Step 1: Understand Fartlek Training.
Fartlek training is a form of exercise that blends continuous running with intervals of varying intensities. The term "Fartlek" is Swedish for "speed play," and it involves alternating fast and slow running over natural terrain like hills, fields, or trails. It is designed to improve both aerobic and anaerobic endurance.
Step 2: Analyze the options.
- (A) Steady pace throughout the session: This does not describe Fartlek training, which involves alternating speeds.
- (B) Alternating fast and slow running over natural terrain: Correct. This is the essence of Fartlek training, which incorporates speed variations over different terrains.
- (C) High resistance exercises with low repetitions: This refers to weight training or resistance training, not Fartlek training.
- (D) Focus on flexibility and range of motion: This refers to stretching or flexibility exercises, not Fartlek training.
Step 3: Conclusion.
Thus, Fartlek training involves alternating fast and slow running over natural terrain.
Final Answer: Alternating fast and slow running over natural terrain.