Beyond the Quick Fix
The Role of HIIT, SIT and Steady-State Workouts in Building Sustainable Endurance
In endurance sports, many athletes are drawn to high-intensity interval training (HIIT) for its efficiency and rapid fitness gains. Others gravitate towards sprint interval training (SIT), which has been shown to drive impressive physiological adaptations through even shorter bursts of intense activity. However, research increasingly suggests that a balanced mix of HIIT, SIT and steady-state training is critical for long-term success and sustainable endurance.
Dr. Stephen Seiler’s recent study, ‘It’s About the Long Game, Not Epic Workouts: Unpacking HIIT for Endurance Athletes’ highlights how HIIT can best be incorporated into a program that emphasizes consistency and longevity. Together with insights from the study ‘Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle’ these findings reveal why varying training intensities is essential to building a well-rounded endurance program.
What’s the Difference Between HIIT and SIT?
Before diving into the benefits of balanced training, it’s essential to understand the distinction between high-intensity interval training (HIIT) and sprint interval training (SIT), two approaches often used in endurance and fitness programs.
High-Intensity Interval Training (HIIT): Involves working at a high but sustainable intensity, typically at 80–90% of an athlete’s maximum heart rate or power. HIIT intervals usually last from 1 to 5 minutes followed by rest or low-intensity recovery periods that allow athletes to repeat the intense effort. Examples include running or cycling intervals at a fast but controlled pace. HIIT sessions stimulate both cardiovascular fitness and muscular endurance, helping improve overall aerobic capacity and VO2 max.
Sprint Interval Training (SIT): Takes intensity to another level with all-out maximal efforts lasting around 10–30 seconds, generally pushing athletes to 100% of their capacity. Rest periods in SIT are longer - often several minutes - to allow complete recovery between efforts, as SIT requires the body to perform at its limit. The goal of SIT is to drive rapid adaptations by activating fast-twitch muscle fibres and encouraging explosive power and high lactate production. Though SIT sessions are shorter, they stimulate quick improvements in anaerobic capacity, strength and even mitochondrial density, especially in less-trained individuals.
In short, HIIT targets a high but sustainable effort for slightly longer durations, whilst SIT emphasizes extremely intense, short bursts with maximum effort. Each approach has its advantages, but neither should dominate an endurance athlete’s training plan alone.
Sprint Interval Training
Why HIIT and SIT Alone Aren’t Enough
High-intensity workouts - both HIIT and SIT - are excellent for short-term gains, improving cardiovascular efficiency and increasing VO2 max relatively quickly. However, Seiler’s research emphasises that successful endurance athletes perform most of their training at low intensities, with HIIT or SIT used sparingly to stimulate specific adaptations. The majority of training should remain low-intensity, building a durable aerobic base that allows athletes to increase their workload over time without risking burnout.
Additional research, including ‘Effects of Exercise Training on Mitochondrial and Capillary Growth’ also underscores the value of varied training intensities. For instance, while HIIT and SIT both target mitochondrial growth by increasing the density of energy-producing structures within the muscle cells, low-intensity, steady-state work promotes capillary growth, improving blood flow and oxygen delivery. This enhanced oxygen supply supports endurance, aids recovery and allows athletes to sustain prolonged effort. Together, these physiological adaptations create a more robust aerobic base that allows for both intense and sustained performance.
Key Insights for Endurance Athletes
Here are the main takeaways from recent research on how to best integrate HIIT, SIT and low-intensity workouts into an endurance training program:
Mitochondrial and Capillary Growth: Both HIIT and SIT stimulate mitochondrial adaptations, but low-intensity training is more effective at building the capillary network needed for efficient oxygen delivery. The combined benefits of these training types help build endurance and recovery capacity. Seiler’s research reinforces that endurance requires not just high-output sessions but the slower, long-term growth that steady-state training provides.
Intensity Balance and Recovery: Dr Seiler’s study emphasizes that HIIT or SIT must be balanced within a low-intensity training base to allow for adequate recovery. Since both types of high-intensity training place significant demands on the body and mind, it’s vital to schedule easier sessions between high-intensity days. Seiler found that elite endurance athletes rely primarily on low-intensity training, adding HIIT or SIT sparingly for specific adaptations.
Progressive Optimization: A well-rounded endurance program is about optimization, not maximization. Seiler’s findings suggest that an 80/20 split (80% low-intensity workouts to 20% high-intensity workouts per week) is optimal for endurance athletes, allowing them to make sustainable gains without risking overtraining. This balance allows athletes to improve over time and provides room for the mental and physical challenges that both HIIT and SIT introduce.
Mental and Perceptual Demands: While HIIT requires focus and sustained effort, SIT is even more taxing due to its all-out nature. Seiler points out that too much high-intensity training can lead to mental fatigue, reducing motivation and long-term enjoyment. By balancing high- and low-intensity workouts, athletes can stay engaged, avoid burnout, and make consistent progress.
Practical Tips for Building a Balanced Training Program
To harness the benefits of HIIT, SIT and steady-state training, consider these guidelines:
Build a Strong Aerobic Base: Most of your training should consist of low-intensity, steady-state workouts to build cardiovascular endurance and improve capillary density. According to Seiler, low-intensity training provides the base upon which all other adaptations rely.
Incorporate HIIT and SIT Strategically: Use HIIT and SIT sparingly - perhaps once or twice a week. HIIT sessions provide cardiovascular benefits and muscle endurance, while SIT sessions enhance explosive power and VO2 max. Both types should be used purposefully to prevent excessive fatigue.
Allow Adequate Recovery: Recovery periods are crucial for adaptation. Both HIIT and SIT require recovery days to allow the body to rebuild and strengthen. Including low-intensity days between high-intensity efforts helps athletes accumulate training volume without risking overuse injuries.
Progressive Intensity Adjustment: As your fitness improves, adjust your training plan to continue challenging your body. For beginners, consistent low-intensity work alone may lead to improvement. More advanced athletes, however, may benefit from a gradual increase in intensity through planned HIIT and SIT sessions, adjusted over time to maintain steady gains.
Long-Term Endurance: The Power of Balanced Training
By blending steady-state training with both HIIT and SIT sessions, endurance athletes can build a comprehensive fitness base. Seiler’s research underscores that long-term endurance success comes not from the ‘epic’ workouts alone, but from training that respects the need for consistency, recovery and purpose. Integrating HIIT and SIT with low-intensity work creates a plan that is sustainable and challenging without leading to burnout.
For endurance athletes, this well-rounded approach offers a path to sustainable performance gains. Emphasizing the long game builds both physical and mental resilience, creating a foundation to reach new levels without sacrificing enjoyment or longevity in the sport.
Bevan McKinnon / January 2025