Biomechanics Week 7.docx

Biomechanics Week 7 Lecture 1: Musculoskeletal Mechanics (ligament and tendon) Tendons and ligaments are dense connective tissue containing 70% water, 25% collagen fibres and 5% elastin. Tendons: parallel collagen fibre arrangement, carry high tensile loads and are stiff but have minimal resistance...

Biomechanics Week 7 Lecture 1: Musculoskeletal Mechanics (ligament and tendon) Tendons and ligaments are dense connective tissue containing 70% water, 25% collagen fibres and 5% elastin. Tendons: parallel collagen fibre arrangement, carry high tensile loads and are stiff but have minimal resistance to compression and shear loads. Ligaments: nearly parallel collagen fibre arrangement, can carry high tensile loads and are less stiff and slightly weaker than tendons. Both have similar microarchitecture. Factors influencing biomechanical function: aging, pregnancy, immobilisation, diabetes, haemodialysis and NSAIDs.   Ligaments Role of ligaments is to provide connect from bone to bone. These are important to look at mechanical and it is the site of transmitting energy to the bone from the muscle. Ligaments can stretch about 8-10% before failure. Bone can only stretch up to 1% Increase mechanical stability of joints Guide joint motion and prevent excessive motion They have viscoelastic behaviour and control the dissipation of energy. Ligaments respond to loads by becoming stronger and stiffer over time, demonstrating both a time-dependent and non linear stress-strain response. Yield point is about 3% of strain, failure/breaking point is 8% Collagen fibres in a ligament are arranged so that it handles both tensile and shear loads, however ligaments are best suited for tensile loading.   Ligament and Injury At the end of the ROM for every joint, a ligament usually tightens up to terminate the motion Ligaments provide passive restraint and transfer loads to the bone (like a seat belt) A ligament can be subjected to extreme stress and damage while overloaded when performing the role of restricting abnormal motion. Injury can be a result to overloaded movements, repetitive poor loading over time, excessive stretching of the ligament or due to an unstable joint. Immobilisation changes the biomechanical behaviour of a ligament. Becomes more elastic but doesn't keep the same level of stiffness. It's yield point is lower than a healthy ligament. Strain Injury Tensile injuries to a ligament Common in adults Tend to occur in high speed conditions Has a 1-3 grading system Avulsion Injury Tensile load causing a bone fracture More common in kids due to growing bones that they're bones are softer This is when a piece of bone breaks off.   Knee ligaments Most common ligament injury The ACL and PCL make the four bar linkage system for the knee joint. During extension and flexion, the ligaments pivot relative to each other but still keep tension. They keep the bones rotating over each other. The knee joint doesn't have a set centre of rotation, it has an instantaneous centre of rotation that moves and stays with the cross over point of the ACL and PCL The shape and tension of the ACL changes from extension to 90 degrees of flexion. When the knee is extended the MCL is taught on both sides to resist valgus and tibial rotation, however when the knee flexes, the posterior side relaxes and the anterior side stays taut. These ligaments have a key role in keep the menisci in place within the knee to sustain shock absorption.   ACL Rupture Common mechanisms Valgus loading in combo with external tibial rotation Knee hyperextension with internal tibial rotation Anterior drawer mechanism. When the tibia is forced forwards relative to the body When loaded at a fast 66% of failure was ligamentous (loading in the ligament). When loaded at a slow rate, 57% was due to tibia avulsion.     Tendons Attach muscle to bone Transmit tensile loads to bones to let us move Help us maintain posture. Large collagen content and are quite strong, relatively stiff. Can act as a dynamic restraint. Failure point is 8%   Achilles Tendon Its ultimate strength is about 350N in cadavers, so in a living person, it is expected to be more. Tendons are also responsive to hormonal changes, the use of the contraceptive pill and pregnancy change the strength of it. Pregnancy reduces stiffness Tendons respond to eccentric training. Hysteresis: 2.5-10%. The amount of energy between when it's loaded and stretched and then when it releases the energy and returns to its resting. Its ability to store energy is limited except when loaded in large forces. Has high tensile strength   Tendon to Bone attachment End of the tendon The collagen fibres intermesh with the fibrocartilage The fibrocartilage gradually becomes mineralised fibrocartilage Merges into cortical bone.     Lower limb strain injury Common in many sports. Calf injury is common in running and sprinting based sports. Thoughts around it is that it's due to acceleration and rapid breaking Lecture 2: Mechanical Problem Solving Methods Movement phases: sequential approach to problem solving. More qualitative or semi quantitative. Mechanical approach: deterministic model. Concept map looking at the primary performance factor and then breaks it down into further sub categories. Mechanical approach: Free Body Diagram. The aim is to reduce the complexity of a mechanical analysis. It defines the extent of the analysis and identifies the significant forces involved in the action. usually uses a version of a stick figure along a set of coordinates, with the forces added as arrows. The stick figure indicates the system involved in the analysis. The forces are magnitude, direction, line of action and point of application.   Types of Forces on the FBD External: weight: combo of mass and gravity (mass x gravity), the vector direction is always down towards earth, originates from the body's COG and that is determined through an analysis of body segments. ground reaction force: reaction force from interaction with the ground. Derived from Newton's 3rd law, usually measured by a force platform. A normal force is perpendicular to the surface. fluid resistance: both human and projectile motion can be greatly affected by this. FR can be split into lift and drag components. Can occur in swimming, cycling and sprinting. You need to account for air or water resistance. Internal: joint reaction force: net force generated by bone on bone contact between adjacent segments when loaded. Loads caused contact forces from muscle, ligaments and bones that are exerted across a joint. These forces are calculated as they difficult to measure. They often act solely on a joint centre so rarely contribute meaningfully to joint torque. Harder to measure. Need to use inverse measures. muscle force IAP: transmits forces from the muscles encasing the IA cavity to the supporting structures f the trunk. Muscles include the TA, RA, obliques and diaphragm and pelvic floor muscles. Voluntary pressurisation is often referred to as the Valsalva manoeuvre.    FBD - Statics A body or object at rest or when moving at a constant velocity Acceleration = 0 Sum of all forces = 0   Inverse Dynamics Modelling technique Kinematic data > kinematic model > dynamic model Kinetic data > dynamic model Anthropometric data > anthropometric model > dynamic model Uses external forces to estimate internal ones.   Advance techniques More invasive that we don't learn about. Skin pin markers. Surgical placed into the underlying bone