Kine305: Ch.2 Biomechanics

Friction Force

Force between 2 surfaces that increases resistance to motion of one surface on another (Skating, snowboarding is low action friction in order to glide)

Linear Force

2 forces acting on the same line

Parallel Force

Occur in the same plane and in the same or opposite direction (x and y working in same direction, z working in opposite)

Force Couple

Two or more forces act in different lines of pull or directions (produces CW or CCW rotation)

Concurrent Force

Two or more forces acting on an object, push or pull in different directions

Resultant Force Vector

Sum of magnitudes and directions of each individual force vector, results from all applications of all forces acting on the object

Traction

Distraction or tension (ex. at your elbow, there can be a little traction if carrying a heavy suitcase)

Compression

Forces that compact, squeeze, or press inwards on tissue, typically acting along the longitudinal axis of bones and joints. (ex. dislocated shoulders- planks create compression force within their joint to help with better muscle activation and stabilization via compression)

Shear

Parallel forces but in opposite directions so if I’m trying to improve glide of joint… I can do a shear (ex. take humerus head and glide it anterior (like jose said) to help with external rotation

Bending

Combination of compression on one side and traction on the other, like when we side bend through the vertebra

Rotary/Torsion

Twisting(torsion) around the long axis, 2 opposing forces create a twist within the object, one force turns one direction and the other the opposite (twisting and rotating(rotary) are not the same *twisting is opposite direction, rotating is same direction) (ex. tibial torsion - the femur twists outward while the lower leg twists inward)

Torque

Torque = F x D

Tendency of a force to produce rotation about an axis. How muscles create rotation around a joint (angular motion)

a twisting force that tends to cause rotation.

Moment Arm

Perpendicular distance between the line of application of force and axis

Longer Moment Arm

= Greater Torque

Force Arm

Where the muscle attaches

Practical applications of torque

Increasing exercise difficulty, choice of exercise can be influenced by understanding how torque affects muscle engagement and joint movement, lifting weights at different angles can change the moment arm and thus the torque experienced by the muscles

Newton’s 1st Law

Law of inertia states that an object either stays at rest or remains in motion in a constant state, unless acted upon by an external force (ex. person standing won’t start walking until muscles generate force agains the ground, muscles have to overcome and apply force or decelerate)

Newton’s 2nd Law

Law of acceleration defines the relationship between force, mass, and acceleration (f=ma) ex. kicking a soccer ball vs a med ball, difference in amount of force required to kick

Newton’s 3rd law

Law of action - reaction states that for every action there is an equal and opposite reaction ex. gravity, jumping we get off the ground but GRF with gravity down into the ground to create that motion, prof Osmar hitting the wall

Equilibrium

Is when the sum of all forces acting on an object is equal to zero, dependent on COM, COG, and BOS

What makes a stable vs. less stable equilibrium?

Smaller BOS, standing off balance, elderly, etc..

Postural equilibrium

Other muscles will start working because your axis (axis of rotation) created a larger moment arm, so created more torque and now working really hard to hold yourself, mechanical advantage goes down

Longer the moment arm

= greater the torque

Rectilinear motion

Same distance, same direction, at same time (straight line path, no rotation) ex. sled across ice

Curvilinear motion

Motion that follows a curved path ex. ski, flight of a thrown ball

Angular motion

Motion that occurs around a fixed point (axis) ex. knee extension during kicking happens around an axis and creating angular types of motion

What do most movements in our body have?

Most have linear and angular motion at the same time

Levers

Rigid bar that rotates around a fixed point (three classes of levers)

bone

lever

joint

axis

muscle

force

When does movement happen?

When the muscle torque overcomes the resistance torque (inverse relationship)

Bw/ gravity / weight

type of force but is over resistance

First class lever

Axis between force and resistance (F - A - R or R - A - F) ex. like a see saw, creating a balance FA=RA

Second class lever

Resistance between axis and the force (A - R - F or F - R - A) resistance is closer to axis which is creating a much longer force arm, ex. wheelbarrow has long force arm FA > RA

Longer force arm

Greater mechanical advantage

Key features of second class lever

Less effort to move a larger load but class 2 levers are more rare in the human body, ex. brachioradialis at elbow joint, ankle plantarflexors, ex. jar openers w/ longer levers (better mechanical advantage)

Third class lever

Force between axis and resistance (A - F - R or R - F - A), mechanical advantage: RA > FA so less mechanical advantage. they do create more speed/distance of movement but you have to create more force to do it. So RA is always longer which requires greater force to move the load but does lead to more motion ex. bicep

Can the class of the lever ever change?

Yes, just put a weight in your hand. change 2nd class to 3rd, increased load increases RA so less mechanical advantage bc RA gets larger. Now, exercise will be harder with weight in hand.

3rd to 2nd class

Change to eccentrically, weight of lowered forearm becomes applied, biceps brachiii now RA

Fixed Pulleys

Single pulley attached to a fixed point, creates a first class lever

Moveable Pulley

Combines a fixed pulley to change direction of force application, a moving pulley to change magnitude of force applied to lift a load (like a third class A - R - F)

Inclines plane

a longer ramp requires less force, a shorter ramp requires more force