The reason why time is more important than miles is because your body has no comprehension of what a mile is; it only knows how hard it's working and how long it's working (effort over time). The duration of effort is one of the key factors that arouses the biological signals that induce physiological adaptations that will ultimately lead to improvements in your running performance.
This concept of training by time should also be applied to individuals training in a group. This is the biggest flaw of group training, during which everyone in the group runs the same workout. A slower runner should not attempt the same number of repetitions of the same distance in an interval workout as a faster runner; otherwise he or she will experience more stress because he or she will spend more time running at the same relative intensity.
For example, an 18:00 5K runner who runs 5 x 1,000 meters at 5K race pace will experience more stress than a 15:30 5K runner who does the same workout. The corresponding times of the two workouts would be 3:37 per 1,000 meters (5:48 mile pace) and 3:07 per 1,000 meters (5:00 mile pace), respectively. For this workout, the slower runner would be running 30 seconds (or 16 percent) longer at the same relative intensity as the faster runner. To make these two workouts more comparable, and therefore to equate the stress experienced by both runners, the 18:00 5K runner should modify the workout by running 850 meters (which would take 3:04) rather than running 1,000 meters. If 850 meters is too awkward of a distance to determine, you can run either 800 or 900 meters. The point is to make the two workouts more comparable by shortening the distance for the slower runner (or, conversely, by increasing the distance for the faster one).
There are a couple of other ways to make these two workouts comparable—the 18:00 5K runner can decrease the number of repetitions or increase the recovery period. For example, if both runners run the same distance (1,000 meters) and the 15:30 5K runner does five repetitions (for a total running time of 15:35 at 5K race pace), the 18:00 5K runner should do four repetitions (for a total running time of 14:28 at 5K race pace).
Alternatively, if the 15:30 5K runner takes 3 minutes of recovery between repetitions, giving a work-to-rest ratio of 1-to-1, the 18:00 5K runner should take 3? minutes of recovery to make the ratio the same. While manipulating the number of repetitions or the recovery periods will make the two workouts more comparable between runners, the best way to equate the stress between these two workouts is to shorten the length of the work periods, since the time spent running at a specific intensity represents the greatest aspect of the training stress. If the 18:00 5K runner runs 1,000-meter repetitions like the 15:30 5K runner, but takes more recovery to keep the work-to-rest ratio the same, it's still a harder workout for the 18:00 runner.
More: How to Create Your Own Interval Workouts
In an effort to equate the stress of workouts between runners of different abilities, here's a hierarchy of strategies:
1) Decrease the length of each work period for slower runners (or increase the length of each work period for faster runners) to make the duration of each work period the same between runners.
2) Decrease the number of repetitions for slower runners (or increase the number of repetitions for faster runners) to make the total time spent running at a specific intensity the same.
3) Increase the duration of the recovery period for slower runners (or decrease the duration of the recovery period for faster runners) to make the work-to-rest ratio the same.
If you stop training by mileage and start training by time, not only will you do the amount of training that's right for you, you may even change your perception of time.
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