PREHENSION SYNERGIRES: EFFECTS OF FINGER MANIPULATION

Poster presentation:
(missing Table 1)

Mark K. Budgeon, Mark L. Latash and Vladimir M. Zatsiorsky

Department of Kinesiology, The Pennsylvania State University, University Park

INTRODUCTION

The purpose of this study was to investigate
how equilibrium of a hand-held object is
maintained when the index (I) or little (L)
finger was removed or added during
different external torques: counter-
clockwise, clockwise and zero torque
(CCW, CW, ZERO). We were interested in
how the finger-tip forces were organized to
maintain equilibrium, whether these forces
followed the principle of superposition
(Zatsiorsky et al., 2004), and if they were
controlled in a feed-forward manner.
Subjects had to exert a pronation effort
(PRO) to resist a CCW torque and a
supination effort (SUP) to resist a CW
torque. The study was inspired by the
literature data on the effects of finger
removal/addition in multi-finger tapping
(Latash et al., 1998) and pressing tasks (Li et
al., 2003). The data were interpreted at the
virtual finger (VF) level for the thumb and
fingers.

METHODS

Seven male subjects with (mean +/- STD)
age = 27.6 ± 4.3, height = 177.7 ± 3.8 cm,
weight = 82.6 ± 12.8 kg, hand width = 9.4 ±
0.4 cm and hand length, measured from
middle fingertip to distal crease while the
hand was extended, = 19.1 ± 1.2 cm
participated in the study. Each subject was
identified as right-handed by their reported
daily activity of the use of their hands. No
history of neuropathies or traumas to the
upper extremities was reported by the
subject. All subjects gave informed consent
in accordance with the Office of Research
Protections of The Pennsylvania State
University.

A handle with five sensors (four for the
fingers, one for the thumb) arranged such
that the vertical location of the thumb was
between the M and R fingers, described in
more detail in Aoki et al. (2003), was used
to collect data. A beam was attached to the
bottom of the handle and a load of 0.55 kg
was suspended from the beam at three
locations to create three torques: CCW, CW
and ZERO.

The data were recorded at 200Hz and low
pass filtered at 10Hz. Steady state values
were calculated as the average for one
second at the beginning and end of the trials.
CW/PRO ZERO CCW/SUP CW/PRO ZERO CCW/SUP
I, normal =1.56 +2.55 +2.77 +3.60 =0.97 -1.60
L, normal +3.02 +2.24 =0.16 =0.12 =0.88 +2.31
I, tangential +2.88 +1.70 +0.70 –3.15 –1.67 –0.64
L, tangential =0.29 –0.30 –0.87 +0.88 +0.96 +1.54
Manipulated
finger Finger removal, 4-to-3 tasks
Finger addition, 3-to-4 tasks

Significance was tested at p < 0.05.
A one-way repeated measures ANOVA was
performed on the factor PERTURBATION
(before, after); one ANOVA per moment
condition and finger manipulation, for a
total of 24.

RESULTS AND DISCUSSION
Figure 2: Thumb vs. VF (dotted and solid
lines) tangential forces during an I addition,
PRO effort trial.

The normal forces significantly changed
depending on the role of the finger, i.e.
whether it was an agonist (helped in the
exerted effort, like the I finger during PRO
effort) or antagonist, see the first two rows
of Table 1. The increases in normal force
were not mechanically necessary because
the force prior to the increase was sufficient
to prevent slipping. The tangential forces
significantly changed depending on the
finger manipulated (I or L), see the last two
rows of Table 1.

The thumb and VF normal forces highly
correlated with each other, as we expected,
but the normal forces did not correlate with
the normal moment. The thumb and VF
tangential forces correlated with each other
as well as with the tangential and normal
moment. The correlations were grouped
into two subsets (see Figure 1), like the
findings of Zatsiorsky et al. (2004), which
support the principle of superposition – two
commands were sent to the hand: “grasp the
object stronger/weaker” and “prevent
tilting.”

The normal and tangential force of the VF
and thumb changed synchronously, see
Figure 2. This was supported by high
Pearson correlation coefficients (~0.99)
between the thumb and VF forces. Also,
standard deviations of the performance
variables — total tangential and total normal
force and total moment — were low (the
highest value for force was 0.41 N and for
moment was 0.24 Ncm). These immediate
compensations indicate that the CNS plans
adjustments of the forces before the
manipulation.

SUMMARY

Changes in normal digit force depend on the
role of the manipulated finger while changes
in tangential force depend on the finger
manipulated. The changes were made
according to the principle of superposition
and in a feed-forward manner.

REFERENCES
Latash, et al.(1998). Exp Brain Res , 122,
131-138.
Li, et al. (2003). Exp Brain Res 150, 230-
236.
Aoki, et al. (2006). Exp Brain Res 172, 425-
438.
Figure 1: Correlations between the final
steady state values for normal and
tangential forces and moments. Zatsiorsky et al. (2004). Exerc Sport Sci Rev
32(3): 75-80.