midterm 2 Flashcards
what is a biomechanical system
- the inputs to a biomechanical system are forced and the outputs are motion
- inside the system are collections of particles and bodies. the can be rigid or not or connected or not
- the rules that govern how a biochemical system responds to the force inputs are “Newtons law of motion”
1. remains at rest or in motion unless acted on by an outside force
2. F=ma
3. every action has a reaction
isolated system
- is isolated in a sense that mass in form of particles and bodies can’t enter or leave a system
- it is removed from its surrounding environment with the environment replaced with the forces that it was applying to the system
system of mass
is amount of matter in the system
external forces
- act on a system
- reaction force from external world that causes movement because “the only way to move is to push on the world and have it push back on you”
internal forces
- act within a system
- don’t contribute to the motion of our system because forces and moments come in pairs (newtons 3rd law: every action (force) there is and equal and opposite action), and those that act between bodies internal to the system have no effect on the system because their effects cancel
forward dynamics
is used to solve for the unknown movement (kinematics) that result given a system model, inertial properties and specifications of the forces
inverse dynamics
used to solve for unknown forces/moments that must have caused a movement given a system model, inertial properties and measurements of movement
system identification
is used to solve for the unknown inertial properties that must be present given a system model, specifications of the forces, and measurements of motion
biomechanical indeterminacy
- when more than one muscle is active, so we have more unknown forces than equations in our force/moment balance equations
how is optimization used to solve biomechanical indeterminacy
- assume constant muscle stress
the brain does forward dynamics
now that you begun to execute the task such as picking up an object. how do we know it’s going as planned?
- you have sensory feedback (touch and proprioception) to tell you if there is an error but sensory feedback can be noisy, delayed and sometimes incomplete
- your brain makes a copy of the commanded muscle forces. the efference copy is sent through a forward internal model to predict sensory feedback. this imperfect prediction is combined with imperfect actual sensory feedback to get a better estimate of the actual movement. the brain can then work to correct the error between the desired movement and predicted using forward model and sensory feedback
the brain does system identification
it experiments my providing known forces as inputs and then measuring the resulting movement. it can then learn the relationship between forces and movements
the brain does inverse dynamics
let’s say you want to pick an object. this task is encoded in terms of motion and the only way to get these motions is to generate appropriate muscle forces which means the brain needs inverse dynamics to solve for an unknown forces given known moment
one way to estimate the center of mass
use a board supported by scales, force or force plate at each end. the COM will lie directly above the middle of the board when the scales read the same force
leg design
a favoured leg design is where the required mass is centred closer to the hip. this reduces MOI of the leg about the hip, reducing the moment of force required to swing it for a given angular acceleration and increasing the achievable angular acceleration for a given moment of force
why do we need biomechanical models
- helping understand the mechanisms underlying a complex biological phenomenon
- helping estimate a parameter that is difficult to measure
Example: the heart as a pump and bones are rigid bodies
