Biomechanics Lecture Notes PDF

STABILITY  STABILITY Is the ability to maintain one’s balance in both static and dynamic situations without use of mechanical devices. FACTORS AFFECTING STABILITY: 1- Center of gravity (COG) height. 2- Base of support (BOS). 3- Relationship between LOG and BO...

STABILITY  STABILITY Is the ability to maintain one’s balance in both static and dynamic situations without use of mechanical devices. FACTORS AFFECTING STABILITY: 1- Center of gravity (COG) height. 2- Base of support (BOS). 3- Relationship between LOG and BOS. 4- Characteristics of supporting surface. 5- Segmentation principle. 6- Subject state. 1 - CENTER OF GRAVITY (COG) HEIGHT  1 - CENTER OF GRAVITY (COG) HEIGHT: The lower the COG, the greater the stability. For example:- 1- standing is less stable than kneeling as in kneeling the COG height is lower than standing. 2-Standing is more stable than standing on toes. 3-Wearing high heeled shoes decreases one’s stability because it displaces the COG upward. 2- BASE OF SUPPORT (BOS)  2- BASE OF SUPPORT (BOS): 1-Is the supporting area under body. 2-It includes the points of contact with the supporting surface and the area between them. 3- These “POINTS” may be body parts such as the feet or hands or they may be extensions of body parts such as crutches or other walking aids. 4- If a patient walks with sticks or crutches, the BOS is the area between the crutches and the feet. Effect of BOS: 1- increase the BOS will be associated with increase in stability. The upright human body is least stable when the feet are close together. As the feet are moved apart, the base is increased and the person becomes less likely to fall. A-Close Standing B-Step Standing C-Stride Standing With Crutch Or Cane 2- The increase in shape of BOS occurs in the direction of force being applied to body. When a force is applied from behind, the shape of the base is enlarged in the forward direction allowing greater stability. STEP FORWARD STEP BACKWARD 3- An increase in the BOS should be within limit ??? Relation Between Angle Of Step And Stability : 1-The angle of the step determine the stability. 2- The force acting at the stepping angle can be resoluted into vertical and horizontal component. 3- If the step length is small and its angle is 30 , the horizontal component (SHEAR )(X) is small. 4- When step length is longer and its angle is 60, X component (SHEAR ) increases. Excessive increase in the X component may cause slipping of the subject unless this component is opposed by frictional force.  EXAMPLES IN PRACTICE 1-The optimum angle for crutch placement is from 60 to 90° between the crutch and the floor.  EXAMPLES IN PRACTICE 2-Tripod and quad canes are designed to give larger base of support for the cane and so providing more stability for the patient. In these cases the slipping force reduces to a minimum value.  EXAMPLES IN PRACTICE 3-In early stage of reeducation of walking in patients, therapist give patient 2 quad or tripod canes at first in order to increase the BOS of patient and also to give patient adequate confidence to be able to walk. 4-Then as the patient is trained to walk, the therapist can give him 2 crutches and later on use one crutch. 3-RELATIONSHIP BETWEEN LINE OF GRAVITY AND BOS  Line Of Gravity Is a vertical line which passes through the COG and falls within the BOS. The line of gravity passes from the vertex, anterior to the 2nd sacral vertebra to just in front of the ankle joint and between the feet. For balance to be maintained, it is essential that the line from the COG should fall within the BOS. The nearer the line of gravity (LOG) to the center of BOS, the greater the stability. For maximum stability, the LOG should intersect the BOS at a point which allows greater range of motion. For Example: - In stride standing, LOG lies within BOS. - If a subject stands on toes, the LOG will become near to edge of BOS so the stability will decrease so, any push to the subject will disturb standing easier than if LOG is in the middle of BOS. 4- SUPPORTING SURFACE  4- Supporting surface A- Friction : The greater the friction between supporting surface and parts of body , the more stable the body. But, this friction should be up to certain limit then the friction will act as a disturbing factor limiting movement.  Examples: - Basketball or football players wears a rubber-soled shoes ???? to increase friction between his body and the floor so increasing stability.  Examples: - Patients use crutches with rubber ends ??? in order to increase friction between the floor and the crutch.  Examples: - The therapist places a blanket on the bed ???? to increase friction between parts of patient’s body and the bed. B- Softness of the supporting surface : In lifting up exercises, more energy will be wasted in case of using soft mattress. So, hard mattress should be used to increase stability and to conserve energy of the patient. C- Inclination of the supporting surface : Is the angle which the supporting surface makes in relation to the horizontal, it is also called the angle of friction. The greater the inclination, the less the stability. For example:- 1-in practice, this concept and the concept of friction are important for constructing outside ramps for wheelchairs and patients who are using crutches. 2-A wet ramp has a decreased friction and if this ramp is steep, wheelchair may not be able to go up the ramp. 3-The wheelchair or patient with crutches will not be able to descend the ramp safely. 5- Segmentation Principle  5- Segmentation Principle: Stated that " If there is deviation of a part of the body to certain direction, there is another compensatory deviation of another part of the body to the opposite direction to maintain balance during this position".  For example 1- When a person carries a heavy weight, the body shifts in the opposite direction in order to compensate. 2- If a weight is carried in one hand, the opposite arm is lifted to help counterbalance a very heavy load held in one hand 3-Carrying a heavy weight behind the back leads to a compensatory movement in the trunk forward to move the LOG to be within the BOS. 4-A load carried as near as possible to the midline of the body will minimize the necessary movements of the body segments and also muscle and ligament strain ???????????? 5-Any change in curvature of spinal column will be compensated by movement of other segment that leads to change in the alignment of the vertebral column. A-Cervical lordosis is compensated by dorsal kyphosis. B- lumbar lordosis is compensated by sacral kyphosis. 6-Any abnormal disturbance in curvatures of spine is compensated by another deviation. For example:- primary C- scoliosis if neglected will be compensated later on by secondary curves leads to S - scoliosis. 6- SUBJECT STATE  6- SUBJECT STATE: a- Mass b- Vision c- Physical and Emotional State d- Pain e- Age f- Speed a- Mass: The greater the mass of the body, the greater will be its stability. For example:- carrying weight in both hands will increase stability than in case of carrying a weight in one hand. b- Vision : A person has a greater balance and stability in locomotion when he focuses his vision on a stationary object rather than on moving or disturbing one. c- Physical and Emotional State : There is positive relationship between one’s physical and emotional state and the ability to maintain balance. For example:- a disabled person is less stable than a healthy one. d- Pain : Pain may decrease stability especially if the pain affects the lower extremities. e- Age : The person will be more stable in adulthood period than in childhood and the stability will also decrease in senile subjects ??????????????? due to the physiological changes occurring as a result of aging process. f- Speed: It easier to balance on a bicycle when it is moving fast than when it is traveling slowly. EQUILIBRIUM  EQUILIBRIUM Is a state of balance in which all forces are equal. A body is said to be in equilibrium when the resultant of all forces acting upon it is zero.  Types of equilibrium: Stable, unstable and neutral.  1-Stable equilibrium: If the object is displaced slightly and it tends to return to its original position, the equilibrium is stable. In this case, the COG raised before the line of gravity will fall outside the BOS. For example:- when you hit the doll , biomechanically the heavy weight located at the base of the doll, its COG is quite low. As you hit the doll it will be displaced but it tends to return to its original position.  2-Unstable equilibrium: If the object is displaced slightly and it tends to increase its displacement, the equilibrium is unstable. For example:- A cone balancing on its apex will tend to continue its displacement once it has been slightly displaced. The COG drops to a lower point when compared to the original starting position, as LOG falls outside BOS.  3-Neutral equilibrium: A body is in neutral equilibrium if it comes to rest in a new position without change in the level of COG either upwards or downwards when it is displaced. For example:- a ball.  Conditions of equilibrium: 1- If the body is at rest, with the velocity equaling zero, it is said to be in static or translational equilibrium. In this case the sum of all forces acting on the body equal zero, F = 0. 2-If the constant velocity is not zero, the equilibrium is called dynamic equilibrium.  Static equilibrium includes: 1-First condition of equilibrium as in linear force system, concurrent force system, and pulley systems. 2-Second condition of equilibrium as in parallel force system, general force system, and lever system.