HMI Exam 2

Definition of Hick’s Law

The time required to make a decision (reaction time) is linearly related to the amount of information that must be processed in making that decision.

Equation for Hick’s Law

Reaction Time = a + b * log₂(N)

• N = number of choices

• H = log₂(N) → H is the amount of information measured in bits

Definition of Fitts’ Law

Fitts’ Law states that movement time is connected to the relative difficulty of the task. The relative difficulty is reflected by the Index of Difficulty (ID). Fitts’ Law takes into account both the distance and the size of the target.

Equation for Fitts’ Law

MT = a + b * log₂(2D / W)

• a and b are empirically derived constants

• ID = log₂(2D / W)

• D is the distance of movement from start to center of the target

• W is the width of the target

Reasons why we can about the musculoskeletal system:

1. If we don’t know our physiological limits, we can’t properly design

2. Provides guidelines

3. Minimizes costs of injuries

4. Helps to account for the older population

Two functions of connective tissues:

1. Force transmission and movement

2. Metabolism

3. Defense/immunity

4. Storage/Buffer

What are the two fiber types associated with connective tissues?

1. Collagen

2. Elastin

Define tendon:

Connects muscle to bone! Mainly longitudinal collagen with some elastin fibers.

Define ligament:

Connects bone to bone. Mainly made up of longitudinal collagen with some oblique fibers and some elastin fibers.

What is Wolff’s Law?

Wolff’s Law states that bone is deposited where needed and reabsorbed where not needed based on the mechanical stresses on it.

How does age affects bone density?

Older bones are less dense, weaker, and brittle.

List one function of joints:

1. Joints hold our bones together, supporting stability and skeleton mobility

List the three types of joints, and list one example for each:

1. Synovial → wrist or knee

2. Cartilaginous → spine, ribs/sternum

3. Fibrous → Skull

List the three types of cartilage, and give examples of where each is located in the body:

1. Hyaline → on bones

2. Fibrocartilage → on disc

3. Elastic cartilage → ear and nose

Explain how the two fiber types affect the mechanical properties of connective tissues

The proportions and geometric organization of the two fiber types determine the mechanical properties of connective tissue.

Describe the stress-strain curves for collagen and elastin

• Collagen has a large elastic modulus because this fiber helps with the transmission of force, and has residual strain that results in plastic deformation (more palabora-shaped)

• Elastin can undergo large strain because this fiber helps with storage and release of energy, but experiences abrupt failure (exponential-shaped graph)

• Strain is on X axis, and Stress is on Y axis

Explain why it’s difficult to recover from cartilage injuries

• No blood supply

• No nerves

• Initial or minor injuries may not be noticed

Explain the two variables that affect spine compressive strength

1. Age → spine compressive strength decreases substantially with age

2. Gender → Males have higher spine compressive strength than females

List three factors that increase the risk of joint injury

1. Aging

2. Inflammation

3. Loss of muscle mass

4. Excess weight

List two functions of muscles

• Skeletal mobility and stability

• Force production

• Fluid control

What are the three types of muscles? Provide an example of each.

1. Skeletal → Quads (somatic/voluntary)

2. Cardiac → Heart ❤ (autonomic/involuntary)

3. Smooth → Intestines, female reproductive system, lungs (can be either)

Describe how muscles are organized, and provide an example to illustrate this relationship.

Muscles are organized in agonist-antagonist pairs, which means there is a muscle causing movement, and the muscle opposing the agonist. An example would be when the bicep (agonist) is activated, the tricep (antagonist) is opposing that movement/activation.

Describe how muscles contract

Muscle contraction is caused by the binding/unbinding of two protein molecules, actin and myosin. Actin and myosin filaments slide past each other when muscles contract, as described by the Sliding Filament Theory.

Define anthropometry

A branch of science originating from anthropology that attempts to describe the physical dimensions of the human body.

What are the two types of anthropometry? Provide an example of an anthropometric dimension for each type.

1. Physical → stature: vertical distance from floor to head (standing)

2. Functional → overhead grip reach (standing)

Explain why caution must be taken when combining body segment dimensions.

Caution must be taken because the data is highly susceptible to variability and inconsistencies. Just because someone is in the 50th percentile for one measurement doesn’t mean they will be in another.

List three ways to design for population variance.

1. Average → Height of visual displays (you’d want to accommodate for the 95th percentile of females)

2. Extremes → Door height (you’d want to use the 95th percentile for men)

3. Adjustability → Seatbelt. You need to accommodate a range of hip sizes, and the belt must be snug for everyone, so it makes sense to use adjustability.

Define work measurement

Involves the use of engineered labor standards to measure and control the amount of time required to perform a specific task or tasks

List 2 reasons why work is measured

1. to discover and eliminate lost or ineffective time

2. to establish standard times for performance measurement

What are the two work measurement techniques?

Time study and predetermined time systems.

Compare and contrast the two work measurement systems.

Time Studies:

• can be less accurate due to human error and variability

• easier for workers to understand

• identify specific waste in a process

Predetermined Time Systems:

• more consistent and objective

• can be expensive and require specialized training

• not ideal for non-repetitive work

How does worker selection affect time study data collection? What worker should be chosen?

The average worker should be selected because if an expert is chosen, the times will be too low, and if a new worker is chosen, the times will be too high. Ratings can be used to even out these effects, but choosing the average worker is best.

List the two important mechanical properties of muscles

1. Length-tension relationship

2. Velocity-tension relationship

Describe the length-tension relationship

Peak muscle force (strength) depends on muscle length (posture).

Define the Sliding Filament Theory and explain when peak muscle force occurs.

SFT: Efficiency depends on the overlap of actin and myosin chains.

Peak muscle force occurs when there’s optimal overlap of the actin and myosin, when the muscle is stretched a little but not to its full length.

Describe the Velocity-Tension relationship. Explain this in relation to power generation.

Velocity-Tension relationship: Capacity for force production depends on velocity. As velocity increases, force decreases. Power is force times velocity.

What are the two muscle fiber types? Describe the difference.

1. Type 1 (slow-twitch) → fatigue resistant, low force

2. Type 2 (fast-twitch) → quick to fatigue, high force

They differ in maximum tension, fatigue resistance, aerobic and anaerobic capacities.

Explain the Rohmert curve, and its implication on design.

The Rohmert curve displays static endurance time vs. % effort.

Y-Axis: Endurance time

X-Axis: Exertion level

As the exertion level increases, the endurance level decreases. In design, we want to stay away from high exertion.

List two effects aging has on muscles

• loss of muscle mass

• general decrease in muscle capacity/strength

• loss of innervation (nerve cell damage)

normal time formula

normal time = observed time * (rating/100)

standard time formula

standard time = normal time * (1 + allowance)

Explain why allowances are accounted for in a time study, and list three allowances that are typically accounted for.

Allowances are accounted for because humans won’t repetitively do the same task at the same rate without distractions or breaks, that’s not realistic. Accounting for allowances helps us to get a more realistic time.

• Personal

• Fatigue

• Delay

What is a PTS, and how do they differ from time studies?

PTS’s break down activities into elements, assign time values to the elements, and the add the time elements together to get total time. This differs from time studies because you aren’t recording the worker actually doing the task, you’re assuming that there are basic, universal units of work with standard amounts of time.

What is a therblig? Give an example and state who coined the term.

A therblig is a set of fundamental motions required for a worker to perform a manual task. Example: grasp. Lillian and Frank Gilbreth coined the term.

What is a TMU?

A TMU is a time measure unit, and can be converted to seconds or hours. Each therblig has an assigned number of TMU’s.

List one benefit and one limitation of PTS’s

pro: in theory, PTS’s can accurately predict task times

con: expensive and significant training required