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by Jonathon Sullivan MD, PhD, SSC, PBC

High Intensity Interval Training remains a go-to modality for conditioning in older adults.

Sled pushes are ideal conditioning for older adults.
Sled pushes are ideal conditioning for older adults.

In The Barbell Prescription, we describe a range of General Fitness Attributes that must be addressed by any comprehensive exercise medicine prescription: strength, power (which includes speed), endurance, mobility (which includes flexibility and agility), balance, and body composition. It should be evident from this list that, contrary to straw-man arguments raised in certain quarters, we do not dismiss the importance of endurance or aerobic adaptations. Endurance adaptations promote a more healthy aging phenotype and promote beneficial cardiovascular and metabolic changes, but even if we set all that aside, we would value aerobic endurance adaptations for their ability to promote training itself. An easily winded athlete has trouble completing a heavy barbell workout. You don’t just train to get in shape; you have to be in shape to train.

Training for endurance has classically been assigned to LSD (long-slow-distance) or what many investigators now call MICT (moderate intensity continuous training). We’ll use this terminology here: Roughly speaking, MICT = aerobic/endurance training. This form of training is exemplified by distance running, cycling, or rowing. It produces profoundly important adaptations for health and performance, and is well-documented to result in increased oxygen utilization and delivery (the parameter we call VO2max), improved cardiovascular function, increased fat oxidation and decreased glycogen depletion at moderate exercise intensities, increased muscle mitochondrial density, increased muscle capillarity, increased insulin sensitivity (hugely important), decreased visceral fat, and increases in certain mitochondrial enzymes like citrate synthase and succinate dehydrogenase.

If you’re keeping score: That’s all good.

Unfortunately for those of us who contend that training for the Athlete of Aging should focus on strength, concurrent training that combines resistance training (RT) for strength and MICT for endurance also demonstrates an interference effect that can blunt strength improvements.

In other words: you can improve your endurance with MICT, and you can improve your strength with RT, but you can’t improve your strength beyond a certain point with concurrent RT + MICT. The reasons for this have to do with “molecular interference,” a kind of biochemical switch that favors one adaptive phenotype over the other. The particulars are addressed in our book, although ongoing research has made the subject considerably more nuanced and elaborate since publication.

The good news is that we have an alternative approach to training for endurance and aerobic adaptations. Instead of working at the more aerobic, moderate-intensity end of the energy spectrum with MICT, we can train at the high-intensity, more anaerobic end of the energy spectrum and get the same benefits.

That’s high-intensity interval training (HIIT).

HIIT represents an apparent violation of the principle of specificity, as I have discussed elsewhere, and this peculiar paradox has not been resolved at the molecular and genetic level to my satisfaction. But that’s an academic rather than a practical consideration. The ability of anaerobic HIIT work to produce aerobic and cardiovascular adaptations similar to those of MICT was already well-documented at the time The Barbell Prescription was published.

Correlating heart rate with energy system and training zone by age.
The Fox and Haskell Formula

Table: The Fox and Haskell formula correlating heart rate with energy system contributions. Credit: Morgoth666. Link to original on wikimedia commons, shared under creative commons license.

In the HIIT regime, the athlete works at a maximal intensity for extremely short intervals, during which the athlete should be in the anaerobic or VO2max range as indicated by heart rate. The Fox and Haskell formula, reproduced above, is nominally useful for this purpose, and heart rate monitors are terrific, but in practice I believe all that is really necessary is for the athlete to “max out” effort during the bout.

A classic approach involves a 30-second all-out effort followed by as little as 20 seconds to as much as 2-4 minutes of rest (depending on the study and the protocol). While any exercise modality might theoretically be used for HIIT, a bit of reflection through the lens of common sense should inform us that the exercise to be used for a high-intensity bout should be a simple, stable, repetitive motor pattern involving a large muscle mass.

Thus, most investigators have used stationary bikes or sprints for investigations of interval training. In the strength and conditioning community, modalities chosen range from stationary bikes to treadmills to rowers to sleds. Some coaches still use barbells for HIIT (a la snatches for time), but I believe this is unwise in the over-50 population. Or rather, just unwise. Period.

As discussed at length in The Barbell Prescription and in this recent video, I believe the sled is ideal for interval training. The movement is profoundly simple and skill-independent, and if the athlete pushes the sled with an upright posture and a heel-toe step, she can garner all the benefits of HIIT without significant risk of injury, in particular the calf and Achilles tendon strains and ruptures seen with a lower, heels-up technique.

Conditioning with sled pushes.

Figure: High vs Low prowler positions. Although more taxing, the low position places more strain on the calves and Achilles tendon, and entails a certain risk of injury to these structures. For Masters over 40, we increasingly recommend the high position, which will still produce an agreeably productive level of cardiorespiratory and metabolic distress. From our book The Barbell Prescription, Aasgaard Press 2016.

So HIIT has been our go-to conditioning modality for the Athlete of Aging who does not engage in a high-power sport or physical discipline outside the weight room. It meets our exercise prescription and selection criteria for conditioning: It’s safe, it has a wide therapeutic window, it’s comprehensive (particularly with regard to its effect on energy systems), it’s specific and effective.

And it’s the ne plus ultra of simplicity and efficiency, yielding the same benefits as distance running or steady-state aerobics in a fraction of the time required to go plodding around the track a few times a week.

But how has the HIIT revolution stood up over time? I’ve recently gone back to this literature, which is too voluminous to either survey or summarize in depth, but here’s a sampling:

On the (slightly) negative side, a 2015 paper published by Lanzi et al in the journal Obesity documented a very short (2 week) study of 19 men with class II or class III obesity assigned to either oxidative (“Fat max”) or HIIT training. Sessions were matched for total mechanical work. Aerobic fitness and fat oxidation rates were measured before and after eight conditioning sessions. Both groups improved fat oxidation rates and aerobic fitness, but only the “aerobic” group decreased their insulin resistance.

This latter finding is unique and in stark contrast to other results, like those of Shoba et al (2015), Alvarez et al (2017), the fairly well-conducted 2015 meta-analysis of Jellyman et al, or the similarly small and short 2017 study by de Souza et al of the impact of HIIT on insulin resistance associated by sleep deprivation, all of which show a positive effect of HIIT on insulin sensitivity. In any event, the Lanzi paper, while you might find it cited as a contrary data point, is too small and too short a study to overwhelm the data indicating an effect of HIIT on insulin resistance.

Another contrary data point you might see is Roy et al, 2018, in the journal Medicine and Science in Sports and Exercise (the most unnecessarily verbose journal title ever) in which an intention-to-treat analysis found no benefits of HIIT…except in those who actually did it. Subjects who actually complied with the protocol demonstrated improvements in weight and visceral fat reduction. The take-home message from this paper seems clear. A meta-analysis published Su et al in 2019 found that both HIIT and moderate-intensity continuous (“aerobic”) training resulted in weight loss and improved BMI, body fat %, and VO2max (a measure of aerobic capacity and cardiovascular fitness), even though HIIT took up significantly less time.

A 2018 study by Grace et al in Experimental Gerontology demonstrated improved metabolic and cardiovascular parameters after low-volume HIIT (once every five days) in elderly men without evidence of cardiac strain or pathologic remodeling, commending this approach as a form of “preconditioning” prior to the institution of a more aggressive program.

In a 2015 systematic review and meta-analysis, Milanovic et al found that while endurance training and HIIT both improved VO2max in young to middle-aged adults, HIIT actually led to larger improvements. On the basis of this cursory and entirely non-systematic review, it appears that HIIT is alive and well. The early observations of improvements in fat mass and oxidation, cardiovascular function, insulin sensitivity, and aerobic-endurance adaptations continue to hold up, even as the literature documents a wide range of effective protocols for the utilization of this powerful modality.

Strength is central for the athlete of aging, but there needs to be some conditioning and “cardio.” If you’re not engaged in a high-power sport outside the weight room, you need to push the prowler or take a ride on the Bike to Nowhere. Do your strength training first, but when you rack that final set of presses or put down that last deadlift, you’re not necessarily done. It’s time to HIIT it.


Jonathon Sullivan MD, PhD, SSC, PBC is a retired emergency physician and research physiologist, and the owner and head coach of the Greysteel Strength and Conditioning Clinic in Farmington Hills, Michigan, which specializes in training adults over 50. He is the author of The Barbell Prescription: Strength Training for Life After Forty, with Coach Andy Baker.

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