L24 - Work & Power

Created by

Hailey Larsen

Cards (44)

Contracting a muscle to move an object:
Force explains movement
We must apply a force to move an object or mass
When we use force to move a mass, we call this work
We measure work:
W = F x s (Jules)
Units are kg m^2/s^2 or Nm but usually represented as Joules (J)
Contracting a muscle to move an object:
Work = force applied muscle thru displacement (movement/distance) of barbell
Have to have a force (F) and a displacement (s)
Only work while force is applied once removed is no longer work
Principles of Work:
A force can only do work if it causes a displacement
No displacement = no work
A force can do work only for the duration of the displacement
Only the force acting in the direction of displacement can do work
Still other forces acting but only force in that direction doing the work (is only 1 component)
W = F * s cos(𝜽) or W = l F * ds
No displacement = no work
Who is doing the work? What is it doing the work
To calculate work done we must define the system
W = F * s cos(𝜽) or W = l F* ds
Who is doing the work? What is it doing the work
Athlete vs bar (slightly different answer for both):
What is the work done by the athlete on the bar?
What is the work done by the bar on the athlete?
Would be different because - where force is directed
Athlete on incline (all force acting on direction of displacement)
But bar has other components of force such as gravity, have to take into account angle of athlete
Can work be negative?
If there is no movement - there is no work
There is effort but no work (isometric contractions)
Can work be negative?
Work may be positive or negative
Positive work = when object moves in direction of applied force
Force vector is in the same direction as the displacement vector
Moving with the work
Negative work = when object moves opposite direction of applied force
Force vector is an opposite direction of displacement vector
eg friction
Can work be negative?
When think in terms of muscle contraction:
Shortening/concentric moving in that direction (pos)
Eccentric is neg even tho force on muscle acting to shorten, acting away from contractile force (biceps)
Friction can apply neg work as acting against movement of object
Example: Net Work
Known:
FP = 600 N (force of person)
FR = 200 N (resistance force)
s = 3 m
Work of the person:
W = F * s cos(𝜽)
W = 600 N * 3 m * cos(0)
Cos(0) = 1 = pos direction of hz
Don’t have to do cos just shows if direction neg or pos (for hz direction)
W = 600 N * 3 m * (1)
W = 1800 J
Work of the resistance:
W = F * s cos(𝜽)
W = 200 N * 3 m * cos(180)
Cos(180) = -1 = neg direction of hz
W = 200 N * 3 m * (-1)
W = -600 J
Total work:
W = 1800 J - 600 J = 1200 J
Total work = each work summed together
The work done on an object may look different to different people:
If we have 2 people moving the same force through the same distance the amount of work is the same
Same work, same height/displacement, but 1 taking more time, need to account for that esp in performance
The work done on an object may look different to different people
W = F x s
Power measures the rate at which we do work
P = W/t (Watts) or P = (F x s) / t
Units Watts; W = work (F * s); t = time
Power = component important for athlete performance is the difference bw/ them
Displacement / time = velocity so P is also W x v (check)
Simple work power example
A 580 N person runs up a flight of 30 stairs of riser (height) of 25 cm during a 15 s period. How much mechanical work is done? How much mechanical power is generated?
Known:
F = 580 N
s = 25 cm (into metres) * 30 stairs
s = 0.25 m * 30 stairs
s = 7.5
t = 15 s
Work:
W = F * s
W = 580 N * 7.5
W = 4350 J (check)
Power:
P = W / t
P = 4350 J / 15 s
P = 290 Watts (check)
Power is ability to apply the highest force in the shortest time:
Ground reaction time for elite sprinters 120-160 m/s
Usain bolt <100 m/s
Power is ability to apply the highest force in the shortest time:
Power athlete = sprinter; because power = an important component
Although way produce power is important in conserving energy for marathon runner
Know how using power to be energy efficient
Power is limited by rate of ATP transition:
Power is limited by muscle contraction, how fast we can shorten a muscle, which is limited by ATP to release myosin head (cycle)
Limited in how fast can form cross-bridges
Little faster in fast twitch muscles, but still limited
However… Human power production has limitations:
Connecting cross-linkages & activating muscle fibres takes time
As load increases so does the number of active fibres
Due to the Force-Velocity relationship, Power is always limited
Maximum power is generated at approximately 1/3 of max shortening velocity
However… Human power production has limitations:
Force-Velocity tradeoff
High force = low velocity
Need more muscle fibres, which takes more time, more you activate at once can only do so many times until get to 1RM (very slow)
Low force = high velocity (15-20 RM)
Not using all muscles, lots of reserve, can cycle thru them quickly
As increase weight slow down as more muscle fibres needed
However… Human power production has limitations:
Tells us that have rate limited force production, limited by speed of contraction
Max power - if increase strength help increase power, only so far can get with velocity (better to focus on strength)
Rules for Power same as for Work:
Because power is derived from the work equation the same rules apply:
No movement = no work = no power
Measured in the direction of movement (velocity vector in rotation)
Power may be Positive or Negative
Instantaneous power - highest power value achieved during the observed movement
Average power - calculated for average force & average velocity forgiven movement
Time dependent waveform
Rules for Power same as for Work:
Because power is derived from the work equation the same rules apply:
No movement = no work = no power
We can’t calculate power for isometric contractions
& only thru the duration of displacement
Rules for Power same as for Work:
Because power is derived from the work equation the same rules apply:
Measured in the direction of movement (velocity vector in rotation)
Won’t have to do any angular velocity vector equations (note)
Rules for Power same as for Work:
Because power is derived from the work equation the same rules apply:
Power may be Positive or Negative
See this as producing (+) or absorbing (-)
Absorbing = normally where injury occurs
bc/ depends on how to train it
Rules for Power same as for Work:
Because power is derived from the work equation the same rules apply:
Instantaneous power - highest power value achieved during the observed movement
A particular moment in time
Force is not linear
Rules for Power same as for Work:
Because power is derived from the work equation the same rules apply:
Average power - calculated for average force & average velocity forgiven movement
Force is not constant, because is changing so is work & power; avg assumes that is same (under the curve)
Assumptions made
Rules for Power same as for Work:
Because power is derived from the work equation the same rules apply:
Time dependent waveform
Power is also time linked because associated with rate (waveform)
A time dependent measure
Displacement is a waveform → can plot it by time point, drawing a line (waveform) bw/ points
Changing as a function time
Power is a time dependent function
Means there is going to be a graph
Power Production vs Absorption"
The power produced by joints of the body can be determined by the product of the net torque & the joint angular velocity
Power = Tω (Torque x angular velocity)
We can derive joint power-time curve by multiplying our angular velocity & angular acceleration curves together
Inverse Dynamics Model
Power Production vs Absorption pt 1:
In rotation (won’t ask to do in exam) is Torque * angular velocity
Graph shows 1 rep, 1 torque over that rep, velocity changes (as different forces/easier or harder at points)
1 graph: flexion → extension
2 graph: angular velocity
Fast at start of lift, to initiate lift
Slow down as get closer to top of movement (flexion → extension)
Move quickly in neg direction, then slow down to stop at end of movement
Power Production vs Absorption pt 2:
1 graph: flexion → extension
3 graph: angular acceleration
Pos acceleration to get bar moving, deceleration because slowing down but still moving up
When move down increase acceleration (as velocity increases)
Slow bar down then have deceleration occur again
Power Production vs Absorption pt 3:
1 graph: flexion → extension
4 graph: angular power
Weather producing or absorbing power
Generating power at start (important)
2 negs (work & velocity) give pos power, to get it going gotta push it out at top of movement → down
1 of each (neg and pos) = neg power = absorbing energy
Power Production vs Absorption pt 4:
4 graph: angular power
Eccentric contractions absorb energy, when going down (triceps contracting), when get to bottom and stop (done by biceps)
At top of movement - triceps absorbs the energy/power produced at biceps
At bottom of movement - biceps absorb the energy/power produced by triceps
Phases of movement important for power = generation/initial (driving force)
Next phase = beginning of downward phase
Look at angular due to the joint
Power Production vs Absorption:
When both the angular velocity and angular acceleration have the same sign (either positive or negative) power will also be positive
It constitutes the energy flow from muscle to arm segment, and is referred to as Power Production
Power Production vs Absorption:
Power Production (+)
2 positives = a positive
2 negatives = a positive
A phase where generating power to cause rotation of segment
Power Production vs Absorption:
When angular velocity and angular acceleration have different signs (one positive and one negative) energy flows from the arm segment to the muscles and is referred to as Power Absorption
Think of it as a braking mechanism
Power Production vs Absorption:
Power Absorption
1 positive & 1 negative = a negative
Braking force
Power Production vs Absorption:
Acceleration (a = F / m)
Is related to the force & mass
Acceleration is the change in velocity
Acceleration is a proxy for force then for angular is a proxy of torque
With angular velocity tell us about power (as in mathematical equation)
Can also just seen it in the graph
Example: Risk of Hamstring injury during sprinting:
Pj = Tω
Power produced at hip & knee changes
Produce at hip, absorb at knee (conflicting action on hamstring)
So want a flexible hamstring
Example: Risk of Hamstring injury during sprinting:
Pj = Tω
Multiple joints:
One can be absorbing & one could be producing + field athletes who sprint (football, rugby)
Power absorption production at hip & knee for different running speeds (graph) pt 1:
Slower speed not much at hip
Absorbed at ankle the most during walking then knee, hip pretty much nothing
As speed increases: hip generating more power with little absorption mostly generation (what hip for); knee just after contact, absorption just prior to contact
When really fast: have little absorption but still mostly generating, just before ground contact largest production/drive at hip; huge absorption
Drive from hip acts a lot of force on knee joint
If don't control that damage knee + not efficient
Power absorption production at hip & knee for different running speeds (graph) pt 2:
So absorption by hamstring to slow knee down so don't slam knee forward (controls knee extension to avoid overextension + to control foot contact)
Greatest knee extension & hip flexion (hamstring @ greatest length) during just prior to ground contact - both feet in air