Finger flexor tendon forces and the control of finger movements during a keystroke

Force is a risk factor for musculoskeletal disorders of the upper extremity associated with typing and keying. External fingertip and internal flexor tendon forces during typing are unknown. The goals of this research were to measure and characterize fingertip force, fingertip motion, and intra-muscular electromyography (EMG) during the keystroke. Each fingertip force history, measured with a load cell mounted in the keycap, contained three distinct phases: (I) key switch compression, (II) finger impact (maximum force ranged from 1.6-5.3 N), and (III) fingertip compression and keyswitch release. The corresponding vertical fingertip motion followed a four-stroke pattern: backswing-downswing (maximum tip velocities ranged from 0.2 to 0.8 m/s)-release-return. The timing of the three-burst EMG pattern suggests: (1) the extensors do not slow the downward fingertip motion; and (2) the flexors overcome the activation force of the key switch but do not propel the downswing as expected.;A new calibration method, developed for tendon force transducers, pre-seats the transducer onto the in vitro tendon, reducing the variability caused by tendon visco-elastic properties. In vivo tendon tension of the flexor digitorum superficialis (FDS) of the long finger was measured using the transducer during open carpal tunnel surgery.;The ratio of FDS tendon tension to force applied at the fingertip during isometric pinch ranged from 1.7 to 5.8 (mean = 3.3, s.d. = 1.4) across nine subjects. The ratio was higher for four subjects using pulp pinch posture (hyper-extended distal interphalangeal joint (DIP)) than for subjects using tip pinch posture (flexed DIP). Isometric tendon force models from the literature did not predict this difference. A new DIP constraint model adds a constraint moment at the DIP for pulp pinch postures and predicts forces correlating well (r = 0.91) with the measured forces.;Tendon force histories during a keystroke did not contain the fingertip impact force and decayed more slowly, remaining elevated after tip force vanished. The maximum tendon forces ranged from 8.3 N to 16.6 N (mean 12.9 N, s.d. = 3.3 N) across five subjects, 4 to 7 times larger than the maximum tip forces. This is greater than the levels predicted using existing models. When the fingertip was not in contact with the keycap, tendon force varied with the two observed fingertip motion patterns.