This study examines the relationship between forearm EMGs and keyboard
reaction forces in 10 people during keyboard tasks performed at a com
fortable speed. A linear fit of EMG force data for each person and fin
ger was calculated during static fingertip loading. An average r(2) of
.71 was observed for forces below 50% of the maximal voluntary contra
ction (MVC). These regressions were used to characterize EMG data in f
orce units during the typing task. Averaged peak reaction forces measu
red during typing ranged from 3.33 N (thumb) to 1.84 N (little finger)
, with an overall average of 2.54 N, which represents about 10% MVC an
d 5.4 times the key switch make force (0.47 N). Individual peak or mea
n finger forces obtained from EMG were greater (1.2 to 3.2 times) than
force measurements; hence the range of r(2) for EMG force was .10 to
.46. A closer correspondence between EMG and peak force was obtained u
sing EMG averaged across all fingers. For 5 of the participants the fo
rce computed from EMG was within +/-20% of the reaction force. For the
other 5 participants forces were overestimated. For 9 participants th
e difference between EMG estimated force and the reaction force was le
ss than 13% MVC. It is suggested that the difference between EMG and f
inger force partly results from the amount of muscle load not captured
by the measured applied force.