3. 200 Mountain Climbers (Core Endurance & Hip Flexor Power) Primary Targets: Rectus abdominis (six-pack), obliques, hip flexors, and shoulders. The Functional Benefit: This is a dual-purpose movement. Your upper body stays in a static push-up hold, building massive shoulder endurance, while your lower body mimics a high-speed sprint. It builds explosive hip flexion and bulletproofs the lower back by forcing the core to stabilize under rapid movement. Note: Counting is usually done as "1-1, 2-2" (left/right equals one rep) or total reps (100 each leg). For a true challenge, count every step to 200.