What is a defining feature of fartlek training?

Fartlek training is characterized by its unique approach to incorporating varying intensities into running sessions, particularly through alternating between fast and slow paces. This method is designed to mimic the natural changes in pace encountered in outdoor running, allowing for both aerobic and anaerobic development. The varied terrain aspect of fartlek training enhances this experience, as it can include hills, trails, or different surfaces, making it adaptable and engaging for runners.

While the other options suggest more narrow or specialized training methods, they do not capture the essence of fartlek training. The focus is not limited to sprinting on a track, nor does it support the idea of solely relying on resistance training or requiring specific equipment. Instead, fartlek training emphasizes versatility and adaptability in training, making it accessible to runners of all levels who want to improve their speed and endurance in a natural setting.