Firm foundation in the main hci principles, the book provides a working

3.3 Human Body Ergonomics (Motor Capabilities)
So far, we have mostly talked about human cognitive and perceptual 
capabilities and how display or input systems must be configured to 
match them. In this section, we briefly look at ergonomics aspects. To 
be precise, ergonomics is a discipline focused on making products and 
interfaces comfortable and efficient. Thus, broadly speaking, it encom-
passes mental and perceptual issues, although in this book, we restrict 
the term to mean ways to design interfaces or interaction devices for 
comfort and high performance according to the physical mechanics of 
the human body. For HCI, we focus on the human motor capabilities 
that are used to make input interaction. We start with Fitts’s law and 
human motor control.
3.3.1 Fitts’s Law
Fitts’s law [23] is a model of human movement that predicts the time 
required to rapidly move to a target area as a function of the distance to 
and the size of the target. The movement task’s Index of Difficulty (ID) 
can be quantified in terms of the required information amount, i.e., in 
the number of bits. From the main equation in Figure 3.19, the actual 
time to complete the movement task is predicted using a simple linear 
equation, where movement time, MT, is a linear function of ID.
MT = a + b * ID and ID = log(A/W + 1)
where A and B are coefficients specific to a given task.
Thus, to reiterate, ID represents an abstract notion of difficulty of 
the task, while MT is an actual prediction value for a particular task. 
The values for coefficients a and b are obtained by taking samples 

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5 4
H U M A N – C O M P U T E R I N T E R A C T I O N 
of the performance and mathematically deriving them by regression 
(Figure 3.20).
Note that the original Fitts’s law was created for interaction with 
everyday objects (in the context of operation in factory assembly lines) 
rather than for computer interfaces. Researchers have applied the con-
cept of Fitts’s law to computer interfaces and have found that the same 
principle applies. For instance, as shown in Figure 3.21, the task of 
“dragging an icon into a trashcan icon” using a mouse can be assessed 
using Fitts’s law [25]. Many other computer interactive tasks can be 
1.4
1.35
A (cm)
16.4
16.4
16.4
24.6
24.6
24.6
2.46
4.92
7.38
2.46
4.92
7.38
6.6666
3.3333
2.2222
10
5
3.3333
Average
STD
1.2455
1.0951
1.059
1.3213
1.2153
1.1522
1.1814
0.0895
W (cm) ID (bits) MT (sec)
1.3
1.25
1.15
MT (sec)
1.05
2
3
4
5
6
ID (bits)
MT = 0.811 + 0.149 log
2
( + 1)
7
8
9
10
1.1
1.2
A
W