Base of Support / Centre of mass relationship

The relationship between our Base of Support (BOS) and our Centre of Mass (COM) may be more important than originally considered; I think of it as “Mass Management”, how we manage our mass is probably more important than what our muscles are doing and what our joints are doing; it has become very clear that this relationship is influencing the internal make up of the body – and must not be forgotten that the internal make up can of course influence this relationship…

The video shows Chris standing on a force plate or pressure platform, the AM3 platform, which records pressure and feeds it into the Footwork Pro software on the computer. It shows his Centre of gravity, a vertical representation of his COM position – which sits at approx L4/L5 in the spine or an inch or so below the naval. When you watch the video you’ll notice why I suggest that “human beings cannot stand still”, the COG on the visual is moving about all over the place. This gave me the original insight into our second Big Rule of motion; “Joints ACT; muscles REACT” part of the WTF process.

The software gives us information such as how the patient bears their weight. Are they weight bearing more left? More right? More forwards in the forefeet or back in the heels? It also gives us percentage weight bearing in the four quadrants of right / left forefoot and right / left rearfoot.

You’ll notice that Chris at the time of filming was holding his weight 53% right foot and 47% left foot, which, believe it or not will be influencing his skeletal make up and global posture.

The Base of Support is the area we get when we draw around both feet and the space in between, the area in which the Centre of Gravity likes to remain. It’s comfort zone ☺

The Base of support is important to understand because the way we change our BOS when we walk / move and how we hold our BOS in stance is a real indicator as to how the body is holding itself above the platform. It gives us insight into how the body is moving.

When Chris moves his body to the right, the COG moves towards the right; as he moves his body to the left, the COG moves towards the left – but no matter how far he goes it never moves outside the perimeter of the BOS.

Interestingly asking a person to stand on one foot changes the BOS from a shape around both feet to a shape around the one foot only. Watch his COG instantly jump inside the BOS and safely back inside the perimeter, back in it’s “comfort zone”.

This is the critical factor for staying upright! Your COG/COM must sit inside the BOS if you are to stay upright.

In the WTF book I outline a couple of reasons. When our COM leaves our BOS one of two things can happen:


  • we fall over OR:
  • We must adjust our stance in order to keep the COG inside our BOS.

What happens is Chris takes his left foot off the ground, going from a large BOS where the COG happily sat to a much smaller BOS inside which the COG must now sit. He adjusts his stance in order to keep the COG inside his BOS.

With that COG representing the position of his L4/L5 it must mean that the whole pelvis and spine have also shifted too – you’ll know this if you have ever attempted to keep your spine and pelvis neutral and upright when taking one foot off the floor… you’ll always fall into the space your foot just left behind… TIMBERRRRR….

When shift from standing on two feet to one foot, there is a whole structural skeletal exchange happening in that moment – when you change ANYTHING; you change EVERYTHING!

You’ll see that when I challenge Chris’s BOS, by pulling him to his left and ensuring the COG leaves his BOS, he quickly reorganizes his posture to bring his COM back inside the BOS to prevent him from falling over. He steps his right foot over his left to do so. I pause the moment in the video where you can see this newly organized COG / BOS relationship in place.

This is a small example of how we strive to maintain our COM inside our BOS in everything we do, from our stance posture to our walking gait and from high velocity movements in our sports to balancing on our hands and other things…

Walking is no different of course, how we choose to take a step forward is actually to send our COM forwards, breaching our BOS, in essence, forcing us to place a leg forward where we can receive the travelling COG in the new BOS under the striking heel. The Flow Motion Model maps the journey of this COG and it’s relationship with the Centre of Pressure (COP) through the weight bearing moments of gait – from Strike through to Propulsion - until the COG again leaves the BOS on one foot bravely seeking a new BOS under the opposite striking heel.

Can your patients comfortably weight bear on both feet? Are they doing their best to keep their weight in their back foot when it should be being transferred to their front foot? Why might they do that? Could it be in relation to their injury history? Turns out that many people who have experienced inversion sprains for instance remain unhappy about transferring their weight fully into that previously busted ankle thus changing the way they hold their Centre of Mass and thus their posture. When this patient complains of opposite side back pain due to this adaptation in their COM, are we thinking about treating the inversion sprain? Even if many years later….?

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About Gary Ward

Gary is driven by the belief that anything is possible. Where some practitioners write you off, Gary has an open door; behind which he intends to create a safe environment where your body can learn to heal itself through the experience of whole and integrated movement

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