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Interpretation of Understanding Intervals

The article below leads on from the first article Understanding Intervals


There are two major points I want to draw from Dr. Astrand’s experiments:

Interval Training

1) Intermittent exercise allows a higher total volume of high intensity work. Performed continuously, the subject could only manage 9 minutes at 350 watts. Performed in 3 minute intervals, he could accumulate over 3 times as much total work (30 minutes, with great effort).

2) When the intervals were 3 minutes in length, the desired work could be accomplished within one hour, at great effort. However, when the work and rest periods were shortened, the physiological strain was dramatically reduced, even though total oxygen uptake during the hour was not markedly reduced. Specifically, if the intervals are less than 2 minutes in length, the physiological workload/stress is severely reduced despite the same total accumulated time (30 minutes here) and same interval intensity (350 watts workload here). If you compare the peak oxygen consumption, HR, and lactic acid concentration achieved during 1 minute intervals with 2 minute intervals in the table above, this difference is easily observed.

Why are the responses so different?

I think the best explanation for the difference was put forward by Astrand. He suggested that during very brief intervals, oxygen bound to myoglobin served as an effective buffer against the accumulation of an oxygen deficit (and lactic acid) during the exercise bout. Therefore after a 30 second bout, myoglobin oxygen stores were repleted during the rest period, and the peak demand on oxygen delivery was not severe. By analogy, the body manages to live expensively, and briefly deplete cash reserves, then always repay the small debt during a subsequent “debt recovery period”. No long term debt accumulates. As the exercise bout lengthens, the capacity of the small buffering myoglobin oxygen store is outstripped, lactic acid production and accumulation becomes significant, and the burden of greater oxygen delivery during the work interval falls on the cardiovascular system.

Subsequent experiments by Astrand showed that if you shortened the work period and rest periods to smaller and smaller intervals, it was possible to perform at even higher power outputs without accumulating lactic acid or severely stressing the cardiovascular system.

Conclusions So Far

For a period of intermittant exercise that approximates a max VO2 workload to overload the cardiovascular system effectively, it needs to be of at least 2 minutes duration due to 1) lag time in the cardiovascular response and 2) the oxygen buffering effect of myoglobin

Now, lets go back to my master’s thesis. The question I set out to address with that study was this: “What is the impact of interval training and steady state training on the performance of the rat heart?” I wanted to evaluate the heart independent of the muscles, and make direct measurements, so I had to use rats. I bred a group of rats in the laboratory, meaning they were litter-mates and therefore, presumably quite similar genetically (they sure looked the same). After they reached adulthood, I divided them into three groups. One group sat around in cages, ate rat chow and got weighed periodically. A second group was gradually adapted over 4 weeks (several months in rat years) to running on a treadmill until they were running for 60 minutes, 5 days a week at an intensity that was equal to about 75% of VO2 max. The third group was adapted to an interval running program. These guys would run two minute bouts at a speed that required just over 100% of VO2 max, then slow down for two minutes, and so on for 10 bouts (after four weeks of buildup). The total running distance was the same for the two groups, only the intensity differed. To evaluate cardiac performance, I develped a surgical procedure under anesthesia (the rats, not me) that allowed me to directly and constantly measure cardiac output, intraventricular pressure development, heart rate etc., while subjecting the heart to a volume overload with saline infusion. What I found and reported at a meeting of the American College of Sports Medicine was this. In these previously untrained rats, 8 weeks of interval training was superior to 8 weeks of steady state training as an inducer of enhanced maximal cardiac performance. Interval trained hearts achieved higher peak stroke volumes during overload.

OK, So What?

Remember VO2 max? We have established that cardiac performance is a primary determinant of the VO2 max. The results of my thesis study suggested that interval training was the best way to enhance maximal cardiac performance, and therefore, presumably, VO2max. In fact, in a study by a different laboratory, a significant increase in VO2 max occurred in previously untrained rats subjected to 5, very high intensity 1 minute bouts of exercise a day. That’s 5 minutes of exercise. This improvement occured without any change in skeletal muscle oxidative capacity. There are other studies, on humans, that demonstrate the same finding.

Interval training allows us to accumulate a greater volume of stress on the blood pumping capacity of the heart. By using a large muscle mass, we promote maximal stroke volume responses. A high heart rate also is achieved as a function of the intensity. Finally, the periodic elevations and decreases in intensity may create special loading stresses on the heart that are adaptive. For example, during an interval, heart rate climbs high, then at the moment you stop the interval, heart rate immediately starts to drop, but venous return remains high. These exposures to additional ventricular stretch may help trigger ventricular remodeling (bigger ventricle volume). In addition, interval training may create a greater signal for changes in the compliance in the arterial system, but that is just more speculation.

Followed by the article : Interval Intensity of Understanding Intervals

source : Copyright Stephen Seiler All Rights Reserved


  1. How wonderful to read something by someone who referenced Astrand. I take my hat off to you. Yes, it all came from the Karolinska Institute, Sweden. As a health professional in human health, anatomy and physiology, of course Per Olaf Astrand and his theory and practice is of much interest. As a trainer of 2 human half marathon athletes with personal bests of 1 hour 12 minutes and 1 hour 4 minutes and change. My own learning was further enhanced by coming to understand that at the same time, 1952, in the next Karolinska Institute building, the equine sports department had word that Astrand was on to something. The equine sports department professors, Sunne Persson and Arne Lindholm started testing Astrands theories on the trotting standardbred.The main results were a match. I started training trotters, finished up moving from Australia to become an international horse trainer in Norway. When I am asked what does health science have to do with horses, the answer should always be the Karolinska Institute. Many individuals like yourself might say you know nothing about horses, my question to you would be how much did you know about breeding rats? Trotting world records are within 10ths of seconds of each other, just the Trotter is twice as fast. For the non believers, just Google search Bengt Saltin Standardbred Trotter. Bengt Saltin, Karolinska Institute. From there, if I were to advise the interested, look at physics, Archemedies, Leonardo da Vinci, Newton. Then go to the winner of the 1952 Olympic of the 5,000m 10,000m and marathon, Thank you Emil Zatopec for saying “I know how to run slow, now I have to learn to run fast, so I ran 100m and came back slow, slow, slow”. It’s a beautiful thing.

    Tak for det, (thanks for that) Per Olaf Astrand.

    • thanks for your input, appreciated


      • Thought i might just put my two cents worth in once again! Little less tired this time.

        With Bengt Saltin at the helm Per Olaf et al. in one hand, Arne Lindholm et al. in the other and Intermittent training being conducted and published in 1952. One might wonder why we call “stop start” training interval training considering Gerschler and Reindel did not publish “Das Interval Training” until 1962. intermittent training were the words used by Per Olaf Astrand.

        Fortunately or unfortunately as the case may be one of Bengt Saltins catch cries was from his mentors (Christensen) mentor August Krogh.

        “There is no value in publishing a finding which is conflict with present thinking, the value comes with finding an explanation for the difference”. Bengt Saltin quoting August Krogh.

        Bengt Saltins mentor Christensen was also quoted at the 18th Annual Congress of the European College of Sports Science, Barcelona, Spain. 20013.

        “When you write an article or present your data, you should refer to other researchers work in the same manner as you yourself want your own work to be quoted and discussed”.( Same reference as above).

        Perhaps it may be prudent that every athlete read “Pace Makes the Race” by Arne Lindohlm even though it is for racehorses.

        There should be no doubt that the team at Karolinska were suspicious to say the least about the theory of Myerhoff and lactic acid. Brooks of course put his reputation on the line and that is what appears to me to be the difference and of course got there first. As much as i am a fan of Saltin and company i am also a fan of George Brooks.

        When we look at “Pace makes the Race” this story then becomes obvious.

        Aer = air and obic means the system closest to = Aerobic.

        An = without and aer = air and obic means the system closest to = Anaerobic

        A = no, not, or against and lactic means lactic acid.

        Just as there is no i in team and considering that creatine phosphate is an alkali or base how in the …… did we mix up the alactic creatine phosphate system with being an aerobic?

        Those pesky little oxygen cells are always hopefully at least hiding in the blood cells.

        There is no obic in ALACTIC

        Good Night.

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