Mechanics4Pods

Hello again guys If you click the link you can view a power point presentation about Levers, forces and moments and how they must always balance or sum to zero. This is the real mechanics of biomechanics and allows us to estimate the forces in tissues of interest. When we know the forces then we can estimate the stress in a tissue of interest. Stress if the force in the tissue or structure divided by its cross sectional area. We're not looking at stress yet but knowing how to work out or intuitively estimate the forces in the tissues of interest during an activity and how to alter those forces to reduce stress and potentially reduce pathology is very useful and the core of the 'Tissue Stress' method of MSK podiatry. Tissue stress method is not a collection of techniques but rather a coherent, logical progression of mechanical principles applied to the body in normal and pathological conditions. Anyway to find out how to balance the levers, forces and moments go her...

Torque, Moments, Axis and Levers - now this is the meat of podiatric biomechanics. Torque and moment are used to describe rotational force, i.e. force applied to a lever about some axis of rotation. Moment is the more usual term in biomechanics and moments of force are in the units of Newton.meters Nm. Why? because Newton is the basic unit of force and metre is the measure of the length of the lever. So the unit of mass is kilogram and gravity (on earth) acting on 1kg is equal to 9.87Newtons, why? Because as we've seen before the acceleration of gravity is 9.87m/s^2 and if you remember Force = Mass x Acceleration (f=ma) Example: a lever 500mm long with a force applied of 250N results in a moment of 125Nm Simple enough of course. There are classically 3 types of lever, 1) force-fulcrum-load, 2) Force-load-fulcrum 3) Load-force fulcrum and they can all be seen in the body (https://www.slideshare.net/spanglerscience/muscle-leversppt-presentation  Although a lever is a ...

Hi again, I'm pushing this understanding of moment of inertia and the torque required to overcome it because it is important in biomechanics.  If you think about it every movement involves levers and pendulums associated with Rotational motion of a mass. The leg about the hip, the lower leg about the knee the foot about the ankle, the hallux about the 1st MPJ, the whole body Centre of Mass travelling over the planted foot. While several bodies might have similar mass, the distribution of that mass has a great influence on the effort required by the muscles to overcome the inertia of the mass in rotation. We don't need to use maths in our daily practice but I think it will help to see the maths to get a more intuitive picture that we can use daily to estimate or visualize what's happening to our patient. So we've seen that a mass has inertia that needs to be overcome by a force to induce motion. In linear problems the inertia can be found by the equation of Newtons...

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 inde...

Sorry to scare you but as we've just taken a look at inertia I thought I'd do this little snippet on 'moment of inertia' its important and useful in biomechanics, why? Well I'll tell you: As you'll remember, inertia is a body's resistance to change in velocity and inertial force is the ficticious force that manifests in the presence of acceleration, proportional to the acceleration ( times the mass) but in the opposite direction. Considering the inertia in this 1st case it is often quite convenient to just ignore inertial force. So say you want to push something in a straight line, like a sledge across the ice with your fat mate sitting on it. The sledge and your mate weigh 200kg and you apply a force of 500N to the centre of mass then the acceleration will be what? Get out the handy equation f=ma - rearrange to find the acceleration a=f/m plug in the numbers we know 500/200 and viola a=2.5m/s/s. After a couple of seconds you stop pushing and he's goi...

Thinking about what I should start with?? Well of course, the root of mechanics    Newtons 3 laws of force and motion . 1) A body remains at a constant velocity unless acted upon by an external force Remember that velocity is displacement (movement) in a direction and that is the direction of the force that was initially applied to start the movement When a body has velocity and mass it has Momentum  Momentum keeps a body going in a given direction until acted upon by an outside force. NB: Assuming a frictionless vacuum, because friction and air resistance are forces 2) Force equals mass x acceleration or f=ma ---- which also means f minus ma = 0 Which is what law 3 says 3) Every action (applied force) has and equal and oposite reacton (resisting force). So the sum of the forces is zero How then can movement occur if all the forces are equal and opposite? READ ON! The n...