Calf Muscle & Ankle Joint Reaction Forces: Foot Up Calculation

Levers consist of a rigid bar pivoting around a fulcrum. Mechanical advantage depends on fulcrum position relative to load and effort. When fulcrum is in the middle (seesaw), no force gain occurs but direction changes. When fulcrum is at one end (wheelbarrow), force gain is achieved because effort arm is longer. When fulcrum is closer to effort (scissors), force is lost but distance moved is reduced. The mechanical advantage equals the ratio of effort arm length to load arm length.

Mechanical advantage for levers is calculated as MA = load/force = distance_force/distance_load. To increase mechanical advantage, increase the distance from the fulcrum to the applied force or decrease the distance from the fulcrum to the load. For complex systems, analyze from the load upward through each component. The total mechanical advantage is the product of individual mechanical advantages. When pulleys and levers are combined, the total mechanical advantage is the product of individual mechanical advantages. For example, a lever with MA = 2 combined with a pulley system with MA = 3 gives total MA = 6.

Levers are simple machines consisting of a rigid bar that pivots around a fulcrum. The mechanical advantage depends on the ratio of force arm to load arm. If force arm > load arm, force gain occurs (MA > 1). If force arm < load arm, distance gain occurs but force is lost. If force arm = load arm, there is neither gain nor loss. Examples include wheelbarrows (force gain) and pliers (distance gain). The fulcrum position determines which type of mechanical advantage the lever provides.

Lever systems work based on torque (force × distance from fulcrum). For equilibrium, clockwise torque must equal counterclockwise torque. Mechanical advantage equals effort arm length divided by load arm length. If the effort arm is twice as long as the load arm, the mechanical advantage is 2, meaning the effort force needed is half the load force. Changing radii proportionally does not change mechanical advantage.

This section covers lever systems and their mechanical advantages. Levers work based on the Principle of Moments. Mechanical Advantage = Load Arm / Effort Arm. First Class Lever: Fulcrum between effort and load (seesaws, scissors, crowbars); MA can be >1, <1, or =1 depending on arm lengths. Second Class Lever: Load between effort and fulcrum (wheelbarrows, nutcrackers); MA always >1, providing force advantage. Third Class Lever: Effort between load and fulcrum (sugar tongs, tweezers); MA always <1, providing speed advantage. Understanding lever classes is essential for analyzing simple machines and their applications in mechanical systems.

Stress is defined as force divided by the area over which it is applied (stress = force/area). Strain is the deformation or change in length of a tissue, calculated as the change in length divided by the original length (strain = ΔL/L₀). When force is applied to a tissue, stress causes strain, which can be measured as the change in length divided by the original length. The distribution of stress across a tissue affects its susceptibility to damage. When stress is concentrated on a small area, it can cause tissue damage even with relatively small forces. Tissues can undergo elastic or plastic deformation. Elastic deformation is reversible - the tissue returns to its original shape after the force is removed. Plastic deformation is permanent - the tissue does not return to its original shape. In the elastic region, tissues can be stretched or compressed and will return to their original shape when the force is removed. In the plastic region, tissues undergo permanent deformation when the applied force exceeds the tissue's elastic limit. When applied forces exceed the tissue's capacity, tissue failure occurs, resulting in rupture, tearing, or complete failure. Tissue failure can be classified into different levels: first-degree (partial tearing), second-degree (complete tearing), and third-degree (complete failure). The severity depends on the magnitude of applied forces relative to the tissue's capacity. Understanding tissue failure mechanisms is essential for injury prevention and designing appropriate interventions in sports medicine, rehabilitation, and ergonomic design.

This section explains fundamental tissue mechanics principles. Stiffness is the relationship between applied forces and resulting displacement (mm or degrees), while hypermobility cannot be quantified as it depends on applied force. Tension is the internal force tissue exerts to oppose external deforming forces. Stress is the internal force tissue supports to resist deformation. Deformation is the amount of change produced by force. Elastic deformation is reversible—tissue returns to original shape when force ceases. Plastic deformation is permanent—tissue does not fully recover. Fatigue explains why tendons and fascia cause pain after years of use, related to viscoelastic properties where force required to deform tissue decreases over time. Individual variability in viscoelasticity explains different patient responses to mechanical loads.

In biomechanics, stress is defined as force divided by cross-sectional area—applying small forces to small areas creates high stress. Strain refers to deformability or how much a material can stretch under force. Stiff materials experience high stress with low strain, while compliant materials experience low stress with high strain. This relationship between mechanical forces and tissue deformation is constantly evolving in the body, as tissues respond biologically to forces and change their stiffness accordingly.

This section covers the foundational principles of material mechanics. Elasticity describes how materials with cohesive molecular forces can deform and return to their original shape when external forces are applied. Stress is defined as force per cross-sectional area (measured in mega pascals), while strain represents the percentage change in length (calculated as (L2-L1)/L1). The video explains that tissues can deform and recover when stress and strain remain below ultimate limits, but fail and break when these thresholds are exceeded.

This comprehensive section covers the fundamental principles of tissue mechanics. The spring-dashpot model explains muscle mechanics through elastic (spring) and viscous (dashpot) components, where the spring stores and releases energy while the dashpot resists motion based on fluid viscosity. Tissue deformation occurs in two ranges: elastic (reversible) and plastic (permanent), with the transition depending on force magnitude and tissue integrity. Tissue compliance is determined by structural components including collagen fibers, which provide tensile strength. Younger tissues differ from older tissues in composition, affecting strength and flexibility. Tissue adapts to mechanical loading through remodeling, changing structure to better withstand forces. Stress relaxation occurs when tissue stress decreases over time under constant strain, as fluid redistributes and structural components rearrange. Loading rate significantly affects tissue response: rapid loading causes stiffer behavior with higher stress, while slow loading allows fluid redistribution resulting in lower stress.

Biomechanics applies engineering principles to the human body to analyze how forces cause injuries. A biomechanist examines injury causation by determining what forces and in what directions were needed to produce specific injuries like fractures or concussions. The goal is to understand injury mechanisms to prevent future accidents. This analysis involves examining how the body responds to forces in different environments, whether in motor vehicle accidents, sports, or industrial settings.

Biomechanics is the study of the mechanical principles underlying biological systems, including how forces cause injuries. The video shows how Dr. Smock used biomechanical analysis to determine that Emily's injuries were consistent with strangulation by hands rather than hanging. He explained that a ligature (such as a USB cable) would not create the specific pattern of injuries observed, as it would not be wide enough to cause simultaneous fractures at multiple points on the neck.

This segment covers the biomechanical analysis of Odell Beckham Jr.'s knee injury. Key concepts include: (1) Non-contact ACL injury mechanisms involve foot position and knee movement patterns; (2) The classic ACL tear position features externally rotated foot with inward knee collapse (valgus), but internal foot rotation limits this risk; (3) Hip rotation affects knee biomechanics and can create visual illusions of knee caving; (4) ACL tears cause rapid joint effusion (swelling) that makes examination difficult after 5-10 minutes; (5) Medical staff perform field exams to assess ACL integrity and other knee structures; (6) ACL tears should always be in the differential for non-contact knee injuries with visible movement.

This segment covers biomechanical principles (biomechanics studying movement and forces on living systems, lever classes in human movement), injury classification and first aid (abrasions from rough surfaces, lacerations from sharp objects, dislocations, green stick fractures in children), and recovery methods. Students learn about proper terminology, injury identification, and appropriate responses to common sports injuries.

Biomechanics is the application of engineering principles to the human body to understand how forces cause injuries. Biomechanists analyze how much force is required to produce specific types of injuries, such as skull fractures or soft tissue damage. This analysis involves examining the injury mechanism, the forces involved, and the body's response to those forces. The goal is to determine whether an injury could have occurred under specific circumstances, which is important for both legal cases and injury prevention.