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The TrackPad(tm) - A Study on User Comfort and Performance

Ahmet E. Çakir Gisela Çakir


ERGONOMIC Institute
Soldauer Platz 3
D-14055 Berlin
Germany
Int + 49 30 302 1050
ERGOINST.applelink.apple.com

Thomas Müller Pieter Unema


Institute of Ergonomics and Industrial Engineering
Technical University of Berlin
Rohrdamm 20-22
D-13629 Berlin
Germany
Int + 49 30 38 00 6202

© ACM

Abstract

The user study on the TrackPad(tm), a new touch tablet technology input device, was designed to investigate the impact of the use of the device on the biomechanical load and postural comfort of the users. In a one day test, the subjects, experienced Macintosh users, performed tests and worked on tasks, using a portable computer, that were organized to resemble normal office tasks and measure performance. The tasks included intensive use of the keyboard.

The performance was measured by text editing tasks and eight Fittsīs Tests with two levels of difficulty. The biomechanical load was measured and evaluated by means of EMG and postural (motion) analyses. General comfort and postural comfort was evaluated with questionnaires.

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