Sprint intervals are the most primitive and effective form of HIIT. Humans evolved to sprint, and the adaptations from sprint training are profound: increased fast-twitch muscle fibre recruitment, improved cardiovascular capacity, enhanced fat metabolism, and hormonal benefits that support muscle growth. Sprinting also develops leg power and athleticism that no other cardio method matches.
For outdoor sprint sessions, find a flat 40-100m stretch. A basic progression: sprint 40m at 80% effort with 90 seconds rest x 6-8 reps (weeks 1-2). Progress to 60m at 90% effort with 2 minutes rest x 6 reps (weeks 3-4). Then 80-100m at maximum effort with 3 minutes rest x 4-6 reps (weeks 5-6). The long rest periods are deliberate - true sprinting requires full recovery between reps to maintain quality.
Always warm up thoroughly before sprinting: 5-10 minutes of jogging, dynamic leg swings, high knees, and A-skips. Cold sprinting is a recipe for hamstring tears, which are painful and take months to heal. Start conservatively in your first sprint session regardless of your fitness level - sprint volume should be earned gradually. Limit sprint sessions to 2 per week maximum, not on the same days as heavy lower body lifting. The combination of sprint training and strength work is arguably the most powerful physique-building and fitness-building combination available.