Interval training: once, twice or thrice per week?

Distilled down to the very basics, ‘interval training’ is a method of training that intersperses intervals of high-intensity exercise, with short periods of rest. Why has this type of training become so popular among elite sportsmen and women? The answer is easy; done right, interval training allows you to get bigger fitness gains, faster than simply ploughing along endlessly at a steady intensity. The popularity of interval training is not based on hearsay but rather on a large body of empirical evidence showing that performing regular bouts of high-intensity interval training is an extremely effective training tool for athletes seeking to maximize performance for a relatively low training workload⁽¹⁾. Put simply, short sessions of high-intensity intervals are a great way of producing gains in aerobic power, enabling an athlete to sustain a higher intensity/pace/workload for longer before fatigue sets in.

The perfect interval recipe

What’s less clear however is how to put together an interval program to deliver the greatest performance gains per unit of time and effort invested. That’s hardly surprising as there are so many variables to consider. Overall, scientists have determined that there are six factors determining the overall ‘recipe’ for interval training. These are^(2,3):

-Total workload of a session
-Duration of each interval
-Interval intensity
-Recovery duration
-Recovery type (ie active or passive)
-Total interval training time per week

Depending on the purpose of the training session, these variables can be manipulated to give countless different combinations. Studies of well-trained endurance athletes indicate that the best training effect, i.e., increased stroke volume and oxygen delivery, is achieved at intensities between 85%–95% of maximum heart rate^(4,5), although studies have also demonstrated positive effects of training at even higher intensities⁽⁶⁾.

Interval session frequency

While there’s a lot of data about the fitness and performance benefits of different interval training regimes, one thing that’s far less clear is how best athletes can integrate their interval training into their weekly endurance training program. In particular, there’s still a lot of debate about the optimum number of interval training sessions per week needed in order to maximally stimulate training adaptations. Retrospective data collected from world-class endurance athletes shows that they typically perform one or two three interval sessions per week, often along with another type of high-intensity session (eg a short but maximal session)⁽⁷⁾. The problem of course is that most athletes who enjoy training and competing are not elite world-class athletes, but rather recreational, weekend warrior or keen amateurs competing at club level.

It’s known that altering the frequency of interval sessions can significantly influence the overall training outcomes – not just in terms of adaptation and performance improvements, but the degree of recovery that athletes can expect^(8,9). However, the data is very mixed; some studies have shown that interval training regimens performed once or twice weekly are not particularly effective for enhancing cardiorespiratory adaptation, specifically in terms of improving maximum oxygen uptake (VO2max)⁽¹⁰⁾. However, other research shows that two interval sessions per week is effective at improving cardiorespiratory fitness, and that adding a third session per week can further improve it⁽¹¹⁾.

Answering the question

The question remains therefore: how frequently should athletes, particularly non-elite athletes, perform interval sessions each week to get the best bang for buck? Too few sessions might not lead to significant gains. However, too many sessions may well risk burnout or injury. What’s needed is some data on recreational athletes undertaking different frequencies of interval sessions, and now a brand new study by a team of German scientists has provided a useful insight⁽¹²⁾. Published in the journal Physiological Reports, this study set out to investigate how different frequencies of interval sessions affect cardiorespiratory fitness such as maximum oxygen uptake (VO2 max), metabolic markers (eg blood lactate and glucose), and running performance (such as time-to-exhaustion and running economy) in recreational runners. The answer to this question is critical for both runners and coaches; interval sessions need to be prescribed in a way that maximizes gains while fitting into a training schedule, and which doesn’t lead to exhaustion of injury down the line!

What they did

The researchers investigated how performing intervals either once, twice, or three times per week affected the performance of recreational runners, and if so, what the differences were between the different training schedules. To do this, 26 recreational runners, aged 18–50 were recruited. All of the runners were already running at least three times per week with a total weekly mileage of around 10-40 miles per week. The participants were then randomized into three different training groups. These were as follows:

• Group 1: One regular run was replaced with one interval session per week. This session consisted of 4 x 4-minute efforts performed at around 90–95% of max heart rate (HRmax), with 3-minute active recovery periods at 60–70% HRmax in between each effort.
• Group 2: As above, but with two regular runs replaced by two interval sessions per week following the format as above.
• Group 3: As above, but with three regular runs replaced by three interval sessions per week following the format as above.

All three groups followed this intervention for a period of six weeks, keeping total weekly running volumes and other workouts consistent. The substitution of regular runs in a training schedule for interval sessions was used by the researchers because it mimics the approach that many athletes use in practice - swapping out an easier run for a high-intensity session in order to add an extra training stimulus - without overhauling the whole plan.

Before and after the six-week intervention (see figure 1), all the runners underwent testing, which included the following:

·        VO2max assessment using an incremental treadmill test (to measure maximal aerobic capacity - an important metric of endurance performance).

·        Time-to-exhaustion testing, where the runners ran at a fixed speed until they couldn’t maintain that speed anymore.

·        Running economy testing, which measured how efficiently the runners used oxygen to cover a given distance while running at a submaximal pace.

·        Metabolic markers, including blood lactate and glucose levels during exercise to assess metabolic stress and efficiency.

·        Body composition, including fat mass, lean mass, and body weight (measure with via bioelectrical impedance).

After the 6-week intervention the changes in the above parameters were compared for the three groups to see how the different frequencies of interval sessions had impacted fitness and performance.

Figure 1: Protocol of the 6-week intervention and before/after testing