Walking lunge with dumbbells overhead
While walking lunges don't appear to work your abs, a 2013 review study suggests that multi-joint free-weight exercises are more effective at activating the deep stabilizing muscles of the core than ab-specific exercises. While these deep muscles may not be the "show me" muscles of the six-pack, they're critical to injury prevention and performance, and they're a great way to warm up your muscles at the beginning of a workout.
The walking lunge with dumbbells overhead is particularly effective at hitting the deep core muscles because it requires good posture and spine stabilization to maintain your balance while you walk forward.
Start with your feet about hip-distance apart, your knees slightly bent. Holding a dumbbell in each hand, press your arms straight up over your shoulders, as if performing a shoulder press. You'll keep your arms extended in this position throughout the exercise. Tighten your core and take a large step forward with your right foot, planting your heel on the ground. Bend both knees, and keeping your torso tall and stable, lower your back knee toward the ground. Just before it touches down, press through your front heel and the ball of your back foot, and return to standing as you simultaneously take a wide step forward with your left foot to continue the walking motion.
Take 16 to 20 total steps (eight to 10 per leg), rest for 60 seconds, then perform another set. Complete three total sets.