This exercise also works the muscles of the arm, not only to stabilize the wrist and elbow but to push the arm out in front of the body while stabilizing against the pull of the band.

UNDERSTANDING WHY A STRONG CORE IS SO IMPORTANT

There are so many benefits of learning to train the core muscles effectively and safely - not only will your midsection look better, but having a nice strong core can improve your walking, running, posture, ability to lift and carry heavier loads, and it can help prevent back pain. How about a stick - if the stick was flimsy and you tied a rock to one end, the stick would just bend. If the stick was stronger it would not bend and the rock would stay up. If you were to take the stick and bend it over and over at one spot - eventually it would crack and break. Imagine being on ice and trying to push a box away, you would move back and it would not budge. But if you were standing on a stable surface like the ground then the box would move. Or the classic example is - a weak core is like trying to shoot a cannon from a canoe. If your arms have a stable base to work off of they are essentially stronger. Think of trying to push a marble with a string- you can't, it just folds up, but you can push a marble with a pencil.

The rock could be your arm, or something you are trying to lift - like trying to catch a big fish with a flimsy fishing pole - the pole would just bend down, so it would be really hard to get the fish out of the water, but if the pole was strong, it would be easy. Same with the marble - it can be anything - imagine pushing a hose, it just bends. But you can push a stick. There are a lot of examples - jumping off of sand vs jumping off of solid ground. The idea is that if you don't have a solid base to work off of then you will not be as strong in your movements/actions. This is true for your arms and legs as well. Imagine trying to walk with really wiggly knees, or trying to write if you don't have control over your shoulder or elbow - they are not stable, or your bone is broken in half - you have no control. I think the message is less about the spine/object relationship and more about a controlled, stable foundation in order to support controlled movement. A bone is another good example - what would happen if you tried to walk on a bone that was broken in half - your leg would just collapse. The muscles support the joints and give them stability - if you were to remove your leg muscles the knee joint would just collapse under your weight, you would not even be able to stand up. The spine is made up of 24 vertebrae, each with 4 joints and muscles that run up and down (multifidi) that compress the vertebra to hold them together - if they are not doing their job the spine just collapses - you would not even be able to stand. This is easy to see in quadriplegics - they don't have muscle tone in their core muscles so they need to be strapped to their wheelchair or they would just fall out.

The oblique muscles run around the sides of the body, from the ribs to the pelvis. There are two layers of muscles, the outermost layer is the external obliques and the layer beneath it is the internal obliques. The two layers run diagonally in opposite directions. The function of the obliques (internal and external obliques) is rotation of the torso, compression of the abdomen (like a corset), flexion (forward bending) and lateral flexion (side bending) of the torso. They also can help in moving the pelvis and ribs.

The quadratus lumborum muscles are on either side of the back. They run from the ribs to the pelvis. The quadratus lumborum and obliques work together with the other muscles of the trunk - transverse abdominis, erector spinae, multifidi, latissimus dorsi,iliopsoas, rectus abdominis, pelvic floor and diaphragm to maintain upright posture and a stable base for the arms and legs to work off of. The function of the quadratus lumborum is side bending (lateral flexion) of the spine. These muscles also work to stabilize the torso against the same forces (flexion, lateral flexion, rotation).

Upright posture and spinal stabilization (good core strength) are needed to prevent injury to the joints of the spine; this is especially important during dynamic (moving) activities that require rotational movements of the trunk with weighted arm movements - shoveling - dirt and snow, vacuuming, mopping, and overhead reaching and pulling.

As a society, we have moved towards a lifestyle that is less physically demanding than that of our ancestors. We do not squat, climb, pull, and push as much. Many people cannot squat all the way down and hold the position - this is a position that people used for going to the bathroom, eating, and even resting. This has led to an decrease in the mobility of our hips and an increase in core weakness. The result is very tight hip joints and weak cores. Just like most things, the body will always take the path of least resistance . So if someone is tight in their hips and weak in their core and they need to bend over - the movement is going to come from their spine - not their hips. Over time, the legs get weaker, and the spine becomes more flexible - too flexible; When the spine bends forward - it is not using muscles, gravity is just pulling it down, so it does not get stronger. This is exactly the opposite of how the body is designed to work - the legs should be strong and move a lot, and the spine should be stable and strong. 

In an exercise world - if you are going to lift weights you need to start by improving your core strength. 

When you are just standing still with a neutral spine, the muscles are the least active. The way the curves of the spine are designed - when upright their shape along with the ligaments and the compressive forces of the muscles, hold the body upright. 

As soon as you lean forward, and tipped the balance, gravity begins pulling down. If your arms are holding something and outstretched the downward pull of gravity is even greater - not only do the core muscles need to hold the spine up but also the object. 

Here is an example that demonstrates this:

If you bend over about 40 degrees from the horizontal with arms stretched out about 18 inches from your center of gravity and lift a 30 pound object - your muscles need to support the weight of your upper body and the object. 

From the equation Torque = force x distance: 

The torque on the spine from the object = 30 lbs x 18 inches = 540 lbs in-lbs

And the torque from the upper body = 60 lbs ( the torso of a 120 pound person) x 10 inches (assuming the top of the head is 10 inches from the center of gravity of the body) = 600 pounds.

So the core muscles would need to be able to produce 1, 140 pounds of force per inch to lift that object without injury.