Endurance Workout: Slow to Fast or Fast to Slow?

by Thomas Kurz

There are ways of arranging exercises in endurance workouts that increase the recovery time for the same amount of work if it were done in a better arrangement. I will use the example from a question I received recently. The question deals with a swimmer’s endurance workout, but the answer applies to any type of endurance activity.

Here is the question:

“I have a question about specific strategies for endurance training. It’s common to do workout sets that move from longer yardage to shorter (e.g., 500 yards, followed by 400 yards, 300 yards, etc.), with shorter lap times as you move down in yardage. Is this the best way to go about this? I’m a novice [in swimming], but doing this kind of workout seems to have increased both my endurance and strength (I’ve gone from being able to swim only 50 yards at a time to 1,200 yards at a good pace), but I’m wondering if this is the best way to do this from a physiological standpoint. I generally alternate during the week between long swims and these kinds of shorter, faster sets.”

Now, my answer:

Doing speed work after endurance work raises lactate levels and increases recovery time more than doing speed before endurance (Wilmore and Costill 1999, McArdle, Katch, and Katch 1996). Prefatiguing slow-twitch (ST) muscle fibers with long-duration aerobic work impairs intramuscular coordination, so a greater mechanical stress is put on the structurally weaker fast-twitch (FT) fibers. This causes greater muscle soreness after a workout that begins with aerobic endurance, proceeds to increasingly anaerobic efforts, and ends with sprints. Mechanical stress, which damages muscle fibers, is the main factor in muscle soreness.

Damage from mechanical stress is compounded by chemical damage from muscle acidosis when a workout ends with exercises of intensity exceeding the anaerobic threshold. Muscle acidosis is caused by conversion of lactic acid to lactate and an accumulation of hydrogen ions–H+. If a high-intensity effort is followed by a long low-intensity effort (for example, 40 minutes at 35% of VO2max) or by an effort of decreasing intensity (for example, 7 minutes at 65% of VO2max plus 33 minutes at 35% of VO2max), there is much less accumulation of lactate than if the high-intensity effort was followed by rest (McArdle, Katch, and Katch 1996).

At low exercise intensities–when you do not exceed your anaerobic threshold and are not very fatigued–going from long and low-intensity efforts to shorter and more intense efforts will not hurt you. As the intensity in shorter yardages rises above your anaerobic threshold, you may feel overstressed after a few workouts with this arrangement of increasing the intensity of effort after fatiguing aerobic work (the longer distances). This is explained in Science of Sports Training and in the books on exercise physiology listed at The Athlete’s Bookshelf.

If this arrangement works for you, either because you do not exceed the anaerobic threshold much or because you are in great shape and can recover quickly after such a workout, then keep doing it. You can also try reversing this “common” order of decreasing yardage and increasing intensity to see how you feel after a workout in which following a good warm-up you begin with short distances at high intensity and then move to lower intensities and longer distances.

I have encountered this topsy-turvy arrangement of exercises (from endurance to sprints) in workouts in other sports, such as ball games. For example, the main part of the volleyball player’s workout would consist of drills and then running 400, 300, 200, 4 x 150 meters or 200, 180, 160, 140, 120, 100 with the goal of, I guess, improving endurance.

Doing long distances and then short may be good for mental toughness if done occasionally, but it is not very good for endurance in the long term and is bad for speed and agility because the short runs are done when you are fatigued and relatively slow. See the chapter “Speed” in the book Science of Sports Training.

It is better to start the main part of a workout (after the warm-up) with short sprints (maximal speed) and end it with longer distances. Volleyball, to continue with the previous example, requires not only endurance but also (if not mostly) speed because the distances to be covered on the court are short and need to be covered fast.

I know two typical explanations for doing sprints when tired by the preceding endurance work. One is that such an arrangement helps recruit fast-twitch glycolytic fibers (FTb or type IIb), which are recruited for high-intensity efforts such as the 100-meter track-and-field sprint or 50-meter swimming sprint (Wilmore and Costill 1999). The other is that sprinting when fatigued gives the ability for a strong finish, the final “kick.”

Well, it is true that during long-duration efforts, as slow-twitch muscle fibers are fatigued, more fast-twitch fibers are recruited to maintain muscle tension. The more slow-twitch fibers are depleted of glycogen, the greater the recruitment of fast-twitch oxidative glycolytic fibers (FTa or type IIa) and then FTb. When there is very little glycogen left in the slow-twitch fibers, the greatest effort falls on the two types of fast-twitch fibers, and eventually there is a considerable involvement of FTb fibers. There is a snag though, as Wilmore and Costill (1999) write: “When glycogen stores in ST fibers are depleted, it appears that FT fibers either are unable to generate enough tension or cannot be sufficiently recruited to compensate for the loss of muscle tension. . . . The sensations of muscle fatigue and heaviness during long-term exercise may reflect some muscle fibers’ inability to respond.” Furthermore, when an athlete’s nervous system is fatigued, the mobilization for maximal-intensity efforts is very difficult and coordination is off. It is much better to work on sprints and other high-intensity efforts that recruit the FTb fibers when fresh and sharp, after a good warm-up and not when fatigued.

As for the final “kick” at the end of a race (or a bout or a round, or whatever), it depends on good aerobic endurance for sparing glycogen in all muscle fibers throughout the distance and on strength and work capacity of fast-twitch fibers used for the final sprint. All these are best improved by rational speed or speed-endurance (a form of anaerobic endurance) workouts. For example, a good warm-up, then intervals followed by a longer run and a cool-down. Even though this arrangement does not mimic what happens during a middle- or long-distance race–when athletes first try to pace themselves and then at the finish go all out with what they have left–it works very well. The reasons are given in books on exercise physiology, in chapters about basic energy systems (ATP-PCr, glycolytic, oxidative) and about glycogen depletion.

The ways of arranging exercises in a workout for best effect and the rationale for these ways are covered in the chapters “Basic Concepts of Sports Training,” “Structure of a Workout,” and “Endurance Exercises in a Workout” in the book Science of Sports Training. Athletes can make progress with bad training but not as much as with good training. Why bad training practices take hold and what their consequences are is well explained by coach Charles Richardson in his article “Bad Training.”

References

McArdle, W. D., F. I. Katch, and V. L. Katch. 1996. Exercise Physiology: Energy, Nutrition, and Human Performance. Baltimore, MD: Williams & Wilkins.

Wilmore, J. H., and D. L. Costill. 1999. Physiology of Sport and Exercise. Champaign, IL: Human Kinetics.