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Understanding Intervals

Understanding Intervals

Understanding Intervals

Matching training characteristics to physiological changes
Which is “better”, Interval Training or Steady State Training? Obviously that is simplistically stated, but I will try to make things more clear as we go. Eight years ago, my master’s thesis asked this exact question. One group of rats “volunteered” to run repeated 2 minute high intensity intervals every day (5 days/week), another group ran 60 minutes continuously each day, and a third group served as cage potato controls. We will come back to the results later. I learned from that study and have continued to learn since.

Interval Training Defined

In a dictionary, an interval is probably defined as a period of time or a specified distance. Well, that’s true, but for us endurance athletes it means repeated bouts of high intensity exercise with intermittent rest periods. In endurance training circles, no matter what the sport, Interval Training has become a standard practice. Since the 1960’s, interval training has come to be thought of as the key to endurance performance success. In some training programs, it accounts for 50-75% of the total training volume. One fellow I know who coaches runners has even written an article espousing “All Intervals, All the Time.” After 10 years of research, reading, observing, and training, I think this line of thinking must be evaluated critically, so here goes. I will hit you with information from a lot of different angles, but hopefully what emerges will be a coherent conclusion.

The Physiology of Intermittant Exercise

In exercise physiology research, one of the true “Fathers” of the field is Per Astrand from Sweden. Today, Dr. Astrand is still showing up at International Sports Medicine research meetings at the age of 80 or so. His best known contribution, “Textbook of Work Physiology”, co-written with Norwegian Kaare Rodahl and now in its 5th edition or so, remains a “must have” for the student of exercise physiology. In the 60s he performed some simple experiments that still have important training implications regarding the physiological impact of continuous versus intermittant exercise.

In a lab setting, a bicycle ergometer serves as a very useful tool for performing exercise studies. We can precisely control the workload that the cyclist must maintain, and since he/she is sitting still, it is easy to perform an extensive array of physiological measurements, such as heart rate, VO2, blood lactate etc. (I prefer not to dwell on just how extensive!) The following experiments were performed using just such an exercise setup.

One subject was made to accomplish a certain amount of work (force x distance, a quantity that can be precisely measured on the ergometer) in 1 hour. This work could be accomplshed either by a continuous bout of exercise at a power output of 175 watts, or by intermittent exercise at a heavier load, separated by regularly spaced rest intervals. A double power output was chosen for this heavier load. Thus the desired work could be acomplished with 30 minutes of exercise at 350 watts within the span of one hour. At a work load of 175 watts, the subject could easily cycle for one hour continuously. Heart rate was only 134, VO2 was only about 55% of maximal, and blood lactate remained near resting levels. When the subject tried to exercise as long as possible at 350 watts, or double that workload, the exercise could only be maintained for 9 minutes. Heart rate was 190 bpm (maximal), VO2 was at max, and blood lactate had risen to 16.5 mM, an extremely high value indicative of extreme fatigue. If, instead, he exercised at the same 350 watts intermittantly for a duration of between 30 seconds and 3 minutes, always with equal rest, he could perform the desired work within the hour. However, the physiological responses differed tremendously depending on the interval duration.

Exercise condition
VO 2
Heart Rate
Blood lactic acid
175 watts
350 watts **
Intermittent ##
30 seconds*
1 minute*
2 minutes*
3 minutes*

** Could be sustained for only 9 minutes

## All physiological values represent peak values achieved during exercise

* Rest duration equaled work duration in each condition

The Interpretation follows on from the above data | Interpretation of Understanding Intervals |

source : Copyright Stephen Seiler All Rights Reserved


  1. […] The focus on lactate levels during recovery made a lot of sense. After all, during short, very intense work intervals, most energy for muscle contractions is produced anaerobically (oxygen- independently). Anaerobic energy production is thwarted by high lactate levels, so the researchers reasoned that the recovery method which did the best job of attenuating lactate would also produce the best-quality work intervals. […]

  2. […] fitness. The different intensity levels experienced when hiking can be compared to high-intensity interval training, which forces the body to work at different intensities instead of one maintained intensity. This […]

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