OPTIMAL UPPER LIMB SUPPORT CONDITIONS FOR MOUSE USE? Richard Wells

1. OPTIMAL UPPER LIMB SUPPORT CONDITIONS FOR MOUSE USE? Richard Wells

2. Rationale • A number of recommendations for support of the mouse arm have appeared in the computer and RSI-related literature. These include: • WRIST; planting the ventral surface of the wrist on the work surface and moving the mouse from the wrist joint • ELBOW; planting the elbow on the chair’s arm rest and moving the mouse from the elbow joint • FOREARM; resting the forearm on the work surface and moving the mouse from the shoulder and • SHOULDER; moving the mouse from the shoulder joint with the arm unsupported R. Wells, I.H. Lee, and S. Bao, 1996 INVESTIGATIONS OF THE OPTIMAL UPPER LIMB SUPPORT CONDITIONS FOR MOUSE USE, in: Proceeding of Human Factors Association of Canada

3. Questions • What changes with different support conditions • Which is optimal? • What are appropriate outcome (dependant) measures? Answers to these questions will inform recommendations on the best practice for mouse use

4. Dependent Measures • Musculoskeletal strain • Electromyography of 8 muscles of the arm and shoulder • Wrist goniometry • Speed/Accuracy • Fitts’ tapping task • Pain and Discomfort

5. Tasks i) Drawing task; ii) Interactive text editing task iii) Fitts’ tapping task iv) Extended game playing.

6. Electromyographic Sites Electromyograms were collected from the right upper limb from surface electrodes overlying • 1st dorsal interosseous (FDI), • finger/wrist extensors (ED), • extensor carpi ulnaris (ECU), • flexor superficialis (FDS), • pronator teres (PT), • a combined flexor/extensor site (Thru) • upper trapezius (Trap) • infraspinatus (Inf).

7. Normalization of the EMG • Against maximal efforts (MVIC), fixed effort contractions and µVolts. Only EMG, quantified by the 10th percentile APDF values for the normalization to maximum efforts (%MVIC) will be reported here.

8. Wrist Posture Measurement • recorded using flexible wrist goniometers (Penny and Giles) • flexion/extension, • ulnar/radial and • pronation/supination • Two calibrations, statically to known angles and dynamically against a fixture instrumented to record wrist postures were made. • EMG and wrist posture were recorded for 3 minutes of continuous task performance.

9. * 5 Elbow Forearm 4 * Shoulder (None) 3 Wrist * 2 * * 1 0 FDI FDS ED ECU PT Thru Trap Inf Muscle Static EMG vs Support Condition for Eight Muscles

10. Discomfort • After 3 Hours Mouse Use: • Support • N=5

11. Discomfort and Mouse Use • After 3 Hours Mouse Use: • Shoulder Support • N=5

12. Discomfort and Mouse Use • After 3 Hours Mouse Use: • Wrist Support • N=5

13. Discomfort and Mouse Use • After 3 Hours Mouse Use: • Elbow Support • N=5

14. Discomfort and Mouse Use • After 3 Hours Mouse Use: • Forearm Support • N=5

15. Findings Based upon the above investigation the following findings are suggested: 1)The elbow support condition appeared to minimize the static load on the shoulder muscles sampled (trapezius and infraspinatus) and the forearm muscles 2)The shoulder support condition appeared to require the highest muscle activity in the shoulder muscles 3)The wrist support condition appeared to require the highest muscle activity in the muscles of the arm sampled