The main purpose of this lab is to demonstrate Fitts’ Law and its applications in motor control and movement execution.
Introduction
Knowledge about the laws of movement control is essential for understanding motor learning principles. One of the most important laws in motor control is the speed-accuracy trade-off, also known as the Fitts’ Law. This principle simply refers to the fact that when movements are produced at faster speeds they tend to become less accurate. In other words, more accuracy is achieved at the expense of speed. This principle can be expressed in a mathematical formula:
MT = a + b log2(2D/W)
According to the formula, MT (movement time) is determined mainly by two factors, D (distance between the target and starting point) and W (width of the target). Fitts (1954) defined the term log2(2D/W) as the index of difficulty (ID). The ID can be used to quantify the level of difficulty for aiming movements.
Equipment and Task
Paper target sheets and pencils will be used. The targets drawn on paper vary in sizes with different distances between the targets creating 9 conditions (see the following table).
Procedures
All students will be tested as subjects. You will work in pairs or threesomes. Before testing starts, make sure that the paper sheets are mounted on the table with scotch tape or held down with the help of another person. The order in which the trials are completed should be randomized. Every person completes 9 trials that differ from one another either in distance or target size. For each trial, the subject completes tapping movements back and forth consecutively for 10 seconds. The requirement for the tapping movements is that no target should be missed even though fastest speed possible is produced. The subject should hold the pencil high enough so that the pencil will not scratch the surface of the sheet and the elbow will not touch the table. Record the number of taps (T) by counting the number of dots on the targets.
Individual Data Sheet
Trial #
Distance (D)
Width (W)
# of Taps
MT
Log2(2D/W)
(Index of Difficulty)
1
8 cm
1 cm
22
0.455
4
2
8 cm
2 cm
30
0.333
3
3
8 cm
4 cm
43
0.233
2
4
4 cm
1 cm
27
0.370
3
5
4 cm
2 cm
39
0.256
2
6
4 cm
4 cm
45
0.222
1
7
2 cm
1 cm
32
0.313
2
8
2 cm
2 cm
44
0.227
1
9
2 cm
4 cm
57
0.175
0
Assignment
Data Analysis
1. Calculate movement time (MT) for each trial by dividing 10 sec by the number of taps.
2. Calculate the index of difficulty (ID) by using the following table
2D/W
1
2
4
8
16
32
64
128
ID (index of difficulty)
0
1
2
3
4
5
6
7
3. Draw a scatter gram (using Microsoft Excel) that displays the relationship between the level of difficulty (ID) and movement speed (MT). Put the MT on the vertical axis and ID on the horizontal axis.
Define the term “ID (index of difficulty.” What factors contribute to the level of difficulty in a task?
· The two factors contribute to the level difficulty in a task are quotient of target width and distance.
- Examine your data and determine what effect distance between the targets has generated on movement time.What effect does the size of the target have on movement time? Looking at your graph, determine if the relationship between MT and ID is direct, indirect, or non-existent. Indicate whether your findings support the Fitts’ Law.
·
- Based on the speed-accuracy trade-off assumptions of Fitts’ law, what do you think would happen in baseball delivery when pitching if the strike zone was made bigger? What if the strike zone decreased? Explain your answer using Fitts’ law related to size of the target.
