Interval training was invented in the 1930s by the German coach Waldemar Gerschler together with his compatriot physiologist Hans Reindell. Their original interval training method consisted of running periods ranging from 30 to 70 seconds at a heart rate of 170 to 180 beats per minute. This was followed by recovery period that allowed the heart rate to decrease to 120 beats. At this point the athlete was ready to perform the next part of the session.
What happens during the recovery period is that the heart rate declines at a faster rate than the return of blood to the heart, resulting in a brief increase in stroke volume (the amount of blood the heart pumps with each beat). This increase in stroke volume places a greater workload on the heart thus making it stronger. Stroke volume therefore peaks during the recovery, and since there are numerous intervals during this type of workout, the stroke volume will peak many times. This provides a stimulus for improving maximum stroke volume and consequently the oxygen transport capacity.
Another phenomenon of the recovery intervals, is that an important part of the muscular stores of rapid energy—adenosine triphosphate (ATP) and creatine phosphate (CP)—that are depleted during the fast work period are replenished through the aerobic system. During each work period that follows a recovery interval, the replenished ATP and CP will again be available as an energy source within the muscles.
Gerschler and Reindell were particularly interested in the cardiovascular aspects of this and believed that the stimulus for cardiovascular improvement occurs during the recovery intervals between work periods rather than during the periods of activity, as the heart rate decreases from an elevated value. For this reason they placed an emphasis on the recovery period or interval as being the most important part of the session and nominated the protocol as an “interval workout” or “interval training.”
But there are other aspects of interval training which are just as important and one of the most interesting is the fact that through intervals you can actually do more intense work at a higher rate or speed than if you were to do a continuous hard workout. For example you could do 4km of running at say 4:00 per kilometre, or you could do 8 x 500m at 3:30” per kilometre (actually you could probably do 10 x 500m at 3:30” per kilometre so both faster and greater total distance). This has all sorts of physiological benefits including, but not limited to, heart rate, stroke volume, neuromuscular efficiency, muscle fibre utilisation, oxygen uptake etc.
Within the broad classification of interval training we can find many different typologies and each has a slightly different purpose, although these purposes or desired effects are often connected to each other - aerobic capacity, anaerobic stimulus, neuromuscular activation and development, muscle fiber development, power production, resistance to power production - and there is the possibility of acting on varying factors to work more on one aspect than another. We can vary the distance or time of the fast section, the recovery period in terms of time or distance compared to the fast section, utilize active or passive recovery, run on slopes (up / down), the intensity of the fast part, the intensity of the active recovery, complete or partial recovery.
The most sought after training benefits that can be obtained from interval training, in particular for trail or mountain running are the following:
Improved aerobic capacity and Vo2max:
It is true that most trail races, and especially the Ultras, are conducted almost exclusively in an aerobic regime and therefore it would seem logical to work only on this aspect. But we know that the more developed (high) our Vo2max is, the easier it will be to keep a certain heart rate even in an aerobic regime. This is because the biggest limitation of the Vo2max lies in the maximum volume that the heart can pump at each beat. Running fast, between 90 - 100% of the Vo2max followed by suitable recovery intervals creates a stimulus that increases this volume capacity. So to transport the same amount of blood to the muscles the heart can beat more slowly, because each beat carries more blood. Alternatively we can maintain the same heart rate but run faster.
Improved Neuromuscular Efficiency:
The better the connection between the brain and the various muscle areas is developed, the better our running and any race will be. And this does not only mean in terms of speed but also in terms of economy of movement at any given pace. To give an example, it should be clear that speed is a function of the number of steps per minute x step length. By improving the neuromuscular connection we can act both on the length of the stride, and on the number of steps we can take in a minute. In this context it is interesting to note that the number of steps also depends on the "Ground Contact Time" (GCT), i.e. how long the foot remains in contact with the ground, therefore a time when we are still, before the push. We are talking about values ranging from 160, for the elite, to 300 thousandths of a second for a beginner, but improving (reducing) the GCT even by very little multiplied by the number of steps in a marathon or any long race translates into a significant time difference. When we run faster we are obliged to run more efficiently than we are on a flat surface. Therefore running fast and powerful intervals can vastly improve our neuromuscular efficiency.
Increased Power Production:
There are those who go very strong uphill but suffer downhill, or go fast downhill but loose ground uphill, there are those who suffer both. Doing interval training on slopes can greatly assist us in improving our performance due to the adaptations in muscle strength production. In order to produce more power or force there must be adaptations in the fibers that constitute the muscles. As always GAS (General Adaptation Syndrome) is the process through which the muscles are stressed (eccentric or concentric forces depending on up or downhill), allowed to rest and recover, and then become stronger than previously.
Improved Muscle Fibre Recruitment:
As we know there are two types of muscle fibres. The so-called "Slow Twitch" (Type 1) and the "Fast Twitch" (Type 2). It is worthwhile mentioning that there are two types of "Fast Twitch" (Type 2 A & B). In actual fact some studies have found that there are up to 50 different types of fibres, but these are details that are not very useful in the present context. When we perform a movement with the muscles (let's say an aerobic run) not all the fibres are involved at the same time but only a part, those necessary for the required effort (harder the effort is, the more fibres are involved). However in time these fibres get tired and need to switch off while other fibres are recruited. The newly dormant fibres are able to rest ready to switch on again when necessary. This is a process that goes on until all the fibres have been totally exhausted - it's a feeling that, I think, we all know rather well. At that point (actually some good time before reaching this point) the fibres of Type 2 (Fast Twitch) are also recruited to lend a hand. But if these fibres have not been trained because we have not practiced fast running (through intervals or other fast training) they will be of little use for the purpose or, at the best, they will be of modest assistance.