Movement

Introduction

In the previous lesson, we learned how human memory handles information. We know how humans store and retain information, but how does this information turn into human actions? What are the limitations and capabilities of movement by a human body?

This lesson gives an insight into the actions a human body can perform and the factors that affect these actions. Recall from the introduction of this chapter that human action is the final stage of Model Human Processor.

Example

Consider the simple task of picking up a piece of bread from the floor. You perceive the image of bread on the floor. Your brain sends a signal to the body, and it performs the required actions to bend and pick up the piece of bread from the floor.

Response time

Each stage of the whole process in the above example takes time. This period of time is called response time. It can be divided into two fragments of time:

• Reaction time: The time taken to react to sensory stimuli. This depends on the stimuli type.
  • Visual: ~ 200ms
  • Auditory: ~150ms
  • Tactual: ~700ms
• Movement time: The time taken by the body’s muscles to respond. This depends mainly on the physical characteristics of the person. For example, age, fitness, and health.

Response time = Reaction time + Movement time

Accuracy

A person’s motor skills depend on two factors; reaction time and accuracy. Reaction time is how quickly the person reacts to sensory stimuli as we have already discussed. Accuracy is how precise and correct the movements are

Fitts’ law

In the world of HCI, movement speed and accuracy play a significant role in designing interactive interfaces. The goal is to reduce the time a user takes to move to a particular position on the screen without compromising accuracy. Fitts’ law states,

“The time required to reach a particular screen target is calculated using the size of the target and the distance between initial pointer position and target position.”

The target can be anything on the screen; a specific position, button, icon, menu item, anything.

The law can be expressed as

Movement time $=$ $a$ $+$ $b*$$log_2 \space({\frac{distance}{size}} + 1)$

where $a$ and $b$ are constants.

Fitts’ law experiment

Let’s clarify the ideas presented by this law using a simple task. There are two calculators and a stopwatch down below. Perform the calculation $(104+47-99*39)$ on both calculators one by one.

• First press the start button. The timer will start. Start inputting the above expression on the left calculator first.
• When you are done, press the stop button and note the time. You will need this later for comparison.
• Now reset the timer, start it again, and this time perform the calculation on the right calculator.
• When you are done, press the stop button and note the time.

Now compare both times, which one took longer? Probably the right one. Can you identify the reasons? If you can, tell us down below.

Note: Fitts’ law limits the type of targets we can design in interfaces. Targets should be large enough, and the distance between them should be small enough to maximize the user experience.

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