Why should you instead train intervals than walk or run far at lower intensity? How many 15-second intervals do you need to have a good effect on endurance? And can it have as good an impact as a 4×4 interval? It's the way your heart works that determines ❤️
BY JAN HELGERUD AND JAN HOFF, professors of medicine
Based on physiological theory and previous research findings, we conducted a study to document the effects of endurance training. The endurance of healthy people limits by our heart function and the heart's pumping capacity of the blood. Previously studies show that large muscle groups that are at work 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 thus, unable to supply enough blood. Our heart rate is innate and cannot change. Hence, we must exercise to increase the size of the pumping capacity. In an average person who does hard work, the heart pumps approx. 0.15-0.20 litres of blood in each beat, in comparison, the heart of a top athlete pumps twice as much.
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 heart size should increase and the highest possible blood flow triggers it. We have the highest possible blood flow when we work with large muscle groups, i.e. we use both legs (or more muscles) and an intensity that is close to maximum oxygen uptake. You must have a heart rate of about 85-95% of your maximum heart rate.
Many people love to train at intervals of less than a minute. However, it takes longer to have a full effect on your heart. We have approximately five litres of blood. It usually takes two minutes for the heart to have a full filling of circulating blood. Others favour long runs, but there is a limit to how long you can maintain such a high intensity. Typically, we are unable to maintain the highest intensity at 95% of maximum heart rate for longer than 6-9 minutes in a row. Therefore, we have seen that effective endurance training should consist of repeated work periods of between 2 and 6 minutes.
Since work in area 85-95% of maximum heart rate causes the muscles to build up lactic acid, you should remove the lactic acid between each work period. This is most effectively when you work at low intensity, about 70% of maximum heart rate. This is a level where you can talk effortlessly with a training partner.
To test the theory that 2-6 minutes at intervals is the most effective in improving fitness, we conducted a new study. We chose to control this by comparing with training with lower intensity, but the same amount of work, and measuring the heart's pumping capacity before and after a period of training.
We set up the training as follows:
Students from NTNU carried out the training three times per week for eight weeks.
We measure the effect on endurance as maximum oxygen uptake (VO2max), and the impact on the heart's pumping capacity measured as stroke volume (the amount of blood pumped for each heartbeat) during work. The effect of the training resulted in changes in maximal oxygen uptake and heart rate as follows:
The figure shows well 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 15×15 training is more effective than calm long-distance and threshold training is that it maintains a higher intensity. The short breaks keep a high blood flow. So that after 8-10 intervals of 15 seconds, and increased blood flow maintains in the 47 intervals, and gives a good effect. Although 47 intervals of 15 seconds have a sufficient impact, it will still not have as significant impact as a 4×4 session.
The conclusion is that frequent intervals of 85-95% of the maximum heart rate of 4 minutes are the most time-efficient way to increase maximum oxygen uptake. If you include such intervals in a walk or a run as a natural interval, one will expect the same effect. A new activity tracker in the app shows you how many minutes you have had in the right zone to change your heart.