Question 1

When using a kinetic link model for planar inverse dynamics, we must make several assumptions regarding the model. Which of the following assumptions also allows the researcher to assume the mass moment of inertia, length and centre of mass of each segment remains constant throughout a movement?

Accepted Answer

Choice D assumes that each segment is a rigid body with its mass concentrated at the centre of mass, meaning that the mass moment of inertia, length, and center of mass remain constant throughout a movement. Choices A, B, and C do not directly address this assumption.

Question 2

A model will generally include several types of forces. Which type of force acts downward through the centre of mass of each segment and has an acceleration of 9.81m/s2?

Accepted Answer

The force that acts downward through the centre of mass of each segment with an acceleration of 9.81m/s2 is the gravitational force. This force is constant and acts on all objects in the same way. Muscle force, net muscle force, and ground reaction force are all different types of forces that may be included in a model, but they do not have a constant acceleration of 9.81m/s2.

Question 3

In a two-dimensional or planar analysis a segment or object has three degrees of freedom, two translational degrees (horizontal and vertical) and one rotational degree (orthogonal to the translational plane) Thus how many equations of motion may be written for each segment in the kinetic chain?

Accepted Answer

Each segment in the kinetic chain has three degrees of freedom, but only three equations of motion can be written for each segment, which are related to the forces and torques acting on that segment. These equations must be solved simultaneously with the equations of motion for all other segments in the chain to determine the motion of the entire system. Therefore, the answer is 3.

Question 4

Unfortunately, because we have many muscles acting on a joint or segment we often end up with more than three unknowns. When you have more unknowns then equations to solve them you face the problem called __________.

Accepted Answer

The correct term for having more unknowns than equations to solve them in the context of forces acting on a joint or segment is "indeterminacy." This situation makes it impossible to solve for all unknowns with the available equations, leading to a problem of indeterminacy.

Question 5

Which of the following is not a known method of solving the indeterminacy problem?

Accepted Answer

The answer of Which of the following is not a...

Question 6

The moment of a force about a point, acts in a direction that is orthogonal to the plane defined by the point of rotation (O), the point of application of force (P) and the force vector (F; Figure 4.2(b)in the text) How do we specify the direction of a moment?

Accepted Answer

The right hand rule is used to specify the direction of a moment. It states that if the fingers of the right hand are curled in the direction of rotation, then the thumb points in the direction of the moment.

Question 7

When studying planar dynamics it is easiest to begin with a static example because the system is in equilibrium. In this context what does equilibrium mean?

Accepted Answer

Equilibrium in the context of planar dynamics means that the system is in a state where the sum of forces and the sum of moments acting on it are zero, which can include scenarios where the system is moving at a constant velocity (linear and rotational) without acceleration. This is because equilibrium conditions can be met not only when a system is at rest but also when it is moving at constant velocities, as the net forces and moments are balanced.

Question 8

What is an inertial reference frame?

Accepted Answer

An inertial reference frame is a frame of reference that is not accelerating. It is a frame of reference in which the laws of physics hold true and objects remain at rest or in uniform motion unless acted upon by an external force.

Question 9

principle of moments, states that the moment of a force about a point is equal to the sum of the moments of the force's components about a point. This principle is also known as...

Accepted Answer

The principle of moments is also known as Varignon's theorem, named after the French mathematician Pierre Varignon who presented it in his work in 1687.

Question 10

A free-body diagram (Figure 4.3(b) in the text) shows the biomechanical system of interest, here the forearm-hand segment, isolated from the surrounding world. Forces acting on the segment are: the weight of the forearm and hand (Fg = m g) at their centre of mass, the muscle force (Fm) and the x- and y-components of the joint force (Fjx, Fjy), producing the three force equations of 4.1. Values that are known are as follows: m = 1.5 kg, r = 0.14 m, rm = 0.05 m and φ = 80Ëš. What is the value of Fjx?

Accepted Answer

From the free-body diagram, we know that:
ΣFx = Fjx - Fm sin φ = 0 
ΣFy = Fg - Fjy - Fm cos φ = 0 
Στ = r × Fjy - rm × Fm = 0

Substituting known values and solving for Fjx, we get:

Fjx = Fm sin φ = (100 N) sin 80° ≈ 7.15 N

Therefore, the best choice is B.

Question 11

Using the joint reaction force simplifies our analysis because it is derived from the net force that is transmitted from one segment to the other. Which of the following statements regarding joint reaction force is incorrect?

Accepted Answer

Joint reaction force is a real occurrence and can be directly measured. The other statements are correct.

Question 12

There are many methods for overcoming the indeterminacy problem. In this method the number of unknowns is reduced by grouping muscles with similar functions or muscles with common anatomical insertion points and eliminating muscles that are considered to be inactive. Which method is this?

Accepted Answer

This method is the Reduction Method which groups muscles with similar functions or anatomical insertion points and eliminates inactive muscles to reduce the number of unknowns.

Question 13

Inverse optimisation uses mathematical optimisation techniques of cost functions and constraint equations to resolve resultant joint moments into individual muscle forces in musculoskeletal models. Which type of cost function uses sum of muscle forces or sum of muscle stresses?

Accepted Answer

The answer of Inverse optimisation uses mathematical optimisation techniques of...

Question 14

When conducting an analysis of a multi-segment chain, which segment do we typically begin the process with when force plate data is available?

Accepted Answer

When force plate data is available, we typically begin the analysis with the most distal segment. This is because force plate data can provide insight into the ground reaction forces, which can be used to estimate joint moments and powers at the most distal joint, such as the ankle or foot. From there, we can work our way up the chain, analyzing the segments proximal to the most distal segment.

Question 15

One of the assumptions of linear inverse dynamics is that the body being studied is a series of rigid segments. However, we can see from high-speed recordings of impacts, the soft parts of each segment of the human body are shifted relative to the bony parts and start to 'wobble' in a complex damped manner. Modelling the body based on the concept of wobble is called what?

Accepted Answer

Modelling the body based on the concept of wobble is called the Wobbling mass model.

Quiz 4: Calculating the Loads on the Body