039 Intervals, Endurance and Fatigue

The Paul Weber Podcast

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There is no apparent reason why intervals would be more fatiguing than endurance work as long as energy expenditure is matched.

The thing is, equating energy expenditure is tough to do, especially in the field.

As an athlete who trains multiple disciplines, the bottom line is the more you do, the more you’ll attenuate strength gains.

Use subjective fatigue measures like libido, resting HR, exercise HR and sleep quality to dial in your aerobic volume.

Also make sure you're retesting your top training priorities at least every macrocycle (~12 weeks) – this is the best way to gauge program efficacy.

MUSCLE FIBER TYPES

Initially, it was hypothesized that intervals would recruit faster twitch muscle fibers than endurance work.

It appears that above 65% intensity, Type IIA fibers are recruited as indicated by fiber type specific glycogen depletion.

HORMONAL STRESS

Salivary stress markers (cortisol and testosterone) indicate that HIIT is no more catabolic than aerobic endurance.

Cortisol levels are acutely higher after HIIT but recover within 3 hours.

This minor hormonal difference does not appear to be significant enough to attenuate anabolic processes or maximal strength expression.

METABOLIC AND CARDIORESPIRATORY STRESS

The metabolic and cardiorespiratory stress of HIIT is higher. As measured by lactate, HR, VO2.

Many years ago, the presence of lactate would be viewed as a mechanism of muscle fatigue, but it appears that is not the case.

Lactate is metabolized as a fuel substrate and, recent experiments in mammal muscles observe no relationship between lactate and muscle fatigue.

Lactate causes acidosis (acute reduction in pH) but that acidosis does not appear to be the cause of muscle fatigue.

Muscle function appears to return before pH has fully normalized.

More likely mechanisms of muscle fatigue:

• The breakdown of CrP which releases inorganic phosphate
• Hydrogen ions inhibit the binding of Ca2+ to Troponin
• Prolongs the rate of calcium reuptake by inhibiting the ATPase pump of the sarcoplasmic reticulum

FUEL SUBSTRATES

Looking at fuel substrates provides the most actionable information.

Acute drops in exercise performance are observed when blood glucose and muscle glycogen drop below certain levels.

Both of these fuel sources are used in intervals AND endurance work – the difference is the rate at which they are used, which is determined by the intensity.

But don’t make the mistake that you are not depleting these glycolytic resources when performing your endurance work – you are.