After years of coaching experience and trying this strategy with all different kinds of athletes, I am convinced that not only short distance runners and triathletes, where the system seems quite logic, also long-distance athletes benefit highly from training short and fast intervals.
Training at (almost) maximum speed improves an athlete's overall running economy, meaning that stride length and stride frequency get better. Whenever you run fast your form gets (automatically) better.
True, long-distance running primarily relies on aerobic energy systems. Still, incorporating short, fast intervals can improve an athlete's anaerobic capacity, allowing them to sustain a faster pace during intense portions of a race (overtaking, finish line, etc.) and respond to race dynamics effectively.
While running fast intervals the anaerobic energy systems, specifically the ATP-CP system and the glycolytic system, are primarily activated. The ATP-CP system provides immediate energy through the breakdown of stored creatine phosphate, while the glycolytic system utilizes stored glycogen to produce energy without requiring oxygen. Running 100m or 200m intervals quickly depletes these energy stores, leading to a buildup of metabolic byproducts such as lactate. Running multiple reps leads to lactate accumulation which is typically associated with muscle fatigue and the "burning" sensation. However, lactate serves as a fuel source for other muscle fibers and can be used for energy during subsequent efforts.
Short and fast intervals kicks athletes out of their comfort zones and challenges them mentally. It helps build mental toughness and the ability to endure discomfort, which is valuable during long-distance races where fatigue and the temptation to slow down can be significant factors.
If you run short and fast intervals, you need different muscle fibers than running slowly. Training these fast-twitch muscle fibers and learning how to recruit and engage them improves an athlete's overall muscular strength, power, and coordination. These fibers have a greater capacity for generating force and are responsible for producing high-speed movements. Running fast intervals particularly trains the quadriceps, hamstrings, and calf muscles. Muscle fibers get damaged and a repair process is stimulated, leading to muscle adaptation and improved performance over time. (Reminder to recover properly and treating those muscles well ;-))
Variability is one of the seven principles of training: incorporating different types of workouts, adds variability to a program and helps prevent overuse injuries by reducing repetitive stress on the same muscles, tendons, and joints that occur during long, slow-distance running.
It's important to note that while short, fast intervals are beneficial, they should be implemented strategically within a well-rounded training program including swimming and running accordingly. Endurance, tempo runs, long steady-state runs, and recovery runs are also crucial components of a long-distance runner's and triathlete's training regimen. The key is to find the right balance between different training intensities and volumes to optimize performance and minimize the risk of overtraining or injuries.
Further questions? Anytime.