Set-up: Attach a bar to a cable tower. Grasp the bar with a narrow underhand grip. With your arms straight, your core tight and a neutral spine, bend at the hips and knees to pick the bar up from the tower.
Lift: Keeping the elbows at your sides and your eyes forward, curl the bar up to your chest. Lower the bar under control. Do not let your upper back round over - consciously pull your shoulder blades together to provide a stable base for your arms to pull from.
Emphasis: Do not swing - keep your core tight and a neutral spine.
Training Impact
Muscle Roles in bicep cable curls - narrow-underhand-grip
Instructions and effects for the Bicep Cable Curls - Narrow-Underhand-Grip exercise 
