21 Day Fix Ab Workout for Flat Abs: Sculpt, Strengthen, and See Amazing Results
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If you’ve ever dreamed of seeing those flat, defined abs peek through—or you’re simply ready to feel stronger from your core out—the 21 Day Fix Ab Workout is your go-to program. Designed by Autumn Calabrese, this routine is a different 30-minute real-time workout that fits perfectly into your daily routine and delivers total-body results. Whether you’re pairing it with the Total Body Cardio Fix or doing it solo as a great addition to your fitness plan, you’ll be amazed at how much stronger, leaner, and more confident you feel in just three weeks.
Let’s dive into everything you need to know about the 21 Day Fix Ab Workout, including warm-ups, exercise breakdowns, core techniques, and how to make the most of every rep for amazing results.
Why the 21 Day Fix Ab Workout Works
The magic of this ab workout lies in its whole system design. Unlike random crunch sessions, the 21 Day Fix program is built to target every muscle group with balance—your upper body, lower body, and core all work together to create synergy.
When you engage your abs while doing compound movements like squats, lunges, and pushups, you get a major calorie burn that continues long after the workout ends. It’s not just about isolated crunches—it’s about using core exercises that stabilize your entire body, improve posture, and sculpt lean muscle.
This 30-minute workout fits seamlessly into your daily routine and pairs perfectly with the other workouts in the system like Total Body Cardio Fix and Lower Body Workout, ensuring full-body engagement from start to finish.
Getting Started: Warm Up for Success
Before jumping straight into ab exercises, always take 3–5 minutes to warm up your muscles. Warming up raises your heart rate, loosens your hip flexors, and prepares your lower back for the work ahead.
Try this simple warm-up:
March in place – 1 minute
Torso twists – 1 minute
High knees – 30 seconds
Standing side bends – 30 seconds per side
Arm circles – 1 minute (engaging the upper body and shoulders)
This short routine activates your entire system, prevents injuries, and primes your core muscles for maximum engagement during your abs workout.
The 21 Day Fix Ab Workout Routine Breakdown
The Ab Fix workout is one of the fan-favorite components of the 21 Day Fix program. It’s only about 10 minutes long, but you’ll feel it deep in your core—and it’s often paired with other routines for a total-body results experience.
Here’s a detailed breakdown of the main core exercises you’ll perform, how to maintain proper form, and where to focus your energy for best results.
1. Flutter Kicks
Flutter kicks are a staple of the 21 Day Fix abs routine. Lie on your back with your legs extended and hands placed under your lower back or by your sides. Lift both legs slightly off the ground and alternate small kicks up and down.
Targets: lower abs and hip flexors
Tip: Keep your core tight and your lower back pressed firmly into the mat.
Time: 60 seconds
Modifications: Bend your knees slightly if your back starts to arch.
These small, controlled movements create tension that strengthens your core muscles while improving endurance.
2. Side Plank Position (Right Side)
Roll onto your right side and lift into a side plank position. Rest your right arm on the mat beneath your shoulder, or support yourself on your forearm. Keep your left leg stacked on top of your right leg, and hold.
Targets: obliques, shoulders, and hips
Hold: 30 seconds, then switch sides
Advanced Option: Raise your top arm or leg for more balance work.
This move sculpts the side waistline and tones the entire core. Remember to breathe evenly and maintain a straight line from head to toe.
3. Oblique Crunches
Lie on your back with your knees bent and feet flat. Place your hands lightly behind your head. Crunch diagonally so your right elbow moves toward your left knee. Then switch sides.
Targets: obliques, rectus abdominis
Reps: 15–20 per side
Form Tip: Keep your chin lifted and avoid pulling your neck with your hands.
These core exercises engage the sides of your abdomen and define that beautiful hourglass line.
4. Flutter Kick to Hold Combo
This challenging combo alternates between quick kicks and a second hold where you pause and keep your legs hovering just above the mat. It’s a true test of strength and mental focus.
Time: 30 seconds of flutter kicks + 10-second hold
Repeat: 3–4 rounds
Focus: Engage abs and press lower back down firmly
This move works every inch of your core—especially the lower abs that are often hardest to tone.
5. Plank with Knee Pulls
From a plank position, bring your right knee toward your chest, then switch sides quickly. Alternate like you’re “running” in place without letting your hips drop.
Targets: abs, shoulders, glutes, and upper body
Heart Rate Boost: Major calorie burn!
Time: 1 minute
This dynamic plank variation boosts your heart rate while carving definition in your abs.
6. Bicycle Crunches
A classic for a reason! Lie on your back, lift your shoulders, and pedal your legs as if you’re riding a bike—touching opposite elbow to opposite knee.
Targets: obliques, rectus abdominis, hip flexors
Tip: Keep movements controlled, not rushed.
Reps: 20–30 total
You’ll feel your entire midsection fire up, building stability and endurance.
7. Reverse Crunches
Lift your legs so your knees are at a 90-degree angle. Use your abs (not momentum) to curl your hips off the mat toward your chest.
Focus: Control the movement on the way down.
Tip: Avoid swinging your legs.
Reps: 15–20
This move isolates the lower abs, helping flatten your stomach and strengthen your lower back.
8. Plank Hip Dips
Start in a forearm plank and slowly rotate your hips side-to-side, dipping toward the mat but not touching.
Targets: obliques, shoulders, and transverse abdominis
Form: Keep a tight core and neutral spine.
Time: 60 seconds
By the end of the workout, you’ll feel every muscle from your ribcage to your hips engaged.
9. Crunch with Reach
Lie flat, extend your legs, and reach your free hand toward your toes. Alternate hands to target both sides of your abs.
Reps: 20–30
Focus: Keep the movement smooth and avoid jerking.
Benefit: Builds upper abdominal definition.
This adds variety while engaging the upper body and strengthening your midsection.
10. Full-Body Bridge Hold
Finish strong by lying on your back, knees bent, and lifting your hips high into a bridge. Squeeze your glutes and keep your abs tight for 30 seconds.
Targets: glutes, hamstrings, and core
Great Addition: Helps release tension from your lower back after all those crunches.
This exercise improves posture and stabilizes your pelvis—essential for a strong, balanced core.
Combining the Ab Fix with Total Body Cardio Fix
The Ab Fix is often combined with the Total Body Cardio Fix for a complete 30-minute workout experience. This pairing ensures your upper body, lower body, and abs all get targeted while maximizing fat burn.
For example:
First Block: Start with cardio moves like jumping jacks, squats, and lunges to warm up your whole system.
Then, finish with core exercises like oblique crunches or flutter kicks for that extra burn.
The combination of high-intensity intervals and core isolation creates an amazing calorie burn that sculpts muscle and reveals definition faster.
Tips for Getting Flat Abs in 21 Days
While workouts are key, achieving flat abs also depends on consistency, nutrition, and recovery. Here are some insider tips to help you get amazing results from your 21 Day Fix program:
Follow the meal plan. The container system helps you balance macronutrients and control portion sizes without guessing.
Stay hydrated. Water supports digestion and helps you feel full.
Use a heart rate monitor. Track your heart rate to ensure you’re working in your optimal fat-burning zone.
Stretch after every workout. Gentle stretches for your lower back and hip flexors improve mobility and reduce soreness.
Don’t skip rest days. Recovery is where your muscles rebuild and your abs get stronger.
Even if you don’t have much time, just 30 minutes a day can transform your body when you commit fully.
Modifications for All Fitness Levels
Whether you’re a beginner or an advanced exerciser, the 21 Day Fix Ab Workout can be adjusted to fit your needs.
Beginners: Keep your knees bent during flutter kicks and take short breaks when needed.
Intermediate: Add light dumbbells to crunches or planks for added resistance.
Advanced: Combine ab circuits with lower body workouts or use ankle weights for extra challenge.
Remember, quality always beats quantity. Focus on proper form, breathing, and control to prevent strain on your lower back.
Tracking Your Progress
One of the best ways to stay motivated is to track your progress. Take photos every seven days and jot down how you feel after each session. You’ll notice:
Improved posture
Stronger core muscles
Increased energy
Visible tone and definition
Even if the scale doesn’t move right away, your waistline will tighten as your muscles strengthen and your body composition improves.
Common Mistakes to Avoid
Holding your breath. Breathing helps activate deep abdominal muscles.
Arching your lower back. Always press it into the mat during ab exercises.
Neck strain. Relax your neck and let your abs do the work.
Skipping the warm-up. This can lead to injuries and tight hip flexors.
By focusing on mindful movement, you’ll build a solid, functional core—not just flat abs.
Making It a Lifestyle
The 21 Day Fix Ab Workout is more than a challenge—it’s a mindset shift. Once you complete the program, you’ll have built consistency, strength, and discipline.
From there, you can:
Repeat the cycle with heavier weights or faster reps.
Incorporate ab-focused days into your daily routine.
Pair the ab fix with other programs like Total Body Cardio Fix for variety.
Think of it as a long-term investment in your health. Flat abs are just the beginning—you’ll also enjoy better balance, reduced back pain, and boosted confidence.
A Few Final Thoughts
At the end of the workout, when you’re sweating and your abs are on fire, remember this: every second counts. Each rep brings you closer to the amazing results you’re working for.
The 21 Day Fix Ab Workout is designed for real people with busy schedules, and it doesn’t require much time or equipment. All you need is a mat, a little space, and the determination to switch sides, stay consistent, and trust the process.
Whether you’re new to fitness or just looking for a great addition to your current plan, this program delivers everything you need for total-body results—stronger abs, a leaner waistline, and a healthier, happier you.
Final Pro Tip:
If you’re following along at home, remember to check your email for shipping notices and program updates if you’ve ordered DVDs or accessories. But even if you’re streaming online, consistency and focus are the real keys to transformation.
So grab your mat, warm up, and press play—because your strongest core starts today.