Metabolic cost of terrestrial locomotion Flashcards
Why are we so focused on forces?
Major source of metabolic energy cost
Source of injury
Can be limiting (top speed, turn radius)
Can’t be avoided, but muscle work can be minimised
- “external work” = work that moves the centre of mass
- “internal work” = work that moves part of the body but not the centre of mass
How does cost relate to ground reaction forces?
Average vertical force must support body weight i.e. take every value and compute an average
How are horizontal (fore-aft) forces balanced?
Must be balanced to maintain a steady speed
- deceleration in the 1st half of the stance (decreasing KE)
- acceleration in the 2nd half of the stance (increasing KE)
- muscles must actively contract to resist these forces - minimise forces to minimise energy cost
Faster speed - shorter contact time - higher force
How does GRF relate to energy?
Metabolic cost “metabolic effort”
Stance frequency (1/tc) “speed of leg cycling”
Cost coefficient Joules/Newtons “cost of force”
- key point to take from this = cost of the force is constant but rate of force is costly
Why not wheels?
Wheels allow constant forces & minimise acceleration and deceleration
- yet no known instances of true wheels in animal locomotion
Wheels could be a difficult mechanisms to evolve
Wheels only work well for movement over smooth surfaces
What are benefits of terrestrial locomotion on legs?
Require deceleration and acceleration within each stride causing fluctuations in mechanical energy
- good for dealing with unknown terrain
How do forces and energy cost relate to step length?
Collisional perspective
- collisions are the main source of energy loss and horizontal forces
collisions determine the external work at step transitions
increase with step length:
- minimised by effective ankle push off - why we have an ankle extensor muscle
- minimised by effective foot rolling
- can be an important factor in changing gait
How does cost relate to ground reaction forces?
Vertical ground force Fv - average vertical force must support body weight
Horizontal fore-aft Fh - balanced to maintain a steady speed - minimise horizontal forces to minimise energy cost cant be entirely avoided however.
What are the energy saving mechanisms of the slow and fast gaits?
Slow - inverted pendulum = walking
Fast - spring-mass (like pogo stick) - running, hopping
- minimise external work within stance but can’t eliminate step-step transitions
What are the energy saving mechanisms of KE and GPE out of phase?
Inverted pendulum
Exchange between KE and GPE
What are the energy saving mechanisms of KE and GPE in phase?
“spring elastic energy” between KE & E spring
- energy cycling within stance, slow gaits, inverted pendulum, walking, fast gaits: running, hopping
Collision reduction at step transitions
- ankle push off
- foot rolling (less in ungulates)
- gait change
How is oxygen uptake measured?
Exhaled gases sampled and pumped to an oxygen analyser
- small animals exercised in a chamber (chipmunk)
- energy demand measured in mL oxygen per minute VO2
How does metabolic energy rate increase with speed?
On a graph of rate of energy use (y-axis) and speed (x-axis) - cost of transport is the slope (gradient) of the line
- (energy cost per unit distance travelled)
How does body size affect cost of transport?
Mass-specific metabolic cost = VO2/Kg
Steeper slope = increased cost of transport i.e. a mouse
Mass specific cost of transport used to compare animals (energy used per meter travelled per kilogram body mass)
Larger animals have a lower cost of transport because they can travel a greater distance per stride and have mass-specific cost per stride
How does change in gait affect cost of transport?
Produces a tangent to the line
