Biomechanics

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Biomechanics Example: A book on the table has two forces 1. Gravity’s downward force = weight of the book 2. Table exerts an upward force = weight of book. Sum of net forces = zero, therefore the book is in equilibrium Primary forces which act on the human body are: Gravity Externally applied resistance Muscle Contraction Friction Force System ● Identification of all the forces acting on an object ● Linear force system = all forces are in the same line ● Parallel force system = two or more parallel forces acting on the same object but at some distance from each other. The lever system is this type. ● Lever = Rigid bar that can rotate about a fixed point when a force is applied (bones) ● Axis = fixed point about which the lever rotates ● Weight = resistance that must be overcome ● Force = usually muscular contraction in the human body ● Weight arm = (resistance) distance between the fulcrum and the weight ● Force arm = distance between the fulcrum and the force Lever Systems – 1 st , 2 nd , and 3 rd class First Class Lever ● The axis is located between the force and the weight (resistance) ● Designed best for balance ● Example: seesaw; atlanto-occipital joint Second Class Lever ● Axis is located at one end, resistance is in the middle and the force at the opposite end ● Designed best for power ● Example: wheelbarrow; rising up on toes; PROM Third Class Lever ● Axis is at one end, force is in the middle and the resistance at the opposite end ● Designed for ROM (mobility); most common lever system in humans ● Example: hinged door; normal muscle contraction Torque – when a force acts on a rigid bar (lever) it may cause a rotary motion around the fixed point (axis). Torque, or moment of force, is the measurement of the ability of a force to cause file:///C|/Documents%20and%20Settings/marsettt/Desktop/Mics%20saves/Biomechanics.htm (2 of 10)2/7/2007 3:30:21 PM
Biomechanics rotation of the lever. T = F x D T = Torque (foot pounds) F = Force (pounds) D = Distance (feet) – the length of a line drawn perpendicular to the action line of the force from the axis Torque is an important measurement because a force applied further from the point of rotation, or axis, requires less force to produce movement. Example: resistive exercise and body mechanics Mechanical Advantage – deals with the relationship between the length of the force arm and the length of the weight arm MA = Force Arm Weight Arm Mechanical advantage is related to the concept of torque. If F = W and dF is greater than dW, then F has the advantage The lever arm will move in the direction of the force (F) Increasing dF or decreasing dW will increase the MA of F Equilibrium in some ways can be said to be a stable situation. A joint is stabilized when the sum of the net forces acting on it equal zero Remember the importance of proximal stability to isolated joint motion and in utilizing body mechanics when we want to increase stability Gravity is probably the most important force. When you consider gravity, its point of application is called the Center of Gravity (COG). COG is a hypothetical point at which all mass would appear to be concentrated and is the point at which the forces of gravity appears to act. In a symmetric object COG is the geometric center In an asymmetrical object COG is toward the heavier end COG can be thought of as the balance point COG in humans is just anterior to S 2 (sacrum) in the anatomical position For an object to be stable, the COG must fall within the Base of Support (BOS) In the principles of body mechanics: ● The larger the BOS, the greater the stability of an object/person ● The lower the COG goes toward the BOS, the more stable the object/person Kinematics ● Concerned with the geometry of motion without regard to the forces acting to produce file:///C|/Documents%20and%20Settings/marsettt/Desktop/Mics%20saves/Biomechanics.htm (3 of 10)2/7/2007 3:30:21 PM
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Biomechanics

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