Ch. 8 - Basic Biomechanics

Unstable equilibrium

Occurs when only a slight force is needed to disturb an object. Ex: a person standing on one leg. Once balanced, it takes very little force to knock over the person.

Force couple

Occurs when two or more forces act in different directions, resulting in a turning effect

Friction

Force developed by two surfaces, which tends to prevent motion of one surface across another

Kinetics

Forces causing movement in a system

Inertia

Property of matter that causes it to resist any change of its motion in either speed or direction

Force

Push or pull action that can be represented as a vector

Scalar

Quantity that describes only magnitude

Parallelogram method

Forces represented as vectors. First, draw in vectors for the two forces (solid lines). Second, complete the parallelogram using dotted lines. Next, draw in the diagonal of the parallelogram (middle line and arrow) which represents the resultant force.

Linear force

Results when two or more forces are acting along the same line

Resistance aka load

Must be overcome for motion to occur and can include the weight of the part being moved (arm, leg, etc.), the pull of gravity on the part, or an external weight being moved by the body part

Gravity

Mutual attraction between the earth and an object

Law of Inertia

Newton's 1st law of motion that an object at rest tends to stay at rest, and an object in motion tends to stay in motion - known sometimes as this law because inertia is the tendency of an object to stay at rest or in motion

Law of Acceleration

Newton's 2nd law of motion that the amount of acceleration depends on the strength of the force applied to an object

Law of Action-reaction

Newton's 3rd law of motion that for every action there is an equal and opposite reaction

Lever

Rigid and can rotate around a fixed point when a force is applied. Ex: a bone in the human body

Fixed pulley

Simple pulley attached to a beam that acts as a first-class lever with F on one side of the pulley (axis) and R on the other. It is used only to change direction.

Mechanics

Study of forces and the motion produced by their actions

Torque aka moment of force

The ability of force to produce rotation around an axis. Greatest when the angle of pull is at 90 degrees, and it decreases as the angle of pull either decreases or increases from that perpendicular position.

Mechanical advantage

Number of times a machine multiplies the force. The load is supported by both segments of the rope on either side of the pulley so it has a MA of 2. It will require only half as much force to lift the load because the amount of force gained has doubled. Although only half of the force is needed to lift the load, the rope must be pulled twice as far. In other words, it is easier to the pull the rope, but the rope must be pulled a much farther distance. The human body has no example of this.

Parallel forces

Occur in the same plane and in the same or opposite direction

Vector

A quantity having both magnitude and direction

Velocity

A vector that describes speed and is measured in units such as feet per second or mph

Mass

The amount of matter that a body contains

Law of Acceleration example

Acceleration is inversely proportional to the mass of an object. If you apply the same amount of force to two objects of differing mass, the object with greater mass will accelerate less than the object with less mass. This can be demonstrated by first rolling a soccer ball, then rolling a bowling ball with the same amount of force. The heavier bowling ball will not travel as far.

Gravitational force

Always directed vertically downward, toward the center of the earth

Linear force example

Two people pulling a boat with the same rope in the same direction

Acceleration

Any change in the velocity of an object

Biomechanics

Application of principles and methods of mechanics to the structure and function of the human body

Dynamics

Associated with moving systems and can be divided into kinetics and kinematics

Statics

Associated with no moving or nearly nonmoving systems

Center of gravity (COG)

Balance point of an object at which torque on all sides is equal; it is also the point at which the planes of the body intersect

Parallel forces example

Body brace: forces X and Y are parallel to each other and pushing in the same direction, while force Z is parallel but in the opposite direction, pushing against them. The middle force Z must always between the two parallel forces, X and Y, to provide stability. If force Z was at either end, instead of in the middle, motion would occur.

Law of Action-reaction example

Can be demonstrated by jumping on a trampoline. The action is you jumping on the trampoline. The reaction is the trampoline pushing back with the same amount of force. This causes you to rebound up in the opposite direction that you jumped. The higher you jump, the higher you rebound.

Stabilizing force

Caused when nearly all of the force generated by the muscle is directed back into the joint, pulling the two bones together

Force aka effort

Causes the lever to move; usually muscular

Scalar examples

Common terms are length, area, volume, and mass with units such as 5 feet, 2 acres, 12 fluid ounces, and 150 pounds

Concurrent forces/Resultant force example

Two people pushing on a dresser at different angles to each other through a common point of application - can be shown graphically using parallelogram method

Law of Inertia example

Consider riding a car. If the car moves forward quickly from a starting position, your body pushes against the back of the seat and your neck probably hyperextends. Your body was at rest before the car moved, and it tended to stay at rest as the car started to move. If the car is moving and then stops suddenly, your body is thrown forward and your neck goes into extreme flex ion, because your body was in motion and tended to stay in motion when the car stopped.

Pulley

Consists of a grooved wheel that turns on an axle with a rope or cable riding in the groove. Its purpose is to either change the direction of a force or to increase or decrease its magnitude.

Force arm (FA) / Resistance arm (RA)

Distance between the force and the axis; distance between the resistance and the axis

Neutral equilibrium

Exists when an object's COG is neither raised nor lowered when it is disturbed. Ex: a ball rolling across the floor or a person moving across the room in a wheelchair; their COG remains the same.

Stable equilibrium

Occurs when an object is in a position where disturbing it would require its COG to be raised. Ex: when the widest part of a brick is in contact with the surface (BOS), is it quite stable. To disturb it, the brick would have to be tipped up in any direction, thus raising its COG. The same rules apply to a person lying flat on the floor.

Inclined plane

Flat surface that slants. It exchanges increased distance for less effort. The longer the length of a wheelchair ramp, the greater the distance the wheelchair must travel; however, it requires less effort to propel the chair up the ramp because the ramp's incline is less. Ex: if a porch is 2 ft from the ground and the ramp is 24 ft long, it would be easier to propel the wheelchair; if the ramp is only 12 ft long, it would be much steeper, requiring more force to proper the wheelchair. Rule: the advantage gained in force (decreased effort needed) is out in distance (longer ramp needed).

Third-class lever

Has force in the middle, with resistance and axis at the opposite ends. Ex: person moving one end of a boat either toward or away from a dock. The axis is the front of the boat tied to the dock, the force is the person pushing on the boat, and the resistance is the weight of the boat. If the person pushes close to the front of the boat, it will be hard to move the boat and the back will swing further away from the dock. Conversely, if the person pushes farther back on the boat, the boat stern won't swing away from the dock as far and will be easier to move. In this case, the RA doesn't change, but the FA does. When the FA is shorter, the boat is hard to push but moves a greater distance. When the FA is lengthened, the boat is easier to push but doesn't move as far. Essentially, any gain in distance is lost in power.

Movable pulley

Has one end of the rope attached to a beam; the rope runs through the pulley to the other end where the force is applied. The loads (resistance) is suspended from the movable pulley. The purpose of this pulley is to increase the mechanical advantage of force.

Kinesiology

How the body moves

Example of when no torque is produced

If the force is directed exactly through the axis of rotation. For example, if the biceps contracts when the elbow is nearly or completely extended, there is very little torque produced. This is because the perpendicular distance between the joint axis and the line of pull is very small.

Friction example

If you slide across a carpeted floor in your stockings, there will be so much friction between the two surfaces that you will not move very far. However, if you slide across a highly polished hardwood floor in your stockings, there will be very little friction and you will have a good slide.

Vector example

If you were to push a wheelchair, you would push it with a certain speed and in a certain direction

Kinematics

Involves the time, space, and mass aspects of a moving system and can be divided into osteokinematics and arthrokinematics

Resultant force

Lies somewhere in between the overall effect of concurrent forces

Arthrokinematics

Manner in which adjoining joint surfaces move in relation to each other - that is, in the same or opposite direction

Osteokinematics

Manner in which bones move in space without regard to the movement of joint surfaces, such as shoulder flexion/extension

Mass/Inertia

Mass is measure of inertia-its resistance to a change in motion

Effect of moment arm on torque

Moment arm and angular force are greatest at 90 degrees; as joint moves toward 0 degrees, moment arm decreases and stabilizing force increases; as joint moves beyond 90 degrees and toward 180 degrees, moment arm decreases and dislocating force increases

Torque example

Muscles within the body produce motion around joint axes

First-class lever

The axis is located between the force and the resistance. If the axis is closer to the resistance, the RA will be shorter and the FA will be longer. Therefore, it will be easy to move the resistance. If the axis is close to the force, the opposite occurs. Ex: Demo with ruler (force), pencil (axis), and book (resistance) where A is closer to R (long FA/short RA - easy to move since the resistance is moved only a short distance, and the force has to be applied through a long distance)) and A is closer to F (short FA/long RA - harder to move since resistance moves a longer distance, and the force is applied through a short distance).

Force couple example

The combined effect of the upper trapezius pulling up and in, the lower trapezius pulling down, and the serrated anterior pulling out, resulting in the rotation of the scapula

Axis aka fulcrum

The fixed point around which the lever rotates. Ex: the joint

Line of gravity (LOG)

The imaginary vertical line passing through the COG toward the center of the earth

Base of support (BOS)

The part of the body that is in contact with the supporting surface

Moment (torque) arm

The perpendicular distance between the muscle's line of pull and the center of the joint (axis of rotation)

Second-class lever

The resistance is in the middle, with the axis at one end and the force at the other end. Ex: wheelbarrow - the wheel at the front end is the axis, the wheelbarrow contexts are the resistance, and the person pushing the wheelbarrow is the force. If all the bricks are placed as close to the wheel as possible, there is a long FA and a short RA making the wheelbarrow fairly easy to move. If the bricks are moved to the end, the FA remains the same, but the RA is longer, making it more difficult to move.

Torque

The tendency of force to produce rotation around an axis

Torque example

The twisting force (torque) exerted by a wrench, which can be increased either by increasing the force applied to the handle, or increasing the length of the handle. Torque is also the amount of force needed by a muscle contraction to cause rotary joint motion.

Concurrent forces

Two or more forces must act on a common point but must pull or push in different directions

Wheel and axle

Type of simple machine; lever in disguise. Consists of a wheel, or crank, attached to and turning together with an axle. Essentially, a large wheel is connected to a smaller wheel and is typically used to increase the force exerted. Turning a larger wheel or handle requires less force. The opposite is true for a smaller axle. Ex: faucet handle. The handle is the wheel and the stem is the axle. Turning the faucet requires a certain amount of force made easier with a longer FA (wheel radius). However, take off the handle and you are only left with the axle, making it much more difficult to turn. In summary, the larger the wheel (handle) in relation to the axle, the easier it is to turn the object. Just like the lever-in which the longer the FA, the greater the force-the wheel and axle provides greater force with a larger wheel.

Determining a muscle's role (force or resistance)

Use the point of attachment to the bone, not the muscle belly, as the point of reference. When determining the resistance of the part, use its COG.

State of equilibrium

When an object is balanced and all torques acting on it are even

Dislocating force

When the angle is past 90 degrees because the force is directed away from the joint

Angular force aka movement force

When the angle of pull is at 90 degrees, the perpendicular distance between the joint axis and the line of pull is much larger, and most of the force generated by the muscle is directed at rotating, not stabilizing, the joint