Interval training has been twisted and manipulated into so many different iterations that people have lost sight of what exactly it is. It’s been a major point of training discussions for decades—for good reason. Interval training is an extremely powerful stimulus that can elicit adaptations to endurance, strength, and power.
- Endurance training is defined as an improvement in aerobic energy production and fatigue resistance
- Strength training is defined as muscle hypertrophy and improvements in force production
- Power is defined as the ability to express maximal force as rapidly as possible
(Before I get angry comments saying, “Hey dummy, interval training isn’t effective at improving strength and power compared to traditional strength training”…yeah, I think we’re all aware of that.)
There are two types of interval training:
- Supramaximal or Sprint Interval Training (SIT)
- SIT results in supramaximal at or near 100% of a person’s power output and >150% levels VO2 max intensity
- High-Intensity Interval Training (HIT)
- HIT results in submaximal <100% power outputs and elicits ³ 80% of heart rate max
That’s it. Everything else is window dressing. Tabatas, Fartlek, and 15 on 15 off are just changes to the work-to-rest ratio. Well programmed work-to-rest ratios are essential for implementing these training types, and I’ll talk more about that later in this post. However, having exact, to the second work-to-rest ratios or using heart rate monitors to ensure an athlete is “recovered” when developing capacity is unnecessary.Using heart rate monitors to ensure work-to-rest ratios are correct is overrated in capacity training but extremely underrated in power training. Click To Tweet
Which brings me to one of my most hotly contested opinions: The use of heart rate monitors to ensure work-to-rest ratios are correct is overrated in capacity training, but extremely underrated in power training.
Capacity vs. Power
If developing capacity is the goal, programming work-to-rest ratios down to the second is a tad excessive. (Capacity is the ability to maintain a specified power output over a set time period. Rest and recovery are not the main focus.) Developing the capacity of a specific energy system requires challenging that system. And just like any other type of training, key variables to consider are volume, intensity, duration, and rest.
- Intensity is critical to understand what energy system you’re trying to develop and how that intensity relates to the system and adaptation. Maximal power output is 100% while maximal fat oxidation is ~50% of maximal power output.
Variations will affect the athletes’ adaptations. As an extremely general rule of thumb, the higher the intensity of work, the shorter the duration the work will be, and the higher the work-to-rest ratio is.
We’ve come to the point, however, that monitoring heart rates when conditioning athletes is considered essential during training. I’ve found, in almost all sport situations excluding track and field, that it is not essential. Typically, heart rate monitoring during training is used to determine if an athlete is rested enough to commence the next repetition. I find this asinine. In no sport scenario will an athlete get to look and see if they’ve recovered enough to perform during their sport.
“Sorry coach I can’t check back in yet, my heart rate is still above the blue zone.”There is no sport scenario where an athlete gets to look and see if they're recovered enough to perform during their sport, says @RealBMike. Click To Tweet
Work-to-rest ratios, however, are extremely underrated if we’re considering improving an athlete’s power (power is defined as performing ³100% power output for a specific activity). Having athletes perform maximal intensity sprints with little rest is a quick way to change your training stimulus from power development to capacity development. If you continue not allowing full rest sets during “maximal” intensity training, you will not improve max power and may cause injuries. This is where heart rate monitoring can be handy. When an athlete has returned to a set resting heart rate, it’s safe to assume their previous max and average power output will be as close to 100% as possible. If you want to train to improve your max power output, you need to reach maximal power during the repetition.
- Sprinters don’t improve their max power output from submaximal intensities.
Interval Training and Corresponding Adaptations
Looking through peer-reviewed literature, you can find a plethora of general population studies touting how SIT and HIT can result in massive improvements in both aerobic and anaerobic performance markers.
- Mitochondrial density (the number of mitochondria in tissues that utilize or produce energy)
- VO2 max (VO2 = a-vO2 difference, O2 levels in arteries vs. O2 levels in veins x Cardiac Output- Heart rate x Stroke volume)
- Lactate clearing/utilization (lactate is the most preferred energy substrate in the body); it’s now considered the upstream signal that stimulates nearly every energy system adaptation (Brooks, 2020)1
- Power output
Almost all of these studies have a control group that performs moderate-intensity continuous training (MICT). In nearly every study, SIT or HIT groups are better than or as effective at improving aerobic adaptation markers (mitochondrial density and VO2 max), and SIT and HIT clearly outperform MICT in improving anaerobic markers (lactate clearing/utilization, max power output, and strength). All while SIT or HIT groups typically perform less volume and spend far less time training compared to MICT groups. Similar aerobic adaptations, better anaerobic adaptations, increase power output, and less volume?
Side note: Recent research in muscle hypertrophy training intensity is eerily similar to this aerobic and anaerobic intensity topic. You can increase muscle hypertrophy from lifting low intensity and high volume (LI-HV). However, if you lift at a higher intensity and lower volumes (HI-LV), you get much stronger and will have similar hypertrophic increases compared to LI-HV.
- Strongmen competitors don’t improve their max strength by lifting light weights.
The Million-Dollar Question
“Why are we not implementing SIT or HIT every conditioning session?”
There are a lot of answers, but here are three that come to mind:
- SIT and HIT freaking suck. If you haven’t done repeated Wingate tests, stop reading this and give them a shot.
- There’s also a point of diminishing returns that’s illustrated in the literature. More SIT or HIT doesn’t = more gains in aerobic or anaerobic qualities.
- We need more research with field and court sport athletes on how SIT and HIT affect recovery, especially in-season when high sport performance levels are the goal.
With these answers in mind, the next logical question would be: What are effective ways program HIT and SIT?
1. What time in the annual plan are you? Are you in the offseason/regeneration period? Or are you at the end of the competitive phase?
- Remember that HIT is a powerful stimulus. If in-game performance is the priority, program low volume with maximal amounts of rest. If you’re at the beginning of the offseason phase, don’t program excessive volumes and vary rest intervals to start. Progressive overload/ periodization is key.
2. What are the characteristics of your sport?
- Baseball players, for example, very rarely tap into any energy system other than ATP-PCr (during the game). However, having the recoverability to play a game nearly every day for 5 months is crucial. This means developing aerobic adaptations and power outputs are critical for success.
- Baseball players (position players, and relief pitchers specifically) usually train ~6 hours before their games (lift @ 12pm, practice @ 3pm, pregame @ 5:15pm, game @ 7pm). Using high volumes of HIT and SIT can harm pre-practice performance. So getting the minimum effective dose here is crucial.
- During the season, we’re able to use a high-low training model because starting pitchers in baseball know exactly when they are playing (see Image 2). The highest volume of training and the lowest level of intensity typically is the day after a game. This allows the athletes the most time to recover from training volumes, along with continuing recovery from the game the day before. The lowest volume and highest intensity (developing power, not capacity) are used the day before they start to assist in priming the CNS.
- Have standards you want your individual athletes to attain. If athletes are reaching the standard at the beginning of the season, hopefully they’re able to exceed (if not maintain) these standards throughout the year. If your athletes are not meeting standards, it may be a good time to de-load (if you don’t have planned de-loads in your program).
Properly periodizing interval training can help improve max power output, sustainability of max power, and recovery. This could be implemented in an in-season high-low training split with a high day taking advantage of the high intensity and low volume of SIT and implementing a low intensity and low volume training on low days. Mixing in HIT and SIT can be powerful stimulus for improved energy system development.Mixing in HIT and SIT can be powerful stimulus for improved energy system development, says @RealBMike. Click To Tweet
- Have a plan
- Know what adaptation(s) you are chasing
- Have a measure of intensity
- Ensure intensity, volume, duration, and rest match
- Be ready to answer, “Why?”
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1. Brooks, G. A. (2020). “Lactate as a fulcrum of metabolism.” Redox Biology, 101454.
2. Esfandiari, S., Sasson, Z., & Goodman, J. M. (2014). “Short-term high-intensity interval and continuous moderate-intensity training improve maximal aerobic power and diastolic filling during exercise.” European Journal of Applied Physiology, 114(2), 331-343.
3. Gibala, M. J., & Hawley, J. A. (2017). “Sprinting toward fitness.” Cell Metabolism, 25(5), 988-990.
4. Gillen, J. B., Martin, B. J., MacInnis, M. J., Skelly, L. E., Tarnopolsky, M. A., & Gibala, M. J. (2016). “Twelve weeks of sprint interval training improves indices of cardiometabolic health similar to traditional endurance training despite a five-fold lower exercise volume and time commitment.” PloS ONE, 11(4).
5. MacInnis, M. J., et al. (2017). “Superior mitochondrial adaptations in human skeletal muscle after interval compared to continuous single‐leg cycling matched for total work.” The Journal of Physiology, 595(9), 2955-2968.
6. San-Millán, I., & Brooks, G. A. (2018). “Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and less-fit individuals.” Sports Medicine, 48(2), 467-479.
7. Weston, K. S., Wisløff, U., & Coombes, J. S. (2014). High-intensity interval training in patients with lifestyle-induced cardiometabolic disease: a systematic review and meta-analysis.” British Journal of Sports Medicine, 48(16), 1227-1234.