Ch. 2 - Loads and Motion

Question 1

To analyse motion of the MSK system, we can view the whole system as a mechanism consisting of which 3 elements?

Answer 1

1. Rigid links
2. Actuators
3. Constraint elements

Question 2

What do we need to determine the deformation and stresses in the elements of this system?

Answer 2

1. Description of the elements in mechanical terms
2. Description of external constraints
3. Application of laws of motion

Question 3

How would you model a bone or limb segment?

Answer 3

Rigid link

Question 4

How would you model a joint?

Answer 4

Standard joint including kinematic constraints (rigid contact surfaces) and force constraints (deformable contact surfaces)

Question 5

How would you model a muscle+tendon?

Answer 5

Actuator

Question 6

How would you model nerves?

Answer 6

Actuator + elastic + viscous elements

Question 7

How would you model a ligament or joint capsule?

Answer 7

Controllers, elastic or viscoelastic springs

Question 8

What are the external boundary conditions in an orthopaedic biomechanics scenario?

Answer 8

1. External forces

2. Kinematic constraints

Question 9

State Newton’s Laws of Motion.

Answer 9

1. A body will remain at rest or moving at a constant velocity unless it is acted on by an unbalanced force.
2. The force experienced by an object is proportional to its mass times the acceleration it experiences.
3. If two bodies exert a force on one another, the forces are equal in magnitude but opposite in direction.

Question 10

What is moment of inertia?

Answer 10

A body’s resistance to change in rotational velocity.

Question 11

Describe the main features of a quasi-static scenario.

Answer 11

1. Geometry is not fixed
2. Accelerations are very small, so inertial terms may be ignored
3. Dynamics (external loads) are not necessarily ignored

Question 12

What is the aim of a static analysis of the MSK system? What assumptions are made?

Answer 12

Aim: determine internal forces in the MSK system when supporting external loads
Assume: acceleration = 0, inertial effects are insignificant

Question 13

Consider performing a static analysis of a skeletal subsystem of n elements. How many equilibrium equation can we write for an analysis in:

a) 2D?
b) 3D?

Answer 13

a) 3n

b) 6n

Question 14

When would a loading scenario be considered a dynamic rather than static or quasi-static situation?

Answer 14

When intertial effects due to acceleration cannot be ignored.

Question 15

What is the difference between direct vs. inverse dynamic problems? Which is more common when analysis MSK systems?

Answer 15

Direct - motion histories are unknown but the forces are known. # unknowns = # equations.
Indirect - motion history and external forces are known. Internal forces are unknown. (Most common for MSK systems)

Question 16

How could you deal with the muscle redundancy problem?

Answer 16

We can lump the unknowns together and represent the net effect of all internal forces across each joint as a resultant intersegmental force vector and a resultant intersegmental moment vector.

Question 17

Name and explain 2 approaches to deal with indeterminate MSK system problems.

Answer 17

1. Auxiliary Conditions: addition of constraint conditions, in order to get an equal number of equations and unknowns
2. Optimization: addition of performance criterion based on the idea that the physiological system performs an “optimized” behaviour. This allows the use of mathematical optimization techniques.

Question 18

Give 3 examples of auxiliary conditions.

Answer 18

1. Force reduction
2. Muscle scaling Fi/Ai = F1/A1 (assumption of equal activation not always true, difficult to calculate cross-sectional areas)
3. Soft tissue force-deformation relations

Question 19

Our rigid body analyses require physical properties of anatomic segments. Name these.

Answer 19

1. Mass
2. Location of mass center
3. Segmental moments of inertia
4. Lines of actions of muscles/ligaments
5. Locations of muscle/ligaments origins and insertions

Question 20

What kinematic data is required for inverse dynamics problems?

Answer 20

1. Position
2. Orientation
3. Velocity
4. Acceleration
   For each body segment as a function of time.

Question 21

What are the 3 steps for kinematic motion analysis?

Answer 21

1. Set up a reference frame with known external markers.
2. Define the internal reference frame with known anatomical landmarks.
3. Locate and track the anatomical landmarks in the external reference fram