A Physics Primer: Part 1

Getting Grounded

     Before entering the training world I had no background in physics and thought it should best be left to the really scary smart people with disheveled hair and far-away stares. Little did I realize how critical an understanding of this cool science was required, not just to be a good fitness pro, but also to be a more effective exerciser. This 2-part series is designed to touch upon some basics concepts that will help you understand a bit more about how we are constantly interacting with the laws of nature, why we move the way that we do, and how to make your movements more efficient and effective.

     In this first article, I want to get you grounded with 2 essential properties of all objects, whether they are animate or inanimate. Base of Support and Center of Gravity are terms that I hear getting thrown around but without a real understanding of their meanings or applications. First, let’s define the terms:

Base of Support (BOS):

     The points of contact with a supporting surface and all of the area within those points. BOS is direction-dependent, meaning that we are only stable in the direction in which we maintain a large base. For example, we can have feet spread wide apart, as in this exemplary horse stance pose, and be quite stable from side to side. It would take a large force from his left or right to knock him over. But if someone were to stand in front of this fierce warrior and push him in the chest with the slightest amount of force, then he would fall backwards. Why? Because his base is very small (length of his feet) in that direction. The solution? Take a look at the photo of the football player in a 3-point stance. He is better prepared to handle forces coming at him from either direction because his stance is both wide side-to-side and long front-to-back.

This gentleman’s horse stance creates a very stable base from side to side but makes him susceptible to losing his balance if pushed from the front or back.

This woman is positioned for a split squat with a large, stable BOS from front to back but this alignment makes her susceptible to losing her balance if pushed from the side.

This football player has created an incredibly stable platform from front-to-back AND side-to-side.

Center of Gravity (COG) :

     The point in the body or object where all the mass is equally balanced, also referred to as the center of the total body mass (1). In the standing human, the COG is usually located around the belly button. It tends to be a little lower for women, who usually carry more mass in the lower body, whereas it tends to be higher in men due to their greater upper body mass. In infants, the COG is also higher because of the larger head size in relation to the rest of the body’s weight.

The red plus signs indicate the location of the COG.

The location of the COG can change if:

You move or lift a body part away from the “home” position (standing still with arms down by the sides). The COG will shift in the direction of the displaced limb or body part.

• For example, little Stevie uses his right arm to reach up to the top shelf in front of him for that irresistible, cuddly teddy bear. That movement will cause his COG to shift up, to the right, and in front of him. Be careful, Stevie!

This position looks precarious and requires exceptional control of the COG in order not to fall.

 You hold a weight. The COG will shift in the direction of the weight.

• For example, if you picked up a bottle of detergent with your left hand then your COG will shift down and to the left.

Now that you have an idea about what BOS and COG are, you may be asking yourself:

What do these concepts have to do with my workouts?

     You may exercise to get stronger, be more agile, function better in your life, stave off the effects of aging or for any number of other reasons. Regardless of your motives, understanding the basic physical properties of objects can help enhance your workouts. By knowing that picking up a weight will shift your COG in its direction or widening your BOS will make you less wobbly on split squats, you will be more productive and safe during your workouts. Below are some examples of exercise goals that will be made more effective with your new knowledge.

Getting stronger

     Increasing your BOS will make you more stable, giving you a sturdier foundation from which you can lift more weight. For example, if you are doing standing shoulder press and you usually use a narrow stance, try widening it from side to side and front to back (staggered stance) to give you a larger BOS. Likewise, doing a deadlift will cause your COG to shift forward so you will need to counterbalance that force by shifting your weight backward to maintain good stability throughout the lift.

Improving balance

     Being well-balanced occurs when the COG remains directly over your BOS. If you “lose” your balance then you’ve just experienced the point at which your COG moves beyond your BOS.

Stan is going to fall because his COG (blue circle) has moved outside of his BOS (yellow circle).

     If you are trying to improve your capacity to remain balanced even when you are in situations that challenge your stability, there are ways to focus your gym efforts to work on this. Hold loads farther away from your body (shifting your COG to the limits of your BOS) and reduce your BOS (bring feet together, stand on 1 foot), creating a balance challenge. You will be forced to recruit all of the stabilizing postural muscles, which tend to get quieted down when you stick to machines or exercises in highly stable positions.

     Some examples of balance challenge exercises: Stand on 1 leg (this alone may be enough for some people) while moving your arms and other leg around you, do conventional exercises holding 1 weight instead of 2 (i.e. chest press, squats) to load you only on one side at a time, lunge with arms overhead instead of down by your sides, do exercises on the cable column to change the line of pull—free weights want to drop straight down to the ground because of gravity’s pull, whereas cables want to retract back to their home position along the cable column, creating unique horizontal and diagonal pulling forces.

     You may be tempted to use wobbly boards, squishy surfaces, and bouncy balls to work on your balance and, while those implements definitely alter your BOS and effect your ability to control your COG, just keep in mind how you move in real life and what gym stuff will have the greatest transfer to those demands.

Functional training

     In the real-world we use our bodies to lift, reach, grab, carry, and transport objects and ourselves, causing shifts in our COG and BOS. In order to do those activities successfully, it might be helpful to implement these same conditions into your training. For example, when we lift bags overhead or pull open doors we have to do so with our feet as our BOS. Therefore, sitting on machines while pressing or pulling creates a more stable base (butt and feet on floor) than we really use when outside of the gym. Try doing the bulk of your lifting in positions that we encounter in life: standing, squatting, kneeling. Reserve seated and lying positions for the hours you spend at your desk or in bed.

Senior fitness

     Some of the effects of aging directly influence one’s ability to remain in control of the COG and BOS. Decreased height (hunched over posture), reduced muscle mass in the upper body, and increased body fat storage in the lower body often lowers the COG in seniors. In addition, changes in the vestibular system and the side effects of prescription medications can make for more movement in the COG even while in quiet stance. Seniors may also experience less security in their BOS due to decreased visual acuity (harder to plant feet when they can’t clearly inspect their surroundings), a slower, shuffling gait, and less sensory receptors on the bottom of the feet (which are used to help determine their positioning in space). (For further reading about changes associated with aging, click HERE)

     For those in need, canes and walkers help increase stability by making the BOS larger (by increasing contact points with the ground).

Larry’s BOS grew substantially with the aid of a walker, creating 6 points of contact with the ground.

When in the gym, where I see many determined seniors (see The Training Diaries: Volume 2), it is helpful to start out with exercises that are performed in stable positions: lying, seated, or standing with a hand on a stable surface (i.e. cane, walker, wall). As you get stronger and more familiar with the movements, challenge yourself by slowly removing those layers of stability (reducing BOS). For instance, lower the weight on the shoulder press and do it without holding on to the bench.

     Also, practice movements that tend to be challenging to stability, such as those found transitioning from one position to another: sitting to standing, kneeling to standing, walking (one foot is in the air swinging while the other is planted—called the swing phase). Those are moments when the COG can teeter close to the edges of the BOS, creating instability and the potential for falls.

     No matter your goals, an understanding of COG and BOS can help you be more successful and safe. Hopefully, you have also come to appreciate how the body orients you (often without conscious awareness) and keeps you balanced, regardless of the demands you place on it during a typical day. In return, please treat it well.

Coming soon…Part 2: Newton in a Nutshell.

Keep on Movin’

-CA

(1)   Shumway-Cook, A., & Woollacott, M.H. (2007). Motor Control: Translating Research into Clinical Practice (3^rd ed.). Philadelphia, PA: Lippincott Williams & Wilkins.