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Kickstart your Heart: Interval Training for Improved Endurance Performance

When developing a training plan for ultramarathons, beginners often scoff at the idea of doing interval training–relegating its usefulness to the likes of Olympic sprinters and triathletes. This is a rookie mistake, however. Intervals and speed are crucial elements of your training system for events of any distance.

Today we’ll take a look at why this is true and how you can incorporate intervals (specifically, hill intervals) into your training.

Joe ran a 4:16 mile in January to win his heat at an indoor mile at Boston University before focusing on his summer ultra season.

What is Interval Training?

An interval is performed by alternating between a high and low/no effort in order to accumulate time or distance at a particular aerobic intensity. By briefly overloading your cardiovascular and neurological system, you prompt an adaptation to the increased stress so that you can improve the time and efficiency with which you can sustain an increased pace, thus improving your overall fitness.

Intervals can take many forms as they target different physiological attributes. For example, there are intervals that target your VO2max, lactate threshold, and endurance systems which can be implemented across all phases of your training. The difference between these intervals is the intensity at which they are performed as well as the duration. We covered these zones and their associated intensity levels in a previous article, so check that out if a refresher is needed. That being said, the most easily understood, useful, and adaptable way to incorporate intervals into any training plan is through efforts aimed at increasing VO2max and lactate threshold.

Sidebar: VO2max vs Lactate Threshold

VO2max and lactate threshold are related yet different physiological structures. When your body breaks down glucose for energy use, pyruvate is created as part of the metabolism and is recycled back into usable energy. As the intensity of your workout and demand for energy increases, so does the production of pyruvate. At a certain point, your body will be producing more pyruvate than it is able to process. When not recycled, the pyruvate converts into lactic acid. The lactic acid will begin to accumulate in the bloodstream, eventually reaching a critical limit that inhibits athletic performance significantly. The intensity (pace) at which this happens is known as your lactate threshold, and training to increase the pace at which you reach this point is one of the most advantageous practices you can do as an endurance athlete.

While lactate threshold demarcates the balance point of your body’s demand for energy and ability to produce it, VO2max is the maximum volume of oxygen that the body can consume. As measured in milliliters per kilogram of body weight per minute (ml/kg/min), it’s your maximum aerobic capacity – your all-out, unsustainable, crash-and-burn pace. For elite endurance athletes, this tends to be around 77 ml/kg/min for men and 63 ml/kg/min for women. While some athletes are born with a naturally higher VO2max, it is a trainable element just like training your gut to handle a variety and large quantity of food during a long event or your muscles to carry you over mountainous terrain for an extended period.

Raising your VO2max allows you to process more oxygen and produce more energy, therefore improving your lactate threshold. With an increase in your lactate threshold, you are able to sustain higher intensities of aerobic exercise for longer periods, which is the definition of improved athletic performance in endurance sports.

Why Cardiovascular Fitness Matters – Even for Ultramarathons

Running the Kepler Track in New Zealand, where cardio fitness was key!

Endurance athletes tend to get caught in a rut of bashing themselves over the head with volume in order to prepare for ultramarathons. I did exactly the same when I first started training, often slogging out more than 70 miles per week in preparation for a 50k race and then wondering why I didn’t seem to be getting any faster and was still trashed by the end. Even though you may spend most of the race at low intensity levels, you can’t expect to improve your timing or fitness if you haven’t trained for it.

By advancing your lactate threshold and VO2max, you improve your running economy and efficiency so that you can finish faster and spend less time on the course being exposed to the elements, carrying nutrition and hydration, and chasing cutoffs.

As endurance coach Jason Koop notes in his book, Training Essentials for Ultrarunning,  “Being more fit isn’t just about going faster or being more competitive. Fitness enables you to run more comfortably, with more control, and with better technique. It not only gives you the ability to get yourself out of trouble if things go wrong but also keeps you out of a lot of trouble in the first place”.

Quality over Quantity

Another reason why intervals work well for ultramarathon training is because they can produce greater cardiovascular progress in less time than regular running. Since there are only so many hours in a week (and for most of us training is often not the top priority), it makes sense to focus the time you have for training on workouts that provide the most benefit to your goals. If using a periodized training schedule as we do, you will have training blocks devoted to specific adaptations such as increasing VO2max, improving lactate threshold, and maximizing endurance.

In most cases, intervals that are designed to improve VO2max will be prescribed furthest from an athlete’s event as they are the least applicable to long-distance efforts. However, they are still a crucial element because improving fitness early on in the training program raises your cardiovascular ceiling and paves the way for further improvements throughout the cycle.

Specific vs. General Fitness

A common coaching dilemma is developing a training plan that either improves an athlete’s specific or general fitness. Would I rather be in peak shape for my upcoming summer 50k, or am I willing to sacrifice some short-term performance gains in exchange for better times in 1, 2 or 5 years?

Many athletes focus on training specificity, and for a good reason. Long, slow easy runs on the trail often emulate the race you are training for. However, if you are racing two or three races in a year, you are probably in a constant cycle of training or recovering for your next race. There isn’t sufficient variety in training that builds your engine for the long term.

Runners like Sage Canaday follow the mantra ‘any surface, any distance.’ Canaday is a 2:16 marathoner but has also won a bunch of competitive ultras like Black Canyon 100k, Lake Sonoma 50m, and The North Face San Fran 50m. Variety in training is the spice of life. I always try to include training that improves my overall fitness (as an ultra runner, that means speed work) 2-4 months out of the year.

Power and Form

This article would be incomplete if we didn’t mention improvements in muscular power and form.

Muscular power, the ability for your muscles to move quickly, is developed. High-intensity training taxes our anaerobic system and increases our ability to buffer against lactate. As the power of each muscle increases, a runner needs to recruit fewer muscle fibers to maintain a certain pace. Essentially, muscles become more resilient to fatigue as you will have a larger reserve of muscle fibers.

Every runner wants to improve their form. Efficiency increases performance and reduces injury. Rather than spending hundreds of dollars on a gait analysis or the right orthotics, speed is our favorite shortcut to efficiency. The problem with consciously altering your gait is that running is a natural subconscious effort. Running requires a lot of coordination. Through decades of being human, we have all developed our own unique way to run. Altering this movement pattern introduces extra stresses that often lead to injury.

Rather, use speed work to get rid of inefficiencies. As a runner hits a higher speed, the foot strikes will naturally fall in a more aggressive spot, the the mid or forefoot, knee and arm drive is more fluid and concise and running cadence (steps per minute) usually increases to 180. This form of training will improve the pace of your easy and long runs. Speedwork, along with running drills and strides, is the best way to improve form for plodding ultra runners.

Hill Intervals

Intervals are essential, but they’re not easy!

Intervals can be performed in a number of ways including on a track, as its own standalone workout, or during a longer endurance effort. However, one of the most advantageous ways to do them is uphill. While this may seem like adding unnecessary torture to an already uncomfortable experience, there are several benefits to performing intervals uphill:

  • It’s harder. Intervals are often prescribed to increase VO2max and lactate threshold. Thus, the intensity must be high (RPE 9 or 10). The fastest and most efficient way to get there is by increasing the slope of your surface. This exponentially raises the required cardiovascular output and elicits the necessary adaptations for increased performance.
  • Specificity. Most ultramarathons and long endurance events in the mountains include elevation change (sometimes significant). Appropriate training is essential to body is prepared for the ups-and-downs. By doing intervals uphill, you are not only training your cardiovascular system but also your climbing skills. You will generally spend much more time going up than coming down during an event, so it pays high dividends to increase your efficiency on the climbs.
  • Less stress on joints and tendons. By going uphill, your target RPE can be achieved at a slower pace which means there is less turnover of your legs and lower impact on joints and tendons. Although your cardiovascular system will be taxed as much as if you were running around a track, you will decrease the potential for strains, sprains, and other impact injuries that are more likely at faster and less controlled paces.

Sample Interval Workouts

7-20 minutes is the ideal amount of time spent at high intensity for a speed workout. If your RPE is below 9, then you are doing it wrong. The workout needs to be at an incredibly high intensity to fully tax your anaerobic system. A 1:1 ratio of work:rest is ideal for speed sessions. You want your body to recover enough to be able to complete the next rep without letting your heart rate settle.

Speed Hills: 12 x 1 min hard with a 1:15 rest at a moderate incline, 5-10%.

A great hill workout that is over quickly. Hills are deceptive; often, runners will feel gassed early, only to realize they have a lot left in the tank on the last 2-3 reps. Focus on knee drive and efficiency in the latter half of the effort.

Power Hills: 20 x 10 sec hard with 30 seconds rest at a steep incline, 15%+.

This is a unique workout Joe uses sparingly, but worth noting. This workout is designed to build muscular power rather than VO2max. Note the short amount of overall high-intensity time. Treat each 10-second interval like you are an NFL lineman trying to tackle a quarterback. Leave nothing out there!

Power Track Intervals: 2 x 6 x 200m hard with 45-second rest between reps/2 min rest between sets.

Designed to be run on a track, 200m is halfway around the loop. For a more experienced runner, I may increase the number of reps and decrease the rest. However, there is a lot to be said to have high quality, high-intensity intervals. 200’s are all about finding efficiency and acceleration in the first 5 seconds.


Speed Track Intervals: 3 x 4 x 400m hard with 90-second rest between reps/2 minute rest between sets.

This workout requires a certain amount of fitness, great at the middle or end of a training block. Joe will set the 400m target times slightly above an athlete’s mile PR (or estimate). The last set will be quite challenging.

Word of Caution

Due to the high-intensity nature of speed sessions, the likelihood of injury will increase. Don’t tweak that hammy! A proper warm-up, including form drills and strides, and cool down are essential for speed work. Also, anticipate muscle soreness for the following day or days, especially if you are new to interval work. Don’t over-prescribe speed work into your training plan at the beginning.

Final Thoughts

When it comes to the efficiency and effectiveness of endurance training, intervals are one of the most important components of a quality regimen. Intervals improve your VO2max and lactate threshold. You can achieve significant performance results not only in speed but also in running economy. That efficiency makes the entire experience of training and racing better.

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Header Photo by Jeremy Lapak on Unsplash.

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