BIOD43H3 Lecture Notes - Lecture 7: Ground Reaction Force, Duty Cycle, Ankle
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Slide 3
We tend to see that in a forelimbs of a horse there are greater peak in vertical ground
reaction force than we do for hind limbs as center of mass isn’t perfectly situated
Faster walking → Duration of time the horse is in contact with the ground is lower during
faster walking (as stride freq and duty cycle are lower with faster walking)
→ Peak gets higher in faster walking because you need to integrate out your body support
over shorter cycle duration.
But how does this change for a horse walking on a leveled treadmill (no change in center of
mass)
Slide 5
But how does ground reaction forces change when an animal is decelerating
→ on incline treadmill, horse must accelerate its center of mass and move up → ground
reaction force shifts reflecting this fact
Mass is shifted towards rear legs, thus shift in vertical ground reaction forces
→ Before, there is uneven distribution in horizontal distribution as fore legs are absorbing
more energy
Rear legs are doing more of the pushing.
In acceleration, both limbs have net positive forelimb reaction forces.
Slide 6
On the incline, there is a bit more vertical ground reaction force (not a strong difference as it
isn’t a horse running up) → comparing the traces with respect to the incline for hindlimbs
there is a noticeable greater area than thicker lines.
Slide 7
On the incline notice the peak ground reaction force when leveled, as it shows the center of
mass being shifted backwards a bit.
→ Shifting of horizontal forces
Slide 9
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For a lab mice, their knee is strongly bent.
Moving from a small animal to large, there is going from a crouched leg to nearly straight leg
with elephant
Slide 10
For hinged joints, one element of the hinge is rotating (thus is it’s a rotational force)
Moment = force around the joint (torqutional force)
Moment arm = how far away from the center of rotation that force is applied
Ground reaction force works to cause out joints to flex (knees, hips etc) if you allow your
center of mass to give into gravity.
Slide 11
Moment arm: perpendicular from long axis of the muscle to the fulcrum
Understand how the numbers relate to each other → IE: based on scenario A expect value to
go up or down
IE (bottom right corner): Fm is the gastrocnemius contraction, draw r signifies moment arm
of the muscle force action this is the perpendicular line next to the fulcrum on the left
→ Ground reaction force is pressing up from the center of the ball of the foot.
→ Ground is pushing vertical force and is directed straight up.
→ Now Big R is the moment arm from the ground vector to the fulcrum
if the joint isn’t moving, force from muscle x muscles moment arm = ground reaction force
from vertical dimension x its moment arm.
Slide 12
When you untighten a screw with your fingers it's hard to generate enough moment due to
the small moment arm
→ Instead using a wrench and applying force distance X away from the nut you wil have a
larger moment arm and apply less force.
The amount of force that effectively gets into twisting around the joint/nut its not just
dependent on ground reaction force but also distance from pivot point.
Slide 14
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Document Summary
We tend to see that in a forelimbs of a horse there are greater peak in vertical ground reaction force than we do for hind limbs as center of mass isn"t perfectly situated. Faster walking duration of time the horse is in contact with the ground is lower during faster walking (as stride freq and duty cycle are lower with faster walking) Peak gets higher in faster walking because you need to integrate out your body support over shorter cycle duration. But how does this change for a horse walking on a leveled treadmill (no change in center of mass) But how does ground reaction forces change when an animal is decelerating. On incline treadmill, horse must accelerate its center of mass and move up ground reaction force shifts reflecting this fact. Mass is shifted towards rear legs, thus shift in vertical ground reaction forces. Before, there is uneven distribution in horizontal distribution as fore legs are absorbing more energy.
