slowest training pace that

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Does intensity matter for capillary development?

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At some level it does, since you require a stronger stimulus to see improvements as you get fitter and significant capillarization won't continue to occur without more time spent at faster speeds, inducing more prolonged hypoxia within the Type I fibers and providing greater activation of the Type II fibers. However, during initial stages of fitness building or following a peaking stage (i.e., during base training), running at moderate intensities (somewhere between a normal recovery day pace and a decent "threshold" pace for most runners) for sufficient duration extends capillaries equally around both Type I and Type II muscle fibers even if it's predominantly the Type I units that are actually recruited. This could possibly be due to the fact that expression of vascular endothelial growth factor (VEGF) messenger RNA occurs both within and between muscle fibers following endurance exercise.

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If I run, and I run long, is there any "bonus" for going faster than whatever feels "easy" that day?

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Yes and no. A possible "no" in the sense that if you need an easy day and a faster pace would just run you into the ground, grinding it out for the sake of development in some other area (capillarization, etc.) makes the negatives outweigh the positives. But if you feel fine, there are quite a few positives associated with picking up the pace to near marathon pace during the last fourth to last half of a long run.

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Also, is there any pace too slow (in the sense of it would take 50 times as long for the same developments)?

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Yes. Capillary development will be minimal to nonexistent if you never operate at any stronger an effort level than about 45% of max VO2. For most competitive runners, this means a pace of roughly 60% of 5k race pace, or in the neighborhood of 3 minutes per mile slower than 5k race pace. This isn't to say that running really slowly on occasion doesn't have some benefits; it's just that the benefits at these super-slow speeds don't extend to capillary development.

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The Japanese tend to "jog" a lot, I'm talking 8:00/mile pace for a 2:06:57 marathoner (Inubushi).

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Yes, and Morceli used to do some easy ambling at 9-10 minutes per mile. Shorter said he couldn't find many people who could run fast enough for him to work out with on hard days and also couldn't find enough people who would run as slow as he did on easy days. Willis once remarked that he hadn't seen college teams that ran as slow as he likes to run on recovery days either. There are plenty of other examples.

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Of course the Japanese do long tempos and such, but those 2-3 sessions are not going to make a 2:06:57 marathoner.

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However, those runners neither sprang forth from the womb able to do 30k at 4:50 pace nor decided one day that they'd jump into running and schedule a few of these long, fast runs during their first season. They had to work up to that level over many years with thousands of hours of running at all manners of speeds, and the cumulative stimulus of all this created the performances they ultimately produce.

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Could you explain the success of runners that use a long slow distance approach, or run most of their mileage slower than 60% of 5k race pace? What other factors are involved that enables these runners to succeed with this type of training?

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The slow running can be helpful, just not in the area of capillarization. As mentioned, the sum total of tens of thousands of miles created all the physical and mental components of success, including the wisdom to find the proper balance between fiery determination, patience and prudence in training and in racing. Most of these runners have already reached their physical limits in many of the measurable physiological facets of running performance before they reach their peak performances, yet they continue to improve in more subtle areas, such as efficient mobilization and sequencing of motor recruitment.

Those extremely slow runs can be very useful for regeneration by keeping connective tissue healthy, providing weight maintenance and promoting running economy through a tremendous amount of repetition in an utterly relaxed state. Even thought the speed seems useless (and would be largely ineffective if this was only speed used), as supplementary running, the sheer act of step after step on autopilot while in no metabolic distress fosters extreme efficiency in the neurons and muscle fibers responsible for fine motor control. Runners who already have a substantial high mileage background have also trained well beyond the point of experiencing a glycogen sparing effect in their long runs; therefore, they may need this easy jogging to provide a fat-burning stimulus to help stabilize weight at an ideal level for running.

In addition, if you're doing a decent number of regular hard workouts during a certain stage of a season, you should run your recovery runs as easy as you need to. While easy doesn't necessarily mean really, really slow, if this is what it takes to recover sometimes, so be it. The time for significant capillary development is probably past by that stage of a season, anyway, so recovery from hard efforts (with maybe a little maintenance of the vascular component as an afterthought) is the name of the game at that time.

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Are there actual studies on humans that actually measured capillary growth NOT occurring below 45% of Max VO2?

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There was at least one in the early 1980s using human subjects. Admittedly, these were already active subjects and the sample size was small (as is the norm for many of these larks). But the conclusion was that capillary development did not take place at 45% of max VO2 and it is therefore prudent to keep exercise intensity from becoming too low to effect change in this area.

There have been other studies within the last decade using groups of rats running for various durations and intensities. But rat muscle fiber composition differs markedly in homogeneity from that of the corresponding muscle groups in humans and rat recruitment patterns are altogether different, often involving Type II fibers where Type I fibers are recruited at the same relative intensity in humans. Interested runners should therefore be skeptical of any sweeping conclusions about the effects of various intensities and durations of exercise on human fiber types if the test subjects are rats.

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So walking wouldn't develop capillaries at all?

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Perhaps it would in completely sedentary subjects who had never undertaken any exercise regime, but that doesn't really apply to any competitive runners that I'm aware of, so I've never thought it was worth looking into.

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Higher-intensity running is actually more conducive to mitochondrial proliferation than low-intensity running is. The main benefit to FT units arising from longer, slower runs is the fact that some glycogen depletion occurs in adjacent fibers whether they are actually recruited during the run or not, which stimulates enzyme production within all the fibers, ST and FT alike, and creates more efficient meshing of glycogen synthesis with other metabolic systems such as lipid breakdown.

For a better long run protocol, one which actually recruits more of the larger (but still oxidative) fibers along the spectrum, sometimes pick up the pace to around probable marathon pace for most of the last third to last half of the run, with a slightly faster last mile or two than that. This will count as a moderately hard effort day due to impact stress and possibly some fuel depletion, so adjust the remainder of the weekly schedule accordingly. Eventually, you'll need to do this (a faster pace for some of the long run) in any event due to glycogen sparing, which puts a damper on the stimulus for aerobic enzyme activity. Maybe alternate long run protocols, with one being a true long run at an easy pace throughout and the next one being 75% to 80% of that length (15 to 16 miles if your longest run is 20) but using the strongly-paced last half.