Fight or Flight Fitness

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Alan Couzens over at endurance corner writes a very good piece (click here for the full article) about the overall importance of including sprint intervals in your training even if your overall goal is an ultra endurance even like an Ironman.

He gets into the detailed physiology as you can see from this passage:

In the figure above (adapted from this study), the x axis represents intensity (%VO2max), the y axis represents aerobic adaptation in the form of cytochrome c concentration (nmole/gm). As you can see, slow twitch fibers were maximally (aerobically) trained at an intensity of ~83% of VO2max (approximately equal to the anaerobic or ‘functional’ threshold. FOG fibers were maximally trained at anywhere from 83-116% of VO2max, while FG fibers weren’t significantly trained until the workload exceeded 100% of VO2max, and weren’t maximally trained until the workload reached 116% of VO2max.

But beyond just the interval work he also discusses the importance of long slow stuff (which I have mentioned previously but haven`t gone into the details).

An additional study that has particular relevance to the optimal duration/intensity combination for slow twitch fibers was that of Harms and Hickson (1983). In this study, the researchers found a near linear relationship between duration of work and adaptation of slow twitch fibers. In other words, providing a minimal intensity of training was maintained (60%VO2max), doubling training volume elicited performance improvements of 40-100%. When we compare to the improvements in Dudley’s study, it becomes clear that in the case of slow twitch fibers duration and frequency trump intensity.

Therefore, you can see the benefits of both sprint intervals and long slow endurance stuff that could even include long serious walks.

Now what does he actually prescribe for sprint intervals (for your base training):

Essen (1978) found that providing the length of the training interval was kept short (15-30s), exercise performed in excess of VO2max elicited glycogen depletion patterns and lactate levels much more in accordance with tempo or threshold training, i.e. 2-4mmol/L lactate even when continued for 30-60 minutes (i.e. 30-60 repetitions). These intervals do not provide the same level of stress on the central systems as long intervals, therefore long intervals or time trials are indicated for a short period of time prior to competition to truly peak an athlete, however, in terms of maximizing peripheral adaptations in FG fibers, 30-60 reps of 15-30s with a 15s-1min rest period is optimal.

This at least give you some of the physiology behind interval training and some guidelines (30-60 reps is a lot so I would say gradually build up to this level), with the additional reminder of also including some low intensity endurance work.