Biological/Structural Bases

1. Biological/Structural Bases Mechanics of the Musculoskeletal System

2. Tissue Loads • When forces are applied to a material they create loads • Axial loadings • Combined Loads

3. Axial Loadings • Compression • External Forces tend to squeeze the molecules of a material together • Tension • The load acts to stretch or pull apart the material • Shear • Right-angle loading acting in opposite directions • Loads are not individual forces, but loads that result from forces from both directions

4. Combined Loads • Torsion • Twisting of the material • Bending • One side of the material is loaded in compression while the other side experiences tension loading

5. Response of Tissues to Forces • Immediate response of tissues structure will change • Size and direction of forces • Mechanical strength and shape of the tissue • Mechanical strength and muscular strength are different • Mechanical Variables that explain how musculoskeletal tissues respond to forces or loading • Stress • Strain • Stiffness and Mechanical Strength • Viscoelasticity

6. Stress • How hard a load works to change the shape of a material is measured by mechanical stress • Defined as force per unit area within a material • Calculation • Similar to the concept of pressure • Tensor quantity • Generalized vectors that have multiple directions accounted for

7. Strain • Measure of the deformation of a material created by a load • Expressed as a ratio of the normal or resting length • Calculated by change in length divided by normal length • Rubber Band • Which can withstand greater tensile strain • Tennis racket, golf club, or diving board

8. Stiffness and Mechanical Strength • Measurement of force and displacement of a material as it is deformed at various rates • Load-Deformation Graph • Elastic Region corresponds to Stiffness • Stiffness- ratio of stress to strain in the elastic region of the curve

9. Stiffness and Mechanical Strength • Plastic region- increases in deformation occur with minimal and nonlinear changes in load • When deformed past the yield point material will not return to its initial dimensions • Biological materials • Normal loadings happen in elastic region • Deformations near and beyond the elastic limits are associated with microstructure damage to the tissue

10. Stiffness and Mechanical Strength • Mechanical Strength- measurement of the maximum force or total mechanical energy the material can absorb before failure • Yield strength • Rehabilitation • Ultimate strength • Largest force a material can withstand • Failure strength • Total amount of energy

11. Biological Tissues • Anisotropic- strength properties are different for each major direction of loading • Nature of protein fibers and amount of calcification all determine the mechanical response • Most soft connective tissue components of muscle, tendons, and ligaments have another region in their load-deformation graph

12. Viscoelasticity • Stress and strain in a material are dependent on the rate of loading • Timing of the force application affects the strain response of the material • High rate of stretch • High stiffness • Slow rate of stretch • Small increase in passive resistance (high compliance) • Silly Putty

13. Properties of Viscoelastic Materials • Creep • Gradual elongation (strain) of a material over time • Stress Relaxation • Decrease in stress over time when a material is elongated to a set length • Hysteresis • Property of materials to have a different unloading response than its loading response

14. Passive Muscle-Tendon Unit (MTU) • Passive stretching is viscoelastic • High rate of passive stretch- stiff • Slow stretch results in less passive tension in the muscle

15. Passive Muscle-Tendon Unit (MTU) • Tendon is the connective tissue that links muscles to bones • Much stronger than muscle tissue • Vascularized, parallel arrangement with cross-links • Great tensile strength • Rupture injuries are rare • Acute overloading of the MTU • Strains and failures at the muscletendon junction or the tendon/bone interface

16. Passive Muscle-Tendon Unit (MTU) • Tendons act as a spring in fast bouncing movements • Short tendons transfers force to the bone more quickly (less “slack”) • Intrinsic hand muscles are well suited for fast movements • Short tendons • Achilles tendon provides shock absorption and compliance to smooth out the forces of the large calf muscle group • Long tendon

17. Biomechanics of Bone • Primary loads are compressive • Response of bone to compression, tension, and other complex loads depends on the bone structure • Strength depends heavily on its density, dietary habits and physical activity • Immobilization and inactivity results in decrease of bone density, stiffness, and mechanical strength

18. Wolff’s Law • Bones remodel (lay down greater mineral deposits) according to the mechanical stress in that area • https://vimeo.com/143468200

19. Biomechanics of Bone- Structure • Cortical (compact) • Dense, external layer • Cancellous (spongy) • Less-dense, internal layer • “Sandwich” construction • Weakest under shearing loads

20. Osteoporosis • Positive stresses of exercise on bone density • Elite women athletes are higher risk for stress fractures • Stress fractures are small breaks in the cortical bone that result from physical activity without adequate rest

21. Biomechanics of Ligaments • Tough connective tissue that connect bones to guide and limit joint motion • Proprioceptive and afferent signals • Like bones, ligaments and tendons remodel according to stress they are subjected to • https://www.physio-pedia.com/Anterior_Cruciate_Ligament_(ACL)_-_Structure_and_Biomechanical_Properties

23. Female Athlete Triad Read and write a summary on the components of the Female Athlete Triad. Use the article posted on the Google Classroom Page and search for new information from other sources.

24. Stretch-Shortening Cycle (SSC) • Countermovement away from the intended direction of motion that is slowed down with eccentric muscle action that is immediately followed by concentric action in the direction of interest • This bounce out of an eccentric results in potentiation (increase) of force in the following concentric action if there is minimal delay between the actions

26. Neuromuscular Control • Muscle activation • The Functional Unit of Control: Motor Units • One motor neuron and all the muscle fibers it innervates • Activation of a motor axon results in stimulation of all the fibers of that motor unit and the resulting twitch • “All-or-nothing” response

27. Regulation of Muscle Force: Recruitment • Activation of different motor neurons within a muscle • 1) Motor units tend to be organized in pools or task groups • 2) Motor units tend to be recruited in an asynchronous fashion • Slightly different times to allow smooth out rise in tension • Too much synchronous recruitment causes problems (fatigue, parkinson's) • 3) Size principle (orderly recruitment) • Progressive recruitment from small (slow-twitch) to large (fast-twitch) • Derecruitment in reverse order • Larger motor units are not needed until maximum effort is required

28. Proprioception of Muscle Action and Movement • Muscle length- muscle spindles • Sensory receptors located between muscle fibers that sense length and speed of lengthening or shortening • Protect from stretch-related injury • SSC • Inhibition of antagonist muscle- reciprocal inhibition

29. Proprioception of Muscle Action and Movement • Force- golgi tendon • Tension developed from an activated muscle is sensed • Located at the musculotendinous junction • Inhibitory effect on the creation of tension in the muscle • Connect to the motor neurons and relax the muscle to protect from excessive loading

30. Summary • Forces applied to the musculoskeletal system? • Based on direction and line of action relative to their direction. • How hard forces act on tissue? • Mechanical stress • Tissue deformation? • Strain • Simultaneous measurement of force and deformation allows to determine? • Stiffness and mechanical strength • Bones are strongest in compression, while ligaments and tendons are strongest in tension

31. Summary • What is the stretch-shortening cycle? • Maximize initial muscle force in most movements with the rapid reversal of a countermovement • Creation of muscular force is controlled by recruitment of what? • Motor units • Proprioceptors provide length and tension information to the central nervous system to help regulate muscle actions • What is viscoelasticity? • Wolff’s Law