Cyclists will incorporate a variety of strategies to assist them in achieving peak performance, but the biggest change in performance will come from their training. High-intensity interval training (HIIT) is an important part of an endurance athlete’s training programme. HIIT is characterised by bouts of high-intensity work separated by periods of low-intensity work or rest. The proposed benefit of HIIT is the ability to perform more work at a high-intensity compared to a one long sustained high-intensity effort (tempo session). Recreational level cyclists are often hesitant to include HIIT, due to the increased levels of physical discomfort, fatigue and high perception of effort. However, 2 – 4 weeks of HIIT can result in large improvements in performance compared to a similar amount of low-intensity training.
Physiological responses to HIIT
The cardiovascular and neuromuscular systems are where the majority of the adaptations to HIIT occur. Performing regular HIIT sessions as part of a well-structured training programme has been shown to improve performance measures, such as VO2max and peak power output, by 2 – 4 %. Some examples of changes in the cardiovascular system include; increases in the size of the left ventricle as well as increased muscle wall thickness, increased compliance of the left ventricle as well as increases in blood and plasma volume (increased oxygen carrying capacity). There is also an increase in the vasodilation of the blood vessels that deliver oxygen to the working muscles.
In addition to the adaptations to the cardiovascular system, it has been suggested that there are also adaptations to HIIT in the muscle cells. Increased recruitment of Type II muscle fibres during HIIT sessions. Type II fibres are recruited at intensities above 90 % of VO2max. Type II fibres are larger and more powerful, but less oxidative and more glycolytic than Type I fibres. Exercising at intensities above threshold causes Type II fibres to adapt and become more oxidative and more fatigue resistance.
Optimising interval training
The intensity of the HIIT training sessions is an important consideration when designing a programme. It is recommended that athletes try and accumulate as much time as possible at intensities close to their VO2max. Type II muscle fibres appear to be recruited at intensities above 90% of VO2max and the intensity of the session should produce a high cardiac output in order to promote the cardiovascular changes mentioned above.
The intensity (power output) associated with VO2max is determined during a progressive incremental exercise test (VO2max test). Once the intensity that elicits VO2max has been determined, it is important to know how long this intensity can be sustained (Tmax or time at VO2max). VO2max is typically reached after 60% of Tmax. In other words, if an athlete’s Tmax is 5 minutes, then they will reach their VO2max after 3 minutes. Tmax should be determined a day or two after the initial maximal test to ensure the athlete is fresh. The athlete should warm up and then try and sustain the target intensity as long as possible. Let’s use an example of a cyclist who reached his VO2max at 400 Watts. Two days later he cycled at 400 Watts for 5 minutes (Tmax). Therefore, the ideal interval duration for a 400 Watt session should be 3 minutes long with either equal (3 minutes) or double rest (6 minutes) periods. This is an effective method for designing HIIT sessions and athletes should aim to increase the time that they can sustain that exercise intensity.
HIIT intervals do not need to always be of a long duration. Supramaximal sprint intervals can also results in significant increases in endurance performance. Intervals as short as 30 seconds at 175 % of PPO have been shown to be effective in improving endurance performance. These intervals require longer rest periods (4.5 minutes) to allow the athletes to recover and ensure that they can reach the target intensity during the next interval. The adaptations that result from low volume, sprint training appear to occur on the cellular level with increases in mitochondria and enzyme activity, rather than in the cardiovascular system.
Manipulating the duration and intensity of the work to rest ratios will determine the effectiveness of the HIIT training programme. Longer intervals of between 3 – 5 minutes appear to be most appropriate for endurance athletes, as these intervals promote both cardiovascular and muscle adaptations. Sprint interval training can still be prescribed to well-trained endurance athletes, after they have done a few weeks of longer duration intervals.
Rest intervals are another important consideration when designing HIIT programmes. The goal of a HIIT session is to accumulate a larger amount of work at a high intensity compared to one long high-intensity effort. It is critical that the rest intervals are sufficient to allow the cyclist to repeatedly hit their target intensity. If the intensity of the recovery period is too high, or the duration is too short, it will result in fatigue, which will prevent the cyclist from adequately performing in the subsequent intervals. Typical work to rest ratios (work:rest) are 2:1, 1:1, or 1:2. The intensity and duration of the intervals will influence the amount of rest required, but the session should be designed to allow the athlete to reach the target intensity in all the intervals.
When designing a high-intensity interval training session, the following factors should be considered:
- The intensity of each interval
- The duration of each interval
- The intensity of each rest period
- The duration of each rest period
- Total work completed (number of intervals x duration of intervals)