Intervals – When, How Often, and Which Type? (#43)

Interval training is one of the most effective ways to boost fitness, but different types of intervals bring about different training benefits. So, the main question isn't just if you should do intervals, but which type aligns best with your goals and how the rest periods between intervals impact the results. The conclusion is that work time, rest time, and total training volume are all important, but you should choose your interval setup based on the specific attribute you wish to enhance. For most endurance athletes, regular interval training with relatively short rest periods is an effective way to improve oxygen uptake, endurance, and performance.

Rest intervals during workouts are crucial because they help optimize your training plan and minimize injury risk. As a runner, your weekly mileage is closely linked to injury occurrences (http://www.ncbi.nlm.nih.gov/pubmed/26203737). Studies indicate that runners logging higher weekly mileage often face a greater risk of overuse injuries compared to those with less intensive training.

We're not going to specify what rest is "best" since it varies based on your sport's goals, activity type, and the time of the season. However, generally speaking:

• Shorter rest enhances oxygen uptake capacity, lactate management, and glycogen burning, while fortifying your mental resilience — it boosts your "man, this is tough threshold".
• Longer rest allows you to sustain a higher pace per interval, crucial for new blood vessel formation, mitochondrial increment, and sport-specific pace training and feeling.

However, the choice isn't always stark. Longer rest can aid in recovering to elevate your heart rate during the next 4-minute interval, thus increasing your VO2max more effectively than shorter rest, where "lactic acid" build-up would prevent full heart stress. More on this below.

Let’s explore the interplay between interval and rest time. What sets apart a 1:1 ratio from a 1:5 one, where you rest five times as long as you train? Consider whether it’s the heart and oxygen uptake, the lactate threshold, or the sport-specific pace sensation that should be the focus of your training.

Shorter rest periods challenge your anaerobic/glycolytic system more intensely. This can improve your body's ability to manage high-intensity efforts and enhance performance at faster speeds.

Longer rest periods allow you to spend more time and volume at speeds exceeding race pace. You'll have greater opportunity to stress the anaerobic system since longer rest often enables you to sustain high to medium-high intensity for a longer duration. Another benefit is dedicating time to high-intensity training at supramaximal race pace, which builds resilience for higher speeds on race day. However, we generally don’t recommend extensive use of long rest periods. Maintain the ratio between training and rest at 1:1 or higher, like 2:1, 3:1, etc.—ensuring rest time is never longer than the work duration.

In an English study from 2015, researchers examined 32 long-distance runners in their 40s (14 males, 18 women) with a weekly volume of about 40 km. They had them run 6 sets of 10-second sprints with either 30, 80, or 120 seconds of rest between each sprint, and then analyzed the effects.

The researchers compared three different ratios of training to rest:

1:3
1:8
1:12

The results showed that only those who used the 1:3 ratio increased their performance in a 3 km time trial (+3.1%), meaning they were the only ones who increased their running speed. A 3 km distance is short enough to exceed your threshold, and this threshold rises when the ratio of work to rest is closer to 1:1 (think 4×4 min, 30/30, etc.), enabling participants to maintain a higher speed. The groups with 1:3 and 1:8 ratios also increased their time to exhaustion more than those who did 1:12 (+6.4%, +4.4%, +1.9%). So over short distances, those with a low ratio between training and rest come out ahead.

However, short rest doesn't lead in all disciplines. Those with longer rests of 80 s (1:8 ratio) and 120 s (1:12 ratio) developed more power during the sprints (4.6% and 5.3%) compared to the 30 s rest where the change was essentially negligible.

This is just one study, but many others indicate the same pattern – unsurprising, as shorter rests optimize the body's glycolytic system. You become more efficient at using lactate and glycogen as fuel and maintain proximity to the “ceiling,” thus pushing your threshold. Meanwhile, longer rests result in higher power output because it takes 3-5 minutes for your most explosive ATP system to recover. This might not be relevant for you as an endurance athlete, but it's useful to keep in mind when observing gym-goers who take long rest periods between heavy, explosive sets.

For this reason, we're not exploring long rest periods further here. The rule of thumb is: work time should always exceed rest time during more intense intervals and training sessions in endurance sports.

The Norwegians are known for their famous 4-minute intervals (http://www.ncbi.nlm.nih.gov/pubmed/17414804) where you train intensely at about 90% of your max heart rate for 4 minutes, take an active rest for 3 minutes, and repeat four times, ending with a calm cooldown. This interval duration appears to be highly effective for boosting your oxygen uptake. A 2013 meta-analysis revealed that most studies had interval times between 3–5 minutes, indicating that this timeframe offers the greatest VO2 max increase (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3774727/). The idea is that this duration allows us to mentally push ourselves to the max; if the time exceeds 5 minutes, the mind tends to give up before the body does.

The rest period between these intervals has also been thoroughly researched. In a study from 2005, 9 well-trained runners with a high VO2 max of 71 ml/kg/min were tested. They performed 6 sets of 4-minute intervals at 15 km/h with a 5-degree incline. In summary, the results showed that 1 minute of rest was too brief to achieve a good effect, and 4 minutes of rest provided no additional physiological benefit compared to 2 minutes of rest—except that the perceived exertion slightly decreased with 4 minutes of rest. When participants chose their rest time, the average selected rest time was 118 seconds, or about 2 minutes, so 2 minutes works well there too.

The work-to-rest ratio is often set to 4:3 in practice, but the study suggested 4:2 as effective. They did not test exactly 3 minutes of rest in that study, so we can't compare all studies perfectly.

In summary: this interval time is highly effective. Aim to work in 4–6 blocks with 4–5 minutes of interval time and 2–3 minutes of rest between each. The key is to be consistent and ensure you can improve your performance from week to week—even with small increments (0.1 km/h on the treadmill or 3 watts on the bike count).

VO2max intervals such as 4×4 or 10–16×1-minute should be incorporated approximately 1 month after the end of the competitive season and continue until 1–2 weeks before the season starts again. They can also be used occasionally during the competitive season if there's a 2–4 week gap between races to maintain high oxygen uptake capacity.

The Norwegians excel in this area, and an interesting and well-controlled study (published in 2016) randomized 63 well-trained cyclists into three groups, all of whom followed a 12-week program with two HIT sessions per week in addition to normal low-intensity training (8–10 hours/week).

The groups had different setups: increasing volume, decreasing volume, or mixed. The increasing volume group started with 4×4 twice a week for four weeks, continued with 4×8 for four weeks, and finished with 4×16 in the last four weeks. The decreasing group did the opposite. The mixed group varied sessions but did the same total amount of intervals.

The researchers believed that decreasing volume (from long volume to high intensity) would yield the best effect. But that wasn't the case. The results showed that it didn't matter much how the participants structured their training — all increased performance by 5–10% (VO2max, peak power, and a 40-minute power test). In practical terms, this means the total amount of HIT is more important than how you periodize it over 12 weeks.

If you're fatigued or mentally tired after work, you can do longer intervals at lower intensity that day and save the really tough 4-minute intervals for another day with a clear conscience.

Volume is incredibly important, but to ensure it doesn't become higher than necessary, you need to achieve the right results from your training based on your athletic goals. If you can reduce weekly volume by streamlining your training, it's a clear win-win — both your body and your family will thank you 😉

Practical advice:

• Plan for 1-2 intense, shorter sessions each week throughout the year.
• Gradually increase the load over time and don't be afraid to adjust the intensity based on how you feel that day.
• If you're feeling worn out, opt for longer intervals at a lower intensity and save the high-intensity intervals for another day during the week.
• Try to standardize your intervals as much as possible and record speed/incline or watts from session to session so you can track your progress.
• Train at race pace or faster during intervals, never slower. You're building a strong engine now, so let's push it a bit (http://www.ncbi.nlm.nih.gov/pubmed/10331896).

Listen to your body, measure progress, and be kind to yourself occasionally — after all, it's long-term development that counts, not who sweats the most on a Tuesday session.