Interval training: extracting the maximum for performance

Interval training is now a staple in the training programs of just about all athletes. With an increasing awareness of the positive impact of interval training on performance, there has been more and more interest on what type of interval training is ‘best’. Of course, understanding which type of interval training is best requires an understanding of two key facts: firstly, an athlete’s goal must be identified, and secondly, the specific physiological impact of different types of interval training must be understood.  Only then can we decide which approach is best.

Goals of interval training

Clarifying an athlete’s goals is typically straightforward, and typically involves enabling athletes to perform faster, longer or both. Identifying the specific physiological impact of different interval training schemes has proven to be less straightforward. Not all interval training is same; for example, it’s intuitive that performing 6 x repetitions of 30 seconds maximal effort with 30 seconds rest is different than performing 8 x repetitions of four minutes with 60 seconds rest. Any athlete who has performed interval training can attest to the different impacts of different interval sessions, and how they can influence performance.

However, while an intuitive understanding of the differences between interval training sessions can be helpful, the more the impact of these training sessions is understood physiologically, the more effective coaches and athletes can be in selecting the most effective training session for the goals at hand. Simple strategies like measuring heart rate and lactate values were initially used to describe these differences, and these measures have been immensely helpful in creating an understanding of how physiology is stressed during interval training.

However, these measurements don’t tell the full tale. Performance in middle distance events requires extremely high rates of energy production, which must be sustained for an extended period of time. These races put maximal pressure on the body’s ability to use oxygen to create energy. The ability to use oxygen to create energy is limited by two factors, the ability to delivery oxygen via the blood, and the ability to muscles to extract oxygen from the blood.

Oxygen use during intervals

To help coaches and athletes maximize the impact of oxygen usage, researchers have typically focused on what they can measure, heart rate and the rate of oxygen usage. It makes sense that large amounts of work at high heart rates and high rates of oxygen usage help athletes develop the ability to use lots of oxygen, and that has indeed proven to be true. However, there’s more to the story. Those approaches emphasize the ability to deliver oxygen. But especially in middle distance events, there’s accumulating evidence that the ability to extract oxygen from the blood is also critical. To determine this, a new measure called ‘muscle deoxygenation’ is being increasingly used by scientists. Muscle deoxygenation measures how much oxygen is extracted from the blood; if more can be extracted, more can be used for creating energy.

Recently, more research has begun to investigate whether different types of training sessions can put more stress on muscle deoxygenation, and if so, which are most effective. Just as spending more time at high heart rates can help train the cardiovascular system to deliver more oxygen, spending more time with greater muscle deoxygenation values can help train an athlete’s ability to extract the delivered oxygen. A key step in improving the effectiveness of interval sessions therefore is to identify which kind(s) of training sessions are effective at producing longer periods of muscle deoxygenation.

New research

To help provides answers, a new study has compared the physiological responses during two different interval training sessions(1). In particular, the researchers were interested in differences in muscle deoxygenation dynamics between the two sessions. If differences did emerge, they wanted to understand if these differences were related to other physiological changes, or if muscle deoxygenation dynamics represented a distinct physiological response.

Thirteen distance middle-distance runners participated in the study. The average best performance time over 1500m for the males was 4mins:00.62secs and the average time for the women was 4mins:53.25secs. All runners had been training consistently for at least two years. The subjects performed two separate interval training sessions within a 3-week window to minimize any changes in fitness or health.

The interval training sessions were as follows:

·        15 x 400-meter repetitions

·        6 x 1,000-meter repetitions

The total volume equaled 6,000 meters in both cases, and the subjects received one minute rest between each repetition during both sessions. The subjects were asked to run each interval at the fastest sustainable speed. In other words, the runners tried to complete session as fast as possible while maintaining performance.

To quantify the physiological response to each session, several measurements were taken. Heart was measured continuously via a heart rate monitor. From this data the highest heart rate during each interval was recorded, as well as the highest heart rate during each recovery period. The time spent above 90% of maximal heart was also calculated from the data to understand how much work the subjects were performing at very high intensities.

Like heart rate, muscle deoxygenation responses were monitored continuously using technology known as wireless near-infrared spectroscopy (NIRS) monitoring. This is an established technology for measuring muscle deoxygenation. A sensor is placed on a given muscle group and oxygen dynamics can be measured through the skin. It’s important to note that the researchers chose to measure the medial gastrocnemius (calf) muscle. This muscle was chosen due to ease of measurement and its importance in running. (Note that NIRS is a local measurement, and data from one muscle cannot be extrapolated to another.)

As well as heart rate and muscle deoxygenation, blood lactate and rate of perceived rate of exertion were also monitored. As these measures can’t be monitored continuously, and some effort is required to take a measurement, they were measured at different points throughout each session. Measurements were taken after every five repetitions in the 400m session and after every two repetitions in the 1000m session, for a total of three measurements per session.

The findings

The results were interesting to say the least. From a pure performance standpoint, the 400m repetitions were run significantly faster than 1,000m, which is not a surprising outcome. Interestingly however, the 400m repetitions became faster over the course of the session whereas there was no change in the speed of the 1,000m repetitions. 

Across both conditions, rating of perceived exertion climbed steadily, as did lactate concentrations. However, there were no significant differences in the values between either training session, nor were there any differences in the degree to which these physiological measures changed. Although there were significant differences in speed, there were no significant differences in overall stress as indicated by rating of perceived exertion or lactate values.

Key differences

When it came to the heart rate responses, differences between the sessions begin to emerge. While there was no difference in peak heart rate between the two sessions, the accumulated time spent at greater than 90% of peak heart rate was significantly longer in the 1,000m session compared to the 400m session. In fact, every runner spent more time over 90% of peak heart rate during their 1,000m reps intervals session compared to the 400m rep session.

Where it gets really interesting however is that the muscle deoxygenation responses were reversed. The magnitude of muscle deoxygenation was significantly greater with the shorter intervals, and the time spent with greater than 60% of muscle deoxygenation was also larger with the shorter intervals (see figure 1). While the heart may have been working harder for longer in the longer intervals, oxygen was being depleted to a great degree at the muscular level with the shorter intervals.

Figure 1: Muscle deoxygenation in 1,000m vs. 400m intervals