RESEARCH ARTICLE
Manipulation of a fragile object
Stacey L. Gorniak
•
Vladimir M. Zatsiorsky
•
Mark L. Latash
Received: 11 September 2009 / Accepted: 15 December 2009 / Published online: 31 December 2009
Ó Springer-Verlag 2009
Abstract We investigated strategies of adjustments in
kinetic and kinematic patterns, and in multi-digit synergies
during quick vertical transport of an instrumented handle
that collapsed when the grasping force exceeded a certain
magnitude (quantified with a fragility index). The collapse
threshold of the object was set using a novel electromag-
netic device. Moving a fragile object is viewed as a task
with two constraints on the grip force defined by the slip-
ping and crushing thresholds. When moving more fragile
objects, subjects decreased object peak acceleration,
increased movement time, showed a drop in the safety
margin (SM) (extra force over the slipping threshold), and
showed a tendency toward violating the minimum-jerk
criterion. Linear regression analysis of grip force against
load force has shown tight coupling between the two with a
decline in the coefficient of determination with increased
fragility index. The SM was lower in bimanual tasks,
compared to unimanual tasks, for both fragile and non-
fragile objects. Two novel indices have been introduced
and studied, the SM due to fragility and the drop–crush
index. Both indices showed a decrease with increased
object fragility. Changes in the drop–crush index showed
that the subjects would rather crush the fragile objects as
opposed to dropping them, possibly reflecting the particular
experimental procedure. We did not find differences
between the performance indices of the dominant and non-
dominant hand thus failing to support the recently formu-
lated dominance hypothesis. The synergies stabilizing grip
force were quantified at two levels of an assumed two-level
control hierarchy using co-variation indices between ele-
mental variables across trials. There were strong synergies
at the upper level of the hierarchy (the task is shared
between the opposing groups of digits) that weakened with
an increase in object fragility. At the lower level (action
of an effector is shared among the four fingers), higher
fragility led to higher synergy indices. Analysis of force
variance showed that an increase in object fragility was
accompanied by exploring a smaller range of equivalent
combinations of elemental variables. The additional con-
straint imposed by high fragility facilitated synergies at the
lower level of the hierarchy, while there was evidence for a
trade-off between synergies at the two levels.
Keywords Prehension Á Fragile object Á Hand Á
Biomechanics Á Safety margin Á Synergy
Introduction
Recent studies of multi-finger prehension have focused on
the coordination of fingertip forces and moments of force
during manipulation of rigid objects (Burstedt et al. 1997,
1999; Shim et al. 2003, 2005a, b; Zatsiorsky et al. 2003,
2005, 2006). While this is the case for some of the objects
humans encounter in everyday life, they also manipulate
objects that are fragile and deformable. Currently, it is not
known how the human prehensile actions change during
manipulation of fragile objects. The aim of the current
study is to fill this gap.
It has been well-established that, during quick move-
ments of a hand-held object, grip force shows changes that
S. L. Gorniak
Department of Biomedical Engineering,
Cleveland Clinic, Cleveland, OH 44195, USA
S. L. Gorniak Á V. M. Zatsiorsky Á M. L. Latash (&)
Department of Kinesiology, Rec.Hall-267,
The Pennsylvania State University,
University Park, PA 16802, USA
e-mail: mll11@psu.edu
123
Exp Brain Res (2010) 202:413–430
DOI 10.1007/s00221-009-2148-z