We talk about bodyweight and body mass interchangeably.
Strictly speaking, mass is how much of us there is.
Weight is the force that our mass exerts on the ground.
Stood on planet Mars our mass is the same.
There's still the same amount of us.
But we weigh half what we do on Earth.
The body mass of John, Mr Jones, George and Iain was each over 120kg.
On planet Earth or planet Mars.
My schoolboy physics describes their mass accelerated by Earth’s gravity (a little under 10m/sec
2) with a force or weight of about 1200 Newtons.
Newton, the same dude who came up with the 3 laws of motion. And who described planetary motion to boot.
But I find it easier to think and write ‘120kg of force’.
That’s the force that John, Mr Jones, George and Iain put through the ground.
And at first read you'd think that's what they put through their joints...
More or less?But do they put more or less than that through their joints?Â
The answer is, not that intuitively, a
LOT more.
Let’s look at the hip...Bodyweight does transmit through the hip joint.
But the muscular pull that balances our bodyweight must also be factored in.
That muscular force also transmits through the joint.
Muscular forces that we don’t much think about. So...
Balance is key to good joint healthThe gluteal muscles of our bum lift our leg out to the side
But they also hold our body to the leg we stand on.
‘Oh, that’s what my bum muscles are for...’ I hear you say.Â
If our bum muscles are weak, our body drops away from that leg...
...and we limp.
Â
How much extra joint force?Well, those muscles pull a lot closer to the hip.
A lot closer than the centre of our mass where gravity pulls down.
The big gluteal muscles of our bum pull near to the hip joint
Whereas our bodyweight acts through the midline of our body.
