Biomech Exam 2 Study Guide Pt. 1

Law of parsimony

We want to be efficient with energy. Conserve energy via efficiency. Used in persistence hunting (when we walked things to death)

Different types of work

external work (effect objects) and internal work (effect self)

External work equation

F*d/t

Fractional approach

cut the body up into little segments and calculated individual mechanical energy done in each part, then added it together.

Source approach

Determined work at a joint as the integral of the product of joint moment and angular velocity over time.

Problems with fractional and source approach

How do you account for eccentric contractions? How do you compute work for muscles that cross over multiple joints? Doesn’t take into account the energy expended to overcome antagonistic activity of muscles, for elastic and nonelastic internal forces (ex: ligament extension). Also- we have features like pendulums that work in our body and mess with physics.

What did Cavagna discover?

Muscles have spring action- when you are moving a muscle you are storing and releasing energy in the muscle. Also- pendulum-like behavior.

What did Winter discover?

transfer of mechanical energy between adjacent and non-adjacent segments. EX: if I do a calf raise I transfer energy to be upper legs, abs, neck, ect.

Workless examples in physics

Static forces. A force acting perpendicularly to the displacement (moving water bottle away from you). VGRF doesn’t actually do work on the performer, internal process do.

Work of a muscle

muscle force*muscle length change. Eccentric contraction= negative work

What two kinds of energy occur during eccentric contractions?

1- to the muscle, external force on muscle

2- the muscle spends energy to resist against external force

Energy in biomechanics

energy represents the capacity to do work and exert muscle force.

Pendulum in body

Pendulum: transformation of KE of body segments into the gravitational potential energy. Body is not an ideal pendulum- during walking KE of our legs far exceeds PE, but the trunk behaves like an ideal pendulum. The legs exert force, increasing KE, but the COM doesn’t change up and down when you walk faster

Ideal pendulum

the total mechanical energy is constant, kinetic and potential energy change exactly out of phase.

Pendulum equations

One way to estimate the possibility of energy conservation during walking is to compare the max values of changes of KE and PE in a part of the legs or the COM during a cycle. If the two values (part of leg and COM), are different, energy must be supplied by external forces. *when the speed of a body link doubles, its KE quadruples (quadratic function)

Intercompensation

two-joint muscles. EX: when sprinting, the hip flexes and the knee flexes. The rectus femoris crosses both joints, and causes hip flexion and knee extension. Since it is concentrically working to flex the hip, but eccentrically working against hip flexion. Therefore, what is the RF actually doing? Problem when calculating work

tendon action of two joint muscles

idea that muscles don’t contract, they just hold isometrically while tendons stretch. This leads to movement. It can save us energy and enhance performance. ???????????

Deformation of tissues

Energy recuperation/compensation- when kinetic energy from deformation of tissues is turned into potential energy, and vice versa. When a muscle develops force eccentrically, the external force does positive work on the muscle. This energy can be stored temporarily as elastic energy and then recoiled and used later. Tissues=big rubber band

Recuperation

The muscle acts like a spring→ eccentric load provides energy and is stored. EX: if eccentric force= -10 J and recoil muscle force=15. Together that only equals 5 J of force needed.

Non-recuperation

cannot passively absorb the force, but expends energy to resist it. EX: -10 from external force + needs 15 to move up. Total 25 J of work needed.

Torque

a force that produces a turning effect.

Centric force

act through the center of gravity of an object and cause linear translation.

Eccentric forces

NOT MUSCLE CONTRACTIONS- do not act through the center of gravity. They cause both linear and angular motion.

Force couples

create angular motion. Eccentric forces that act in opposition directions, spins object around.

Torque equation

T=F*ma (ma=moment arm)

External torques in human movement

loads and gravity provide the external force in most human movements. Act at some distance from out COM, or from the joint center they are acting around. External loads impart torques upon us.

1st class lever

create HUGE forces, but the distance is really small. EX: car jack. Large mechanical output. 

2nd class lever

kind of in the middle, output forces are moderate and distance is moderate. EX: wheelbarrow

3rd class lever

output forces are weak but distance is HUGE. Really good for fast movements. Small mechanical advantage. Most joints, in human body we sacrifice MA for ability to move light loads with high speed and distance.

Mechanical advantage equation

F output/F input

Center of gravity equation

Sum of weight force*moment arm= sum of the weight*R(center of gravity)

COG and stability facts

Stability is dependent upon height of center of gravity, size of base of support, and the weight of an object.

Low man wins (lower center of gravity)

Why canes are helpful (widen base of support)

Why you sit down or take a large step you feel like you’re going to fall (COG goes over BOS).