In vivo finger flexor tendon force while tapping on a keyswitch

Force may be a risk factor for musculoskeletal. disorders of the upper extr emity associated with typing and keying. However, the internal finger flexo r tendon forces and their relationship to fingertip forces during rapid tap ping on a keyswitch have not yet been measured in vivo. During the open car pal tunnel release surgery of five human subjects, a tendon-force transduce r was inserted on the flexor digitorum superficialis of the long finger. Du ring surgery, subjects tapped with the long finger on a computer keyswitch, instrumented with a keycap load cell. The average tendon maximum forces du ring a keystroke ranged from 8.3 to 16.6 N (mean = 12.9 N, SD = 3.3 N) for the subjects, four to seven times larger than the maximum forces observed a t the fingertip. Tendon forces estimated from an isometric tendon-force mod el were only one to two times larger than tip force, significantly less tha n the observed tendon forces (p = 0.001). The force histories of the tendon during a keystroke were not proportional to fingertip force. First, the te ndon-force histories did not contain the high-frequency fingertip force com ponents observed as the tip impacts with the end of key travel. Instead, te ndon tension during a keystroke continued to increase throughout the impact . Second, following the maximum keycap force, tendon tension during a keyst roke decreased more slowly than fingertip force, remaining elevated approxi mately twice as long as the fingertip force. The prolonged elevation of ten don forces may be the result of residual eccentric muscle contraction or pa ssive muscle forces, or both, which are additive to increasing extensor act ivity during the release phase of the keystroke.