Why Do HIIT when Hypertrophy is the Goal?

I recently reached out to my good friend Marc Lewis with a question regarding muscle hypertrophy and interval training. With Marc's background in exercise physiology and research interest in concurrent training, I knew he'd have the answer.

When I received Marc's incredibly thorough response, I realized just how lucky I am to have such a smart friend. I learned so much, I figured I'd share it here. If nitty gritty exercise science tickles your fancy, read on!

Hey Marc,

Even when someone’s primary goal is muscle hypertrophy, we still recommend a combination of cardiac output training and high-intensity metabolic training (i.e. HIIT).

In really simple terms, cardiac output training improves a person’s ability to recover, which helps them train with more volume and more frequently (thereby promoting hypertrophy). 

Why, though, do we recommend the high-intensity component? The argument you see a lot of the time is “just look at a sprinter’s body,” but you could also make the case that elite sprinting selects for muscular body types. Is there research showing that high-intensity metabolic training builds muscle?

Thanks a bunch,

Travis

Marc's All-Star Response

Greetings, Travis!

You bring up a great point that is missed by many fitness professionals. In theory, HIIT is actually utilized more as a conditioning tool due to its efficiency in terms of the amount of caloric expenditure per unit of time (i.e. HIIT for 20 min versus CO for 20 min). The downside of CO in this context is that the amount of time that it would take to expend calories would require you to engage in steady-state exercise for greater periods of time, which in turn, would trigger the activation of kinases such as AMPK and SIRT1 that are triggered with lower glycogen levels, and these have been shown to inhibit the mTORC1 complex via rapamycin. HOWEVER, what people forget is that higher intensity interval training triggers these same kinases in a shorter period of time due to relying primarily on muscle glycogen for energy. Therefore, it is all about managing the trade-off of volume/intensity. In addition, increased levels of metabolic stress can keep AMPK and SIRT1 levels elevated, which is why we try to keep it interspersed especially when the primary goal is to build muscle.  

In the context of hypertrophy, the only research showing HIIT increasing myofibrillar size is in the untrained (i.e. any stimulus will do this). HIIT is not really useful for building muscle in trained individuals. The reason it is used more is due to the aforementioned rationale, as well as the potential power and speed benefit from tapping into those higher threshold motor units and improving frequency modulation. 

Practical application: If you are performing HIIT, go ahead and put it on an off-day from lifting, as this will maximize your ability to build muscle due to the cellular milieu. If you have to perform HIIT on the same day as lifting, then keep it short-and-sweet (i.e. <20 minutes) in order to reduce upregulation of AMPK, SIRT1, etc. Finally, perform cardiac output training in a non-depleting fashion (i.e. keep the pace conversational) and for no more than 40 minutes at once. Obviously, the time-frames can change due to diet, training status, etc., but these are good recommendations for the average gym-goer.

Also, I would mention the modality effect. If building muscle is the primary goal, then ditch the running for cycling, rowing, sled dragging, etc. This was brought out nicely in the Wilson et al. (2012) meta-analysis. Muscular parameters were significantly altered when aerobic training was >3 days/week, independent of modality. However, when they controlled for modality as a moderator, no significant interference in muscular parameters was seen up to 5 days per week. Most research has used cycling, but more research is coming along using rowers, sleds, etc. (me! haha). Finally, cycling and prowler pushes are good due to being primarily concentric contractions (i.e. less muscle damage) and assisting with type 1 fiber hypertrophy.

Side note: AMPK and SIRT1 levels return to baseline in about 3-4 hours, so if needed, you can do a same-day split session to reduce interference. Additionally, intake of 20-25 grams (0.25 g/kg) of protein immediately after training is of GREATER importance if conditioning and resistance training is performed in the same session due to the activation of the leucine transporter LAT1, which upregulates mTORC1 complex and assists in activating p38 kinase, which upregulates PGC-1a (i.e. the main regulator of mitochondrial biogensis). So these little nutritional nuggets help maximize growth and blunt any negative cellular milieu.

Cheers,

Marc