Movement Mechanics – The Essentials Uncovered

Base of support

A base of support is quite simply wherever your body touches the ground.  It’s called the base of support because with gravity acting straight down at all times we have a base of support created whenever we are in contact with the ground.

The base of support for the squat is the area from the outside of one foot to the other.  For a push up the base of support is very large - it encompasses the area from the outside of each hand as well as the area from the outside of each foot.

Here is the base of support when using crutches.  You can see it relates to the points that are in contact with the ground and their position at any moment in time. But you can see why crutches are useful for helping balance and support as they make the base of support larger than feet alone.

Centre of mass

The centre of mass is simply the middle of an object.  For example in the human body when standing with arms hanging by the side the centre of mass is at about the top of the hips as shown by the dot on the adjacent image.

If you re-distribute a mass its centre moves.  For example if I raise my arms over my head I’ve moved the mass of my arms almost a metre from my side to above my head.  My centre of mass will now be higher and more around the bottom of my rib cage.

Similarly if I lower my body my centre of mass gets lower.  Please note that centre of mass is also known as 'centre of gravity'.

Line of force

A line of force simply refers to the direction that a load is acting in.  For instance, gravity is always acting straight down.  Whereas the line of force of a punch is virtually straight out (horizontal to the ground)

Axis for rotation (joint axis)

An axis is simply a point around which movement is happening.  The movement that happens around an axis is called rotation because the axis is the centre of the movement and anything that moves around that centre must move in an arc, that is be rotating.

How the centre of mass, base of support and line of force relate

In a standing position your base of support is where your feet are.  So if I was standing upright my base of support would be the area between and including my feet as marked by the red line on the adjacent image under the feet.

My centre of mass is represented by the big red dot just above the hip.

The line of force is the line of gravity as it acts down through my centre of mass to the ground. 

The centre of mass is about or just below my tummy button (in males it’s about 57% of their height and females about 55%)

 If I were to tip off balance then the axis would be where my feet contact the ground, that is the rotation would be around the fixed point, my base of support in this case.

As long as I keep my centre of mass over my base of support then I can stay balanced.  If my centre of mass gets outside my base of support I won’t be able to stop the rotation around the axis and I’ll tip off balance.

Where the load is within my base of support will depend on where my body parts are.  For example the adjacent image looks at a person doing front raises.

You can see here that raising the weight up is putting more mass forward of the body and has shifted the centre of mass to the front of the base of support. 

As the centre of masses moves forward the weight will be felt through the forefoot and a number of supporting muscles in the spine will be working extra hard to stop the body tilting forward as the load is moved. 

In the image to the right you can see the front raise back in the starting position again.  With the centre of mass directly over the base of support, the weight will be felt back in the middle of the foot.

Doing heavy two armed or barbell front raises is not recommended because the load you are lifting in front of you will increase and often move your centre of mass outside of the base of support.  To compensate you will need to change your posture and put more of your mass behind the base of support (for example sway backward and lean back when the weight is raised) thus bringing your centre of mass back over your feet.  This is where you will see swaying or people starting the exercise with the body weight seemingly on their heels.

The other option to manage the load is to change the shape of the base of support such that the centre of mass can stay within it. 

For front raises this would involve putting one foot in front of the other which would mean the centre of mass could move forward and backward within the base of support during the exercise.

Why not always have the biggest base of support possible?

It’s common for exercises to be manipulated so that more load can be lifted without the exerciser losing their balance and tipping over.  But, before you change the base of support of an exercise you must first re-visit some fundamentals relating to movement.

Remember, movement patterns are created and refined all the time through repetition.  One underlying objective for most training is to create and reinforce safe long term movement patterns.  That means reinforcing the correct posture, ensuring inner (deep stabilising muscles) and outer (superficial moving muscles) units are working together and ensuring the load is not beyond the person’s ability to stabilize in a good postural position.

Also remember, that when the muscles fatigue the brain will work feverishly to try and make life easier and quite naturally will want to cheat on safe technique.  A personal trainer’s job is to condition clients to stay in the safest possible posture when they are tired – anyone can stay in a safe position when they are fresh.

So, before you change the base of support of an exercise to allow more load to be lifted you must consider whether any change in technique will compromise the best overall posture your client could be in.

Here’s an example.  The barbell bicep curl (suns out, guns out) can be done with a flat stance (feet side by side) or a split stance (one foot in front of the other).

This is the flat stance bicep curl.

1.      You can see the centre of mass (COM) starts over the pelvis and shifts as the load moves out from the body.

2.      Once the line of force is outside of the base of support the person will rotate over their feet (ie tip over) as an axis of rotation is created.

3.      To prevent this the person has to re-distribute some of their mass backward to move the COM back over the base of support.

4.      They usually do this by leaning back, which then loads the lumbar spine (usually L3/L4) with significant forces.

Here you can see the split stance bicep curl.

1. This time the person has solved the COM problem by changing the shape and size of their base of support.
2. In doing so the line of force stays within the base of support and there is no need to lean back.
3. However, their lifting posture is poor.  In order to place one foot in front of the other they will need to rotate the pelvis which will rotate the lumbar spine and counter rotate the cervical spine.
4. They will also be pulled into trunk flexion from the shoulder girdle as the load at the shoulder joint won’t be able to be held in position despite them being able to maintain their standing balance.
5. Under high load the brain is now learning a very dysfunctional posture – one which it will learn to adopt whenever it is heavily loaded blindly presuming this is the best way to deal with excessive loads.
6. Tissues will strengthen into this pattern over time and some tissues will lengthen and weaken to accommodate this.
7. Injury risk is high and gets higher over time with this approach.

Here is a flat stance bicep curl again.

 1.      This time the person is lifting the bar up there body allowing their elbows to move back and forward beside their body to achieve this.

2.      They are able to keep their COM within their base of support at all times.

3.      The loading on the shoulder girdle and pelvic area will be emphasized as fatigue sets in remembering the trainer will spot the tempo change and support and cue technique in those areas first as they know in order for technique to change stabilizers must first ‘let go’.

4.      This approach trains the brain to put the body in a healthy posture and maintain it during fatigue.  It also works the biceps fully and protects the shoulder joint from capsule loading which occurs when the arms are out in front of you with a significant load.