If you read training literature or talk with coaches and athletes, the topic of interval training often comes up. Interval training, the alternation of work and recovery phases within a workout, is a proven way to improve your performance. But not all interval sessions are created equal. The athlete has a number of variables that can be manipulated to give a completely different training effect. To reach a desired effect, these specific variables must be planned with a given purpose in mind. The athlete can control the intensity (how fast they run), the length of the interval (time or distance), the terrain, the amount of recovery, the type of recovery and the number of repetitions in any given interval session. The goal of this article is to discuss types of intervals, the length of the repeats, and how long the recovery phases should be done for each. We'll start with short and fast intervals, then move into more aerobic intervals.

Pure Speed: Anaerobic Alactic intervals: These intervals are characterized by being very short and extremely fast with lengthy recovery phases. The term alactic is referring to the fact that these intervals are so short that there is little to no accumulation of lactic acid during the interval. These intervals can be divided into two different components, alactic power and alactic capacity. When an athlete is training alactic power they are training to increase the top speed reached, where as training alactic capacity is training the athlete to maintain the top end speed for a longer length of time. Who needs to train this system? Any sport where pure speed is needed: football, soccer, sprint races, longer running races with a sprint finish or short hills, or running and cycling races where an athlete is having to open a gap on competitors.

Alactic power intervals are very short: only 2- 10 seconds in length, with a lot of recovery, including a work to rest ratio of at least 1:10, up to 1:20. Recovery needs to be almost inactive, just light movement to keep the body moving. This allows for the replenishment of ATP-CP, the sprinting fuel of the body and also partial recovery at a neuromuscular level. Alactic capacity intervals are a little longer: 7-15 seconds in length, with a shorter work to rest ratio of 1:5-1:10.

For all pure speed intervals, the athlete needs to be going as fast as possible. How much is too much? A good rule of thumb is when an athlete is taking 10% longer to cover the same distance, they are fatigued and need to call it a day.

Prolonged High Speed: Anaerobic Lactic intervals are longer and have different goals, work, and recovery periods. Like alactic intervals, there is also a power and a capacity component. The term lactic in these intervals means that the intensity and duration results in a high production of lactic acid during the interval. Lactic power refers to the speed reached or distance that can be covered in a set time, whereas lactic capacity refers to the ability to maintain that sub-maximal speed for a longer period of time. The anaerobic lactic system is used to a large extent by athletes in events of 45 seconds to 2 minutes, such as 400-800 meter runners, or in events that require longer bursts like surging up a hill in a road race, pulling away from a pack in running or cycling, or the finishing kick in a track race.

Anaerobic lactic power intervals are 10-40 seconds in length. This is much longer than the pure speed intervals to allow the generation of high amounts of lactic acid and other anaerobic byproducts. The athlete is working at nearly maximal effort for the length of each interval. An athlete will have a higher absolute speed in a 15 second interval than a 40 second one, but regardless of length, the interval is near maximum effort for the majority of the interval. The work to rest ratio is about 1:10, but should include a more active rest period than when working on pure speed. The athlete needs to jog or spin for the recovery phase, which will not allow the sprinting fuel (ATP-CP) to be replenished.

To work on anaerobic lactic capacity, the ability to hold a just below sprint speed longer, the intervals get longer (30-90 seconds) and the work to rest ratio drops to 1:5 and the athlete "paces the intervals", so that they can just about maintain the pace for the entire workout. Athletes will start with 3 or 4 intervals and eventually build up to 12 to 15 intervals, often broken into sets. As the athlete improves, the intervals are run faster or more intervals are run, but the work to rest ratio stays the same.

Lactic Intervals: The goal of the lactic interval is to create a lot of lactic acid and at the same time reach maximal aerobic stress through intense efforts with short rest. These intervals are critical for sports where there are very intense efforts requiring a large aerobic capacity which also create a lot of lactic acid on a regular basis. Sports such as alpine skiing, criterium cycling, cross-country running, soccer and basketball are all examples of sports that can benefit from training this system.

Intervals should be 20-60 seconds in length with the work to rest ratio being 1 to 1. Intervals are done as intensely as possible, with very little pacing, keeping in mind the goal is to create as much acidic build as possible and reach maximal aerobic contribution. You are training the muscles to function with high acidity levels, while also training the body to use lactate more effectively. Yes... use lactate, as lactate is a useful fuel, not just an end "waste- product" as some incorrectly believe. The total work time is usually in the 6-15 minute range, with a sample workout being: 2-4 sets of 6 intervals with each interval being 30 seconds long. The break between intervals is 30 seconds of light activity. Between sets the athlete needs to stay active for 4-6 minutes so that they are warm and ready to give 100% on the next set. These intervals are very intense. A good warm up and cool down is critical.

VO2 Max Intervals: These are a little longer than lactic intervals. Interval length is in the 2-4 minute range, with athletes completing 3-6 intervals per workout. Intervals are paced, but very fast. By halfway through the workout, the athlete needs to be struggling to cover the same distance in the time period. The workout is finished when an athlete is no longer able to continue or is showing a 5-10% drop in the distance covered in the interval time. These workouts are very intense, often rating 8-10 out of 10 on the effort scale. Recovery time should be equal in length to the interval, up to 1.5 times the length of the interval.

Lactate Threshold/Race Pace Intervals: For longer races (10K to marathon), the race pace is often going to fall near one's lactate threshold. Sustained workouts just below this point are often referred to as tempo efforts. These intervals are critical in several ways. They develop a good feel for race pacing and they are used to prepare the body to biomechanically and physiologically become more efficient at race pace.

The athlete works for 3-15 minutes at race pace then actively recovers for half to an equal length of time. There are 3 variables to sequentially increase workout efforts: The number of intervals can increase up to or just above race distance, the pace can increase to just faster than race pace, and the interval recovery phase can drop significantly to intensify the workout.

For example: for a 10K runner trying to break 40 minutes (6:13 per mile) but currently with a PR of 42 minutes, this type of workout may start with 4x1 mile at 6:25 pace with a 1:1 work to recovery ratio. Gradually, over weeks or seasons, the runner should bring the speed up and the interval length up to where they are running 6-7 by 1 mile at 6:00 to 6:10 pace, with 2-3 minute of active recovery between repeats. A workout is done when the athlete can no longer maintain race pace for an interval length.

Tempo Intervals: As mentioned above, tempo intervals are performed just below the LT point - closer to half marathon race pace for highly trained athletes and closer to 10K race pace for less well-trained runners. In performing these intervals, you should start at a heart rate or pace indicative of overdistance (aka long slow distance) pace and build speed until the heart rate is 5 beats below LT or the pace is 5-10 seconds/mile below LT. The goal is to sustain the higher HR and/or pace for 10 to 30 minutes. If performed as an interval, the recovery phase is "active" but really easy, with the goal of decreasing the HR as quickly as possible. Once the HR reaches the OD level the athlete begins the next interval. Interval length can vary between 5 and 20 minutes, and the overall workout length can range from 30-60 minutes in length. It should be noted that it may take several minutes until the athlete is getting close to the upper HR range during these longer intervals. This is important to realize, otherwise the athlete may start too hard and making the workout more "anaerobic in nature".

The benefit of this workout is that it allows the athlete to stay mostly in the "aerobic zone", but it's more taxing. It trains the athlete to recover after moderate efforts, and allows the athlete to train at closer to race speeds without as much fatigue as other intervals.

As with all training, interval sessions need to be planned well in advance. While it is a proven method of improving performance, it is also one of the most abused forms of training. It is very easy for an athlete or coach to overdo intervals with disastrous effects: injuries, overtraining and physical and mental burnout. Intervals should be planned for 1- 2 sessions per week for portions of the training year. Always plan on the side of caution, as intervals are strong medicine.

Author Lester Pardoe BA, CPCA is the Coaching Specialist at the Boulder Center for Sports Medicine and a former National Team and Olympic Coach in Speed Skating. Lester is also a USA Triathlon certified coach and helps many local endurance athletes involved in mountain biking, road cycling, triathlon and running.