The analyses of the EMG-measurements yielded no indication of progressive fatigue or increased muscular
load from one session to the next. On the contrary, the recorded EMG-levels showed a decrease in muscular
activity. The postural analyses indicated that undue deviation, extension, or flexion of the hands, which may
cause discomfort, generally did not occur. The average values were within the limits given by the
physiology of the human arm. However, personal preferences for the arm posture were highly different.
When performing the text editing task with the TrackPad(tm), during the training session, the subjects had
already achieved a performance equivalent to 65% of mouse performance. A performance of more than
90% was achieved after two hours and 100% in the fifth hour session. This means that the learning period
for such tasks will in practice be accomplished within one working day. The average performance achieved
with the TrackPad(tm) for the eight tasks with Fittsīs Test, during the last session, was lower than that with
the mouse, but the difference was not statistically significant.
The results of this study indicate that the TrackPad(tm) can be used for everyday tasks without causing
postural discomfort or fatigue. In some respects, this device may even be preferable to the mouse, if the
users can achieve the same level of performance.
Keywords:
Input device, TrackPad, Touchpad, postural discomfort, EMG
Introduction
The TrackPad(tm) is a new input device for the Macintosh PowerBook(tm) 500 series computers which
replaces the trackball input device used in previous Macintosh PowerBook(tm) computers. The user study
was designed to investigate the impact of the use of the device on the biomechanical load and postural
comfort of the users. Since the ease of learning and the achievable performance play an important role for
the acceptance of an input device, the study was designed to capture user performance and usage learning
progression data.
THE TEST PROCEDURE
The subjects had to perform tests and work on tasks during a five hour period, using a portable computer.
The tasks, such as entering data in a database or selecting information from a database, were organized to
resemble normal office tasks and measure performance. The overall tasks of each session were to be
performed within about 40 minutes. The tasks included intensive use of the keyboard in combination with
the TrackPad(tm), as would be the case during a normal office work environment. For another 8 to 10
minutes, the subjects completed performance tests, a text editing task and eight Fittsīs Tests. During this
time period the subjects performed approximately the same number of pointing, clicking and dragging tasks
as they would have performed within an hour of normal office work.
The text editing task was similar to normal editing work with a text editor; it included pointing at a certain
location on the screen, dragging the cursor over a given space to highlight the selected portion and deleting
it. The only difference (between normal editing work and the text editing tasks) was that the subjects did not
enter any new text. Thus, the task primitives involved in text editing (pointing, dragging and selecting) had
to be performed for five minutes without changing to other tasks such as typing. Such a concentration on a
specific task primitive within a short time period is likely to cause more discomfort than an even
distribution over time.
The Fittsīs Tests help to determine the performance of the subjects in pointing. The tests were to be
accomplished in four directions with two levels of difficulty; one level corresponding to the difficulty of
selecting menu items or pointing at virtual buttons etc., the other level corresponding to the difficulty of
selecting single characters or pointing at radio buttons or the like.
During the performance tests, the muscle activity of the relevant muscles of the right arm were measured by
EMG. In addition, the postures of the arm were recorded on video for later motion analyses. Both EMG-
measurements and motion analyses were also performed using a mouse for the same tasks.
The subjects completed a questionnaire on general comfort both at the beginning and at the end of the test.
The questionnaire helps to compare the level of perceived fatigue and general discomfort in comparison
with the normal work of the subjects. In addition, they assessed their level of postural discomfort after each
session. A further questionnaire on the level of (inner) activation completed the end of the assessments. This
last questionnaire helps to obtain a stress score on a scale which allows a comparison with different tasks.
The results were compared with the results of a four hour keyboard test under similar conditions, as well as
with the results of a recent Swedish study on hand and arm posture using a mouse.
The test panel included 20 experienced Macintosh users (average experience of 3.28 years).
RESULTS
Postural and General Comfort
It is believed that prolonged working with constrained postures is likely to cause postural discomfort or
fatigue. The degree to which unfavorable effects occur after a certain time period depends upon how
inconvenient the posture is. Recent research has demonstrated that within a four hour test period, of
performing computer tasks, levels of discomfort increase even for those parts of the body not involved with
the operation of the specific device [2].
Both direct ratings of postural comfort (wrists, forearms, back region) and ratings of fatigue and related
symptoms (e.g. headaches, back aches, neck aches) gave no indication of discomfort or fatigue related
symptoms after five hours of work. In comparison to a four hour keyboard test, conducted under similar
conditions, the results obtained from the questionnaires of the TrackPad(tm) study were significantly better.
Instead of increasing, most indicators for postural discomfort showed a decrease.
Results of EMG-measurements and Postural Analyses
The analyses of the EMG-measurements yielded no indication for progressive fatigue or increased
muscular load from one session to the next. On the contrary, the recorded levels showed a decrease in
muscular activity. This may have been caused partly by learning effects. Surprisingly, the impact of the task
difficulty on the muscular load was not significant.
The postural analyses indicated that undue deviation, extension, or flexion of the hands, which may cause
discomfort, generally did not occur. The average values were within the limits supported by the physiology
of the human arm. However, personal preferences of arm posture were substantially different. Thus, some
users may perceive discomfort due to the hand position they choose. This is not a design flaw but simply a
matter of user education.
Interestingly, the hand deviation (bending the wrist towards the little finger) of the subjects using the mouse
was greater than those using the TrackPad(tm) (18 versus 11.3 ). The average hand deviation angle with the
mouse was almost the same as in a recent Swedish study (18 versus 17.6 , [1]). From this point of view,
the TrackPad(tm) even seems to be preferable to the mouse.
Performance (Speed and Errors)
In the Fittsīs Test, the errors of the subjects (missing the target, double-hit) occurred only sporadically and
therefore could not be analyzed. Since this was true for the entire test from the training session until the end,
the performance of the subjects with respect to accuracy was at an adequate level in all sessions.
With regard to speed, the performance was direction dependent. The average performance with the
TrackPad(tm) for all eight tasks at the end of the test was lower than the performance with the mouse. But the
difference was not statistically significant. However, the performance in the vertical direction (moving the
cursor on a vertical line) remained remarkably low even after five hours of testing (less than 80% of mouse
performance). This means that the learning period for pointing tasks will be longer than one day if the same
level of performance as that with the mouse is to be achieved.
Surprisingly, the difference of TrackPad(tm) to mouse performance was smaller for the tasks which had a
higher level of difficulty. Both the subjects and the experimenters tended to underestimate the level of
precision that was achieved with the TrackPad(tm).
When performing the text editing task with the TrackPad(tm), during the training session, the subjects had
already achieved a performance equivalent to 65% of mouse performance. A performance of more than
90% after about two hours and 100% in the fifth hour session. This means that the learning period for such
tasks will in practice be accomplished within one working day.
DISCUSSION
The results of this study indicate that the TrackPad(tm) can be used for everyday computing input tasks
without causing postural discomfort or fatigue. In some respects, this device may even be preferable to the
mouse, if the users can achieve the same level of performance. With respect to the levels of accuracy
required during the test, a sufficiently high performance of the subjects was observed from the beginning.
For some tasks, it only took the subjects five hours to become as proficient with the TrackPad(tm) as that
achieved with the mouse after many years of mouse experience. Given the fact that the test was not
designed to exploit the features of the software for optimal learning progression, this is a very positive
indication. In practice, the users can select the appropriate control/display ratio to their preference and
change it as they become more proficient.
For other tasks, the learning period may be longer than one day. However, it should be kept in mind that
users of other input devices also need a lengthy period of familiarization.
In this study, the TrackPad(tm) was not tested for tasks which require very high accuracy (e.g. CAD-tasks,
manipulation of graphics etc.). The reason for this was the manufacturerīs intention of utilizing the device
for a portable computer. Provided that further studies confirm our assumption that even higher levels of
accuracy can be achieved, the TrackPad(tm) can be utilized in different types of devices to the benefit of the
users.
References
1. Karlqvist, L.: Variation in upper limb posture and movement during word processing with and without
mouse use, ERGONOMICS, 1994, Vol. 37, No. 7, 1261-1267
2. Karwowski, W.; Ray, E.; Salvendy, G.; Noland, S.: The effects of computer interface design on human
postural dynamics, ERGONOMICS, 1994, Vol. 37, No. 4, 703-724
