Core, abs, six-pack—while all three are located in your trunk, they're not made of one muscle.
Your core is split into four major muscles. Each one provides a different service to your body, from stabilizing your spine to giving you that six-pack look. Knowing where each muscle is, and what it does, will help you plan a more efficient and successful core workout. The four main muscles are:
- External abdominal obliques: run along the sides of your abdomen and are contracted during lateral extension (side bend) and rotation (bicycle crunches).
- Internal abdominal obliques: are below the external obliques and both muscle groups work in conjunction during rotation. For example, when you rotate to the right, your right external oblique contracts along with your left external oblique.
- Rectus abdominis: helps you safely flex your spine and is the muscle that forms the six-pack "shape." It also pulls your pelvis up so it doesn't tilt anteriorly (to the front), which can cause lordosis, an abnormal posture position.
- Transverse abdominis: stabilizes your lumbar (lower) spine region. This is the muscle you're contracting when you "suck in."
A strong core protects the spine from injury and builds good posture. That means focusing on all the muscles, many of which are neglected with traditional exercises like crunches. Switch those out for smarter moves that target all the right muscles for a strong and slim midsection.
More: Interval Workouts That Fire Up the Metabolsim
The Exercises
All four muscles are contracted by different exercises. Your obliques are targeted in rotational movements while the transverse abdominis benefits most from planks. Add the following exercises into your workout to ensure you're strengthening all the right muscles.
Plank
Works: Transverse abdominis 
Rest on your knees and place your hands shoulder-width apart on the ground. Rise up onto the toes, pull the stomach in and hold for 30 to 60 seconds.
More: 30-Day Plank Challenge
