Hmmm! I'm realising as I get into this blog writing, how difficult it is to be both useful in concept and be entirely correct without using advanced physics and maths that would be confusing.
So, the topic on CoM and moment of inertia was meant to convey the concept of how the effort require to rotate an object, in this case a leg, becomes greater the further away the CoM is from the axis of rotation, e.g. the hip joint, even tho the total mass remains the same. In fact the maths of that is I (moment of inertia) = mass (CoM) x radius squared. Which means that the effort required to rotate (change its angular velocity) an object increases by the square of the radius of rotation of the centre of mass about the axis of interest.
This is demonstrable practically: take a sledge hammer and hold it at the head, its fairly easy to hold up. But, hold the sledge hammer at the far end of the handle and extend it so the handle is parallel to the ground and then it becomes very difficult indeed to hold it up.
Why!
Because the hammer head represents most of the mass so in the 2nd condition the CoM is a long way from the rotational axis ie your wrist. The radius of rotation is much larger on the same CoM position. The muscular effort required is much greater to stop the rotation because the CoM has a greater lever and the mass is being accelerated (angular change in velocity) by gravity and you are trying to resist this acceleration in the opposite direction.
Look here for a good intuitive explanation
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