Many workers with repetitive motion tasks develop work related musculoskeletal diseases. In this study, the impact of required forces in dynamic key touch pressure and key displacements on forearm extensor and flexor muscles was investigated. The aims were to evaluate the co-activation of forearm extensor and flexor muscles during a 2 min tapping task at 4 Hz and to assess possible changes in maximal surface electromyography (sEMG) activation during tapping using different keys with variable force–displacement characteristics. 13 subjects took part in the experiment and performed ten tapping sequences, using a different key make (with different force-displacement characteristics) each time. Two channels of sEMG were used to monitor forearm muscle activation. Results showed (a) that the co-activation of forearm extensor and flexor muscles increases with rising key makes force, and (b) that coordination between extensor and flexor muscles improves over time.
The findings indicate that keyboards with make-force levels over 0.59 N are ergonomically inadequate. Differences in key displacement were less important.