AppliedErgonoraics 1983, 14.2, 117-122

Keyboard design through physiological strain measurements P. Zipp, E. Haider, N. Halpern, and W. Rohmert

I nstitut f~ir Arbeitswissenschaft der Technischen Hochschule, Petersenstr 30, 6100 Darmstadt, F.R. Germany

The physiologically tolerable range of positions for the joints of the upper extremities have been investigated for typing tasks by recording the myoelectric activities of the involved muscles. For long-term typing tasks a split keyboard is recommended allocating a key field to each hand. The fields should be rotated against each other in the horizontal plane and inclined laterally.

Keywords: Typewriting, joints (anatomy), physiological effects

Introduction

The massive introduction of visual-display units in the last decade has exposed a larger number of people to the daily interaction with keyboards, and revived the interest in the physical well-being of the operators.

The records of the Orthopaedic Polyclinic of the Free University of Berlin show that 7% of the patients treated between 1957-61 were clerical workers, a higher percentage than their share in the working population. Moreover, numerous complaints of shoulder-arm-syndrome have been noted (Mittelmeier, 1963). Further analysis of complaints among 532 typists, clinical treatment records of 600 clerical workers and electromyographical studies of about 30 typists revealed that the constrained sedentary posture maintained for typewriting should be regarded as the primary cause for complaints (Menschig, 1967; TiSnnis, 1965). Alleviation of the constrained posture by changing the position of the keyboard resulted in productivity improvements as well as health improvements (Yll~i, 1962). Complaints among clerical workers have been epidemiologically examined lately in Japan, where it was termed "Occupational Cervicobrachial Disorder" (Hiinting et al, 1980), and in Switzerland (Hiinting et al, 1981). These investigations confirm the view that the introduction of electronic keyboards and visual-display units has not reduced the static work-load imposed on clerical workers.

Bearing these results in mind, one may conclude that the finger muscles are the only actual effeetors in typewriting, while the body as a whole functions as a support holding the fingers in their working position. The static tension produced while maintaining the sedentary posture in typewriting endured over some time may lead to strain and, hence, to fatigue (Rohmert, 1960).

The investigations were sponsored by Normenausshufl Maschinenbau (DIN). Responsibility for the contents of the paper is with the authors.

Previous studies have already determined two postural constraints which are directly affected by the design of the available keyboards:

(a) The angle between upper arm and forearm is directly affected by the height of the keyboard above the floor (Langdon, 1966). This aspect has been treated elsewhere (Zipp et al, 1980).

(b) Typists must abduct their hands at about 200-26 ° and occasionally up to 40 ° (Fig. 1). The degree of ulnar abduction correlates with operator complaints (Duncan and Ferguson, 1974; Hiinting et al, 1981). This position is further aggravated by pronation of the forearm close to the anatomical limit. Klockenberg (1926) and Kroemer (1972) also pointed out that the pronation and the entailed ulnar abduction are being compensated by lifting the upper arm laterally and raising the shoulders.

Kroemer (1972) suggested that these postural constraints could be relieved by a split keyboard inclined horizontally

Fig. 1 Hand abduction at a common keyboard

0003-6870/83/02 0117-06 $03.00 (~) 1983 Butterworth & Co (Publishers) Ltd Applied Ergonomics June 1983 117

and la%rally. However, his recommendations were based on operators' subjective judgements, theoretical considerations and productivity tests; muscular strain measurements directly related to keyboard operations were needed to substantiate Kroemer's proposal.

Studies and literature review were recently conducted by this institute on behalf of the German Institute of Standards (DIN), aimed at optimising the spatial arrangement of keyboards and establishing recommendations for an ergonomically designed keyboard (Rohmert and Haider, 1981). This paper reports the results of electromyographic investigations into the optimal pronation of the forearm and ulnar abduction of the hand and the implications for keyboard design.

M e t h o d

Three subjects, two males of 19 and 25 years of age, 180 and 185 cm height and one female 21 years of age and 163 cm height, were used for the separate experiments to investigate the EMG arising from continuous keying, from various degrees of forearm pronation and from changes in the lateral inclination of the keyboard. One subject took part in each experiment and they are identified in the captions to the figures giving the experimental results.

Recording a n d p r o c e s s i n g o f the myoelectric signal (EMG) The muscle action potentials were recorded by means of

surface electrodes and amplified. The signal processing involved rectification of the analogue signal and integration at a determined sampling rate, thus providing a value in arbitrary units for the mean signal amplitude. This electrical activity (EA) reflected the degree of muscle tension. Moreover, being a time series, EA provided insight into the endurance of muscle tension. An increase of EA over time, work-load being constant, was interpreted as an indicator of motor unit recruitment in order to compensate for a degraded performance due to intramuscular metabolic imbalance. The increase of EA over time was quantified as a regression coefficient. With a coefficient significantly higher than zero, one may assume localised muscular fatigue (Rohmert and Laurig, 1975; Laurig, 1976).

Selection of muscles The investigations into isolated movements of specific

joints consisted of EMG recordings of all synergic muscles involved. On the other hand, when studying the muscular interplay of the upper extremity, only typical protagonists. were recorded.

E x p e r i m e n t s

E x p e r i m e n t N o I The application of electromyography for typewriting

was at first investigated in the laboratory, recording muscle activity produced during long-period continuous keying.

Fig. 2 shows the results of one session.Five out of the six muscles recorded simultaneously show a statistically significant increase of EA, reflecting the fatigue of the shoulder and arm muscles. These results emphasise the need for reducing the static tension imposed by the constrained posture. It should be noted that the strain assumes a sine-

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Fig. 2 Time series of myoelectric activities during keying. Subject (male; 19 y; 180 cm; 67 kg) was performing continuous keying on a numerical keyset (ten keys in a row). Posture: Upper arm vertical, forearm horizontal . Muscles: M trapezius (TRA), M biceps (BIC), M flexor carpi ulnaris (FCU), M extensor carpi ulnaris (ECU), M pronator teres (PRT), M flexor carpi radialis (FCR). Correlation coefficient r0.05 = 0.25

wave pattern. The drawn segments of linear regression indicate that phases of fatigue have occurred with increasing frequency and within shorter periods. Although the subject managed temporary phases of relief through slight postural changes, a cumulative fatigue is discernible.

The rate of increase-per-minute of EA produced by M trapezius and M biceps would have theoretically made it endurable for about 70 min only (Laurig, 1974).

Experiment N o 2 Next, an attempt was made to establish in the laboratory

tolerable ranges for pronation and ulnar abduction.

Figs. 3 and 4 present the mean values of EA as a function of forearm pronation and ulnar abduction respectively. It is obvious that the EA increases as extreme positions are approached. This is due to the non-linear increase in the elastic resistance of the ligaments. However, it should be

118 Appl ied Ergonomics June 1983

Hyoelectric Activity I cV

0 20 ~0 60 80 100

Angle of Pronation f /o__~

pronation may be interpreted as the effect of the reduced muscle length (Grieve and Pheasant, 1976). Fig. 6 shows that by alleviating ulnar abduction, not only is the strain of the genuine hand abductor muscle (M extensor c u) reduced, but also the strain of most of the neck, shoulder and arm muscles.

Discussion

When judging the muscular load associated with different body postures from the recorded myoelectric activity, one should keep in mind the confounding effect of a variation in muscle length. For a constant muscle load (absolute or relative to the maximum strength) the myoelectric activity decreases as the muscle length increases (Grieve and Pheasant, 1976). Thus in this study some of the observed reduction in myoelectric activity must be attributed to the increase in muscle length as the postural constraints are alleviated. This is especially true for the muscles directly involved with pronation and abduction. However, one may also assume that the data reflect genuine reductions in muscle load, since the induced variation in posture was modest compared with the total range of motion.

Normal keyboard design dictates an ulnar abduction of the hand from 20 ° to 40 °, depending on the anthropometry of the operator. Since these angles approach the limit of the joint range, the maintenance of this posture requires a considerable degree of static muscular work. The ulnar

Fig. 3 Myoelectric activity of the main pronator muscles as a function of forearm pronation. Subject (male; 25 y; 185 cm; 78 kg) had his right hand with fingers extended fixed on to a rotatory pad, mounted on a protractor. Zero position: Upper arm vertical, forearm horizontal half-way between full pronation and supination.

noted that Figs. 3 and 4 do not exactly reflect the increase in the required muscular tension, since the relationship between EA and muscle tension changes with the length of the muscle (Grieve and Pheasant, 1976). The optimal ranges derived from the curves are 0-60 ° pronation and 0-15 ° abduction, where EA remains low. The normal position for typewriting, with pronation of about 90 ° and abduction of 20-25 ° (hatched area), is clearly beyond the optimal range.

Experiment No 3 In order to gain insight into the strain imposed on the

upper limb muscles and the effect of an altered keyboard position, additional experiments were conducted. Figs. 5 and 6 give the changes in myoelectric activities of the upper limb muscles as the keyboard is inclined laterally or rotated horizontally in a series of steps, thus reducing forearm pronation or hand abduction.

Fig. 5 clearly shows that relieving pronation by a mere 10 degrees has already caused a significant reduction in the EA, not only of the forearm pronators but also of the shoulder-arm muscles. It should also be noted that when reducing forearm pronation (ie, the angle of keyboard lateral inclination becomes greater), M biceps is being recruited for maintaining the elbow flexed. The increased level of EA of the M biceps which accompanies reduced

Hyoelectric Activity/cV

10 15 20 25

Angle of Ulnar Abduction rio

Fig. 4 Myoelectric activity of the main abductor muscles as a function of hand abduction. The set up is given in caption to Fig. 3. Zero position: Fingers continuing the axis of the forearm.

Applied Ergonomics June 1983 119

Fig. 5 Changes of myoelectric activities of upper limb muscles for various degrees of key- board lateral inclination. The changes are expressed as percentage of the myoelectric activity found in the zero position which corresponds to the horizontal position of a normal keyboard (3' = 0°) • A thin bar fixed in various angles between two stands simulated the home row of a keyboard. The subject (female; 21 y; 163 cm; 48 kg) grasped the bar slightly without resting on it.

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M Pr0nator q. M Extensor ~.u

Fig. 6 Changes of myoelectric activities of upper limb muscles for various degrees of keyboard horizontal rotation. For the set up see caption to Fig. 5. The zero position corresponds to the home row position of a normal keyboard (a = 0 °).

abduction can be reduced by employing a split keyboard where the right half is rotated in the horizontal plane counterclockwise and the left half clockwise. The degree of rotation determines the degree to which abduction is being alleviated. Optimally, ulnar abduction should be reduced to 0 °, requiring a keyboard rotation from 0 ° to 20 °. However, our results indicate that even a sub-optimal keyboard rotation of 13 ° reduces the static muscular work of the upper extremities.

Normal keyboard design dictates also a forearm pronation, until the palm is almost in a horizontal plane. Again, this posture is close to the limit of anatomical feasibility and requires, therefore, a high degree of static muscular work. Forearm pronation is affected by a lateral inclination of the keyboard halves. A lateral inclination of approximately 60 ° of the right and the left keyboard halves (in a clockwise and counterclockwise direction respectively) yields an optimal degree of pronation. However, this keyboard design impairs the visibility of the key symbols. Since our studies showed pronounced effects on the muscular load even at sub-optimal degrees of keyboard inclination, we attempted to estimate the legibility of key symbols in the studied angles of inclination. The peripheral

keys, those normally allocated for the little finger, may lie at a critical range of legibility. Assuming a viewing distance of 400-600 mm and a key symbol 2 mm wide, located at the periphery of a key field 150 mm long and inclined laterally at 10-20 degrees, the visual size of the symbol in the worst case is reduced to approximately 10 min of arc (Fig. 7). As shown in Fig. 8, the rate of correct identification for a visual angle of this size is between 80% and nearly 100%. (The identification rates are only reservedly applied here, since the legibility of key symbols is superior to that of dot matrices displayed on CRT screens.) Therefore, the lateral inclination of 10-20 degrees, which alleviates the muscle strain considerably, does not impair key symbol legibility too much.

Moreover, it is doubtful in the first place whether a touch typist gazes at the keyboard in order to read the key symbols; it is more likely one glances at the keyboard for orientation. Secondly, the visual disadvantages may be easily corrected by printing the symbols on the peripheral keys 3 mm wide in order to provide a visual angle of 15 min of arc. Thirdly, some of the peripheral keys allocated for the little f'mger could be removed to the area between the two keyboard halves, where visual conditions are unimpaired,

120 Applied Ergonomics June 1983

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Angle of Keyboard Inclination 5,/*

Fig. 7 Symbol visual angle ~ calculated for various angles 3' of keyboard inclination and viewing distances 9. Symbol width 2 mm (x) and 3 mm (I-I).

and be actuated by the thumb. The latter arrangement has been recently promoted by Marsan (1979) and by Malt for the PCD Maltron keyboard. Fourthly, it is the core field which should be affected by the proposed inclinations; less frequently used keys may naturally be allocated to the periphery, outside the inclined fields.

A split keyboard has already been offered by Rhein. Metal in the 1930s and by Kroemer in 1964. The superiority of a split keyboard, with horizontal rotation and lateral inclination, has been proved for postal coding at visual- display units of the German Post Office (Rohmert and Luczak, 1978).

Conclusions

An ergonomically designed keyboard should take,into account the tolerable angles of the joints of the shoulders, arms and hands. Once a tolerable range has been determined, ie, the positions at which static muscle strain is tolerable, provided that the elbow is flexed at 90 degrees, the resulting keyboard assumes the following shape: two symmetrical fields, each rotated in the horizontal plane at an angle of a = 10 ° -20 ° and inclined laterally at an angle 7 = 10 ° -20 ° (Fig. 9).

The work load should be the decisive factor for determining where to introduce the proposed keyboard. Evidently, the proposed keyboard is assumed to alleviate the postural constraints imposed on full-time touch-typists. The acceptability of a new keyboard may be better tested then at workplaces employing full-time typists and data- entry operators.

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Applied Ergonomics June 1983 121

c~

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c~

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I Fig. 9 Axes of rotat ion for a split

keyboard, a : horizontal rotation;/3 : frontal inclination; 3, : lateral inclination.

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122 Applied Ergonomics June 1983