Detection threshold and mechanical impedance of the hand in a pen-hold posture

We report position and force detection thresholds for sinusoidal waveforms in the frequency range 10-500 Hz delivered through a stylus. The participants were required to hold the stylus in a way similar to that of holding the stylus of a force-feedback device. A minishaker moved the stylus along its length so that the majority of vibrations were presented tangentially to the skin of the hand. The measured position thresholds decreased initially with an increasing stimulus frequency and formed a U-shaped curve in the high frequency region. The thresholds of high frequency vibrations were lower than those reported previously for vibrations that were perpendicular to the skin, but were similar to the thresholds reported earlier using vibrations that were tangential to the skin. A similar force threshold curve was obtained using a force sensor attached to one end of the stylus. Mechanical impedance of the skin derived from velocity estimates and force measurements indicated that the skin and tissues in the hand holding the stylus can be modeled with mass-, damper- and spring-like elements. A comparison of the mechanical impedance from the present study with those reported previously showed similar results for vibrations delivered in the tangential and normal directions to the skin.