Friction in human movement

1. Friction in human movement • All movements contain relative motion of various parts of the skeletal system. • This creates friction, which requires work to resist, which in turn reduces the effectiveness of our muscles in transmitting force (and doing work) on the external world.

2. Friction • Defined previously, and calculated for your shoes. • Values for person standing barefoot on a wooden board - 0.8 • We need high values so that we don’t fall, can hold objects with our fingers (1.0), etc.

3. Reducing friction • We reduce friction in our joints by lubricating them, so that the surfaces will easily slide over each other.

4. Lubrication: manmade Oil Hydrodynamic lubrication rotating shaft = 0.2 dry metal surfaces = 0.01 with h.l.

5. Lubrication: natural • We need to lubricate our joints. • How do we do that? • Surfaces of bones that come in contact with each other are covered in cartilage: • One or two mm thick on the joints of our fingers • Five mm or more on larger joints found in hips and knees

6. Synovial Joint Articular Cartilage Synovial Fluid Capsule Bone Collagen Protoglycan

7. Collagen - 20% Proteoglycans - 5% Water - 75% (contains ions such as sodium and chloride) Cartilage: components

8. Collagen fibers attached to bone, but loop up to the surface Proteoglycans (combine proteins and polysaccharides) Proteoglycans have the property of attracting water, so as the water moves into the cartilage it swells But collagen, which is stiff, resists the swelling. Cartilage: organization/function

9. Cartilage/Synovial fluid lubrication: = 0.2 dry metal surfaces = 0.01 with h.l. = 0.008 for finger joints

10. Cartilage in action Remove force = 0.008 Collagen resists swelling = 0.008 Proteoglycans restore shape

11. Control force and displacement electronically Servo System Stainless Steel Plate Cartilage Temperature Controlled Bath

12. Stress Relaxation Experiments for Vertebrate Cartilage