Cable Crunching with Just One Arm

To get truly awesome abs, you have to attack your abdominal wall from all angles with a vengeance. Understanding the structure and function of muscles involved in a well-defined torso helps you identify exercises that are highly effective. One of the best exercises for challenging and sculpting your mid-section is a one-arm cable crunch.
Muscles of the Abdominal Wall
The primary muscles responsible for creating impressive ripples in your trunk are your rectus abdominis and external obliques. The rectus abdominis is the long muscle dissected by a membrane that spans from your ribcage to your pelvis, with tendinous attachments that create the impression of a six-pack. The job of the rectus abdominis is to flex your trunk, drawing your ribcage and pelvis together. Your obliques begin at the side of your ribcage and hug your trunk diagonally. They are responsible for trunk rotation and lateral flexion, or side bending. While classic cable crunches target mainly the rectus abdominis, one-arm cable crunches recruit both the rectus and the obliques.
Overload and Abdominal Muscle Hypertrophy
While doing a variety of abdominal exercises will tone your muscles and give you a cinched waistline, highly defined abs require overload that leads to muscle hypertrophy, or growth. According to University of New Mexico researchers Richard Hernandez and Len Kravitz, Ph.D., overload causes muscle growth by forcing muscle fibers to adapt, increasing the size and number of contractile proteins that make up muscle cells. Exercises like cable crunches that utilize resistance equipment enable you to overload muscles at an intensity that elicits abdominal muscle hypertrophy. In his e-book "Stubborn Muscle Hypertrophy Workouts," professional bodybuilder Derek "Beast" Charlebois says that to maximally showcase your abs, endurance cardio exercise is also essential to reduce overlying body fat.
Equipment Setup
Using a cable pulley machine, set the pulley at the top notch and attach a U-grip or soft strapping grip. Cable crunches are typically done kneeling on the floor, but you can also do them straddling a bench with your hips fully extended, knees pointing straight down with the tops of your feet facing downward. This position may make you less likely to recruit your hip flexors. Position one end of your bench close to the machine, or place a mat or towel on the floor in front of the machine to protect your knees.
Execution
Facing away from but close to the cable machine in either the kneeling or bench-straddling position, grip the handle with your right hand and draw it to your shoulder with your palm facing your body. Hold your upper arm and elbow close to your ribcage. Pull your navel in toward your spine and exhale as you flex your trunk forward, drawing your bottom right rib toward your right hip. Inhale and slowly return to your start position. Perform reps to fatigue, then repeat on the left side. You can also perform crossover one-arm cable crunches by drawing the ribcage toward the opposite hip.