased and this is triggered by the highest possible blood flow. We have the highest possible blood flow when we work with large muscle groups, i.e. we use both legs (or several muscles) and an intensity that is close to maximum oxygen uptake. You must have a heart rate of approximately 85-95% of your maximum heart rate.
Why 4x4?
Many people are happy to train intervals of less than one minute, but it actually takes longer to have the full effect on your heart. We have approx. 5 liters of blood to be put into circulation, and then it normally takes two minutes for the heart to have a full filling of circulating blood. Others favor long runs, but there is a limit to how long you can sustain such a high intensity. Normally, we are unable to maintain the highest intensity of 95% of the maximum heart rate for longer than 6-9 minutes straight. Therefore, we have seen that effective endurance training should consist of repeated work periods of between 2 and 6 minutes.
Active breaks remove lactic acid
Since work in the area of 85-95% of maximum heart rate causes the muscles to build up lactic acid, the lactic acid should be removed between each work period. It is most effectively done with work at a low intensity, approx. 70% of maximum heart rate, which is a level where you can chat effortlessly with a training partner.
Here is the conclusion:
To test the theory that 2-6 minute intervals are the most effective for improving fitness, we conducted a new study. We have chosen to check this by comparing it to exercise with a lower intensity, but the same amount of work, and to measure the heart's pumping capacity before and after a period of exercise.
In order to compare training with the same amount of work, the training sessions were set up as follows:
Long-distance running at 70% of maximum heart rate for 45 minutes
Threshold training at 85% of maximum heart rate for 24 minutes and 25 seconds
15x15, 15 second intervals and 15 second active break for 47 work periods with an intensity corresponding to 90-95% of maximum heart rate 4x4-minute intervals with a 3-minute active break at 90-95% of maximum heart rate. The training was carried out 3 times per week for 8 weeks. The training was carried out by students at NTNU.
4x4 is most efficient
The effect on endurance is measured as maximum oxygen uptake (VO2max) and the effect on the heart's pumping capacity is measured as stroke volume (the amount of blood pumped for each heartbeat) during work. The effect of the training produced changes in maximum oxygen uptake and the heart's stroke volume as follows:
The figure clearly shows that, in line with physiological theories and previous research, work periods of around 4 minutes are more effective than other forms of exercise. The reason why 15x15 training is more effective than quiet long-distance running and threshold training is that it maintains a higher intensity. The short breaks maintain a high blood flow, so that after 8-10 intervals of 15 seconds, a high blood flow is maintained in the 47 intervals, and gives a good effect. Although 47 intervals of 15 seconds have a good effect, it still won't have as much effect as a 4x4 session.
Being out of breath has a good effect
The conclusion is therefore that systematic intervals of 85-95% of maximum heart rate for 4 minutes are the most time-efficient way to increase maximum oxygen uptake. If such intervals are included in a walk or a run as a natural interval, the same effect can be expected. A new activity meter in the app shows you how many minutes you've been in the right zone to change your heart rate.
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