Every time resistance is lifted or moved, core activation should be present. Being aware of core activation is important for spinal protection, improved posture and healthier movement. The core active technique used for lifting heavy weight is called blocking. Heavy weight is any resistance that will naturally stop your breathing while lifting.
If I line dumbbells of increasing weight in a row and proceed to pick each one up beginning with the lightest. The dumbbell that stops my breathing during the attempt to lift is heavy weight or non-breathing weight. Lighter weight that allows for breathing during the movement is considered breathing weight.
Blocking for non-breathing repetitions is a combination of three different elements; the Valsalva maneuver, abdominal hollowing and isometric tension. When you take a deep breath and hold the Valsalva maneuver occurs. The glottis, an opening between the vocal cords in the larynx, is closed against pressure in the lungs. The pressurized lungs act as braces for the thoracic vertebrae located in the middle of the spine, creating thoracic pressure. The Valsalva maneuver reduces the amount of blood flow to the coronary arteries in turn raising blood pressure.
People with history of heart disease are advised to do breathing repetitions.
After thoracic pressure is gained the next element added is abdominal hollowing. Abdominal hollowing moves organs to create empty space in the abdominopelvic cavity. This is achieved by pulling the bellybutton in toward the spine and then elevating it up and holding. The last element added is isometric tension. Isometric is a held, static or still muscle action. During isometric tension the muscle does not change length and the joints do not change angle. This form of tension should be applied to the external abdominal muscle and the oblique muscles. Combining all these elements creates blocking against pressure of heavy resistance to the vertebrae. During heavy weight movements the air pressure in the lungs should be held until the last three-fourths or the end of the positive contraction. A positive contraction is moving toward or into the resistance. If the choice is made to release air during the positive contraction it should be released with force through pursed lips.
During breathing repetitions only one of the elements, the isometric tension, can be applied. Breathing should remain three-dimensional. The belly rises outward, the lower back widens, and the chest lengthens upward. Inhaling and exhaling are ultimately governed by the spine. Inhaling accompanies spinal extension (spine straightened) and exhaling should accompany spinal flexion (spine curved forward). Likewise, abduction and external rotation you should inhale; adduction and internal rotation you should exhale.
However, every movement can be over exaggerated with the entire body to identify spinal movement, whether the spine straightens (extension) or curves (flexion), and the spine has the final say so.
After identifying the breathing pattern for the particular movement desired, maintain isometric tension in the abdominal muscles while still allowing the belly to rise outward during the exercise. The constant awareness of tension in the abdominals is core activation. Core activation is important for spinal protection, improved posture and healthier movement.