The effects of "lifting height" and "lifting velocity" on timing and manipulative forces of the precision grip / 体力科学

ABSTRACT

The effects of“lifting height (2 cm, 6 cm, and 10 cm) ”and“lifting velocity (natural and fast) ”, and influence of reaching action on timing and manipulative forces of the precision grip were examined while lifting a test object. Five adult males volunteered as subjects for the study. Grip forces, load force (the vertical lifting force) and vertical position of the test object were measured and time derivatives of them were computed using laboratory software. It was found that lifting height and velocity exerted significant interaction effects in movement time of the object during lifting phase (T3), peak load force (PLF) and peak velocity (PVel) . Significant main effects of height and velocity were as follows. The higher the lifting height (2 cm, 6 cm, 10 cm) was, the longer T 3 as well as larger PGF, PLF, PdGF/dt, and PVeI were. For the fast velocity trials, T 2 and T 3 were shorter, and PGF, PLF, PdGF/dt, PdLF/dt, and PVeI were larger than the natural velocity condition. The influence of the lifting velocity was stronger in the height of 6, 10 cm than in the height of 2 cm. For all trials with different heights and different velocity, the force rate profiles (dGF/dt and dLF/dt) were continuous, bell-shaped and single-peaked during the loading phase. There was a parallel increase of grip force in relation to an increase of load force. To reveal the height effect further, the target height was unexpectedly changed in some trials at the moment of finger contact with the grip surface. It was found that both timing and force actions were similar to the initial target height condition followed by an apparent correction of gripping and lifting actions to reach the secondary target height.<BR>Results of this study suggest that both lifting height and velocity were important factors to determine the force actions when manipulating an object. Both grip force and load force seem to be well-programmed according to intended height and velocity conditions.<BR>When the lifting action was made following the reaching action, T 3 became shorter, PGF became less, and PVeI became longer than those without the reaching action. The reaching action thus seems to facilitate the force coordination during the lifting action.