\
PERGAMON
Neuropsychologia 26 "0888# 640Ð645
Note
Impact of the surface slipperiness of grasped objects on their
subsequent acceleration
Pierre Saelsa\ Jean!Louis Thonnarda\\ Christine Detrembleura\ Allan M[ Smithb
a
Unite de Readaptation et de Medecine Physique\ Universite catholique de Louvain\ 42 Ave[ Mounier B!0199 Brussels\ Bel`ium
b
Centre de Recherche en Sciences Neurolo`iques\ Departement de Physiolo`ie\ Universite de Montreal\ Canada
Received 13 September 0886^ accepted 5 August 0887
Abstract
Seven subjects were asked to reach and grasp an object between the thumb and index _nger\ lift it about 29 cm high and 14 cm
forward from one table to another\ at their preferred speed[ The perpendicular grip force and the tangential load force applied to
the contact surface were digitized at 499 Hz and stored on a laboratory computer[ The trajectory of the wrist and of the object was
recorded using four infrared cameras tracking the movement of re~ective markers attached to the distal styloid process of the radius
and on the top of the object[ The aim of this study was to demonstrate the in~uence of low friction "i[e[ surface slipperiness# on the
acceleration of the wrist[ Friction was reduced by coating the smooth brass grasping surface with talc[ The seven subjects had skin
to surface coe.cients of friction which ranged from 9[41Ð0[07 for dry brass and 9[13Ð9[23 for talc!coated brass[ Two weights "307
and 0969 g# were used with each surface[ The results indicated that with the slippery surface the necessary higher grip force:load
force ratio was produced by an increase in the grip force and by a decrease in the wrist acceleration and a consequent reduction in
the load force[ This strategy was observed for both weights over a range of grip strengths between 10Ð87) of the individual|s
maximum voluntary contraction "MVC#[ This implies that even with adequate grip force reserves the reduction in acceleration is an
acceptable and probable alternative solution to the force control problem[ Our results also suggested that the loading rate and the
object acceleration were planned and controlled together which emphasizes the role played by a predictive mechanism in organizing
the kinematics of movements involving hand!held objects[ This study shows that friction of the grasping surface not only a}ects the pre!
hensile force dynamics\ but it also in~uences the kinematics of the entire upper limb[ Þ 0888 Elsevier Science Ltd[ All rights reserved[
Keywords] Motor control^ Precision grip^ Hand movement^ Friction^ Kinematics^ Dynamics
0[ Introduction
Grasping and lifting small objects requires a prehensile
force greater than the opposing forces of gravity and the
inertial load of the object to be moved[ Johansson and
Westling ð4Ł showed that a fairly constant ratio exists
between the grip force applied normal to the contact
surface\ and the load force tangential to the contact
surface\ in order to insure secure object manipulation
without slip[ The cutaneous feedback from slip!sensitive
receptors in the glabrous skin of the hand provide the
essential stimulus for adapting grip and load forces to
the friction at the skin!object interface ð4\ 8\ 09Ł[
A common feature of the previously mentioned studies\
was that the changes in load forces was applied to the
subject|s hand as the object was being held stationary[ In
Corresponding author[ Tel[] ¦21 1 653 4264^ fax] ¦21 1 653 4259^
e!mail] thonnardÝread[ucl[ac[be
a logical extension of these studies\ Flanagan and Wing
ð1\ 2Ł examined the grip force modulation as the subjects
performed either point!to!point or cyclic arm movements
with a hand!held load[ They found that variations in
inertial forces caused by the subjects| own arm move!
ments over a range of accelerations produced parallel
changes in grip forces that rose and fell with the changes
in tangential load forces on the _ngers[ Regardless of the
object|s surface friction or the frequency of movement
applied to the object\ the grip forces were modulated in
parallel with the load forces\ that is\ the grip forces
re~ected an anticipation of the ~uctuations in inertial
forces[ It can be concluded from these studies that the
nervous system not only can accurately predict the iner!
tial properties of familiar stationary objects\ but in
addition\ it can compensate for variations in inertial loads
due to changes in acceleration resulting from self!gen!
erated movements[
In the present study\ we set out to document a well!
known but unexplored corollary of this principle which
9917!2821:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved
PII] S 9 9 1 7 ! 2 8 2 1 " 8 7 # 9 9 0 0 2 ! 3
641
P[ Saels et al[ : Neuropsycholo`ia 26 "0888# 640Ð645
is that when accelerating a slippery grasped object\ the
higher grip force:load force ratio necessary to prevent
slip is produced by decreasing the movement acceleration
to reduce the tangential load on the skin as well as by
increasing the grip force[
1[ Methods
1[0[ Experimental task
Seven subjects "four men\ three women# between 12
and 44 years of age gave informed consent before par!
ticipating in this study[ The subjects sat on a chair beside
two small tables\ shown in Fig[ 0\ with the elbow ~exed
at 89> near the body and with the right hand about 04
cm from the test object[ On instruction\ they reached
forward\ grasped the test object between the thumb and
index _nger and lifted it from one table to a second
surface 29 cm higher and 14 cm forward[ Subjects were
instructed to accomplish the task spontaneously at their
preferred speed[ The two parallel vertical grip surfaces of
the test object were spaced 29 mm apart and were made
of smooth brass[ The coe.cient of friction was modi_ed
by applying talc to the brass grasping surfaces[ The sub!
jects were instructed to wash and dry their hands before
beginning the experiment[ Two experimental conditions
were carried out for each subject[ In the _rst condition\
each subject was asked to perform a series of _ve upward
movements when the object surface was dry brass[ In the
second experimental condition each subject was
requested to execute another series of _ve lifts with the
grip surfaces coated with talc[ Each experimental con!
dition was performed with the test object weighing either
307 g or 0969 g[ The lighter weight was tested before
the heavier weight in all cases[ However\ because of the
known anticipatory e}ects of object weight ð5Ł\ each sub!
ject was given three practice trials with the new inertial
load before data collection began[
The experiment ended by measuring the mean maximal
voluntary strength of the precision grip for each subject
on three successive trials ð7Ł[
1[1[ Kinematic data
The trajectory of the wrist and of the test object was
recorded using passive re~ective markers "9[3 cm diam!
eter# secured to both the distal styloid process of the
radius and on the top of the test object[ The kinematic
data were collected by an Elite System 4[9þ\ using four
infrared cameras tracking the coordinates of the re~ective
markers at 49 Hz[ The cameras "see Fig[ 0# were placed
above the calibrated working space and were inclined at
an angle of 34> to the vertical\ 0[4 m above the test object
and spaced 0[9 m apart[ The working space was calibrated
Fig[ 0[ Schematic illustration of the experimental task with the dynamic and kinematic data for _ve successive movements executed by the same
subject[
P[ Saels et al[ : Neuropsycholo`ia 26 "0888# 640Ð645
using a grid of 6×3 markers spaced at 09 cm intervals[
The grid was shifted within a parallelepiped 59 cm long\
29 cm wide\ and 43 cm high between 47Ð001 cm from the
~oor in steps of 07 cm\ to yield an overall error of 9[3)[
The coordinates were smoothed and derived by the Elite
System 4[9þ software using a _nite impulse response lin!
ear _lter ð0Ł\ and the displacement\ velocity and accel!
eration of the wrist and of the test object were determined
in the sagittal plane[
slipped due to gravity[ The measure of friction was cal!
culated as half the load force:grip force ratio at slip onset
ð4Ł as detected by the accelerometer[ The _ve measures of
friction taken before the sequence of upward movements
were averaged with the _ve measures of friction per!
formed afterwards\ in order to provide a single\ reliable
coe.cient of friction for each subject and for each surface
condition[
1[4[ Statistical analysis
1[2[ Dynamic data
The forces perpendicular to the grip surfaces of the
apparatus "i[e[\ the grip forces# were measured with
strain!gauge force transducers[ Although the grip forces
"GF# were measured for the thumb and the index _nger
separately\ in this study a single grip force was taken as
the mean value measured at the two grip surfaces[
The vertical component of the force tangential to the
grip surfaces\ was computed from the following equation]
LF m" `¦av#−Fr
642
"0#
For each subject\ the mean values of the dynamic and
kinematic variables were calculated for each series of
_ve trials[ A two!way repeated measures ANOVA was
performed to determine the e}ect of object weight\ sur!
face friction and the interaction between these two vari!
ables on the dynamic and kinematic variables[ Simple
linear regression analysis was performed to determine the
relation between the peak vertical acceleration of the
wrist and the peak load force rate[ A P!value of ³9[94
was accepted as indicative of statistical signi_cance[
Where]
LF is the load force
m is the mass of the test object\
` is the gravitational acceleration\
av is the vertical acceleration of the object\
Fr is the table vertical reaction force[
The vertical reaction force of the table "Fr# was mea!
sured by strain gauge transducers "which were glued on
a horizontal steel plate integral with the test object#[ An
accelerometer was mounted on the test object and pos!
itioned to record the vertical acceleration "av#[ The con!
tribution of the horizontal acceleration to the total force
tangential to the grip surfaces never exceeded 2) in any
subject or condition[ Therefore we decided to omit the
horizontal acceleration in the computation of this force
and we measured solely the vertical component of the
lifting force\ which is termed the load force "LF#[
The _rst time derivative of the load force "i[e[\ the load
force rate^ dLF:dt# and the _rst time derivative of the
grip force "i[e[\ the grip force rate^ dGF:dt# were computed
using a 24 point numerical time di}erentiation[ All
dynamic signals were sampled at 499 Hz with a 01 bit
analog to digital converter[
1[3[ Measuring the coef_cient of friction
The coe.cient of static friction between the _ngertips
and the grip surfaces was measured for each subject both
before and after each sequence of _ve upward movements
in a separate series of _ve lift and drop maneuvers[ The
subjects were instructed to lift the test object from the
table[ The object was brie~y held stationary in the air
and then the grip was gradually released until the object
2[ Results
Five successive lifts by a single subject for each of the
two surface conditions are displayed in Fig[ 1[ The _gure
shows typical vertical acceleration traces as well as the
grip and load forces with their respective derivatives and
the movement shown as changes in horizontal and ver!
tical wrist position[ Each trace encompasses the time
between the grip force onset and the _nal touchdown of
the test object[ All the traces in Fig[ 1 have been aligned
on the instant of object lift!o}[ The peak vertical accel!
eration and peak load force rate were reduced with the
talc coating[ The peak grip force\ unlike the peak load
force\ tended to increase with talc but the peak grip force
rate was not in~uenced by the coe.cient of friction[ The
hand paths in the vertical and horizontal directions were
the same for the two grip surfaces\ although the velocities
decreased with the coe.cient of friction[
The means and standard deviations of the dynamic and
kinematic variables for both object weights and friction
conditions are presented in Table 0\ along with the results
of the statistical analyses[ An ANOVA was performed to
evaluate the e}ect of object weight and surface friction
on the dynamic and kinematic variables[ As might be
expected\ the weight of the object signi_cantly decreased
the peak vertical acceleration and increased the grip force
parameters[ The most notable _nding of the present study
was that the surface friction signi_cantly in~uenced the
vertical acceleration[ In fact the surface friction had sig!
ni_cant e}ects on all the variables listed in Table 0 except
the peak grip force rate[ There was no signi_cant inter!
action between the surface friction and the object weight[
The seven subjects had skin to surface coe.cients of
643
P[ Saels et al[ : Neuropsycholo`ia 26 "0888# 640Ð645
Fig[ 1[ Five successive records from one subject with the two contact surfaces[ The coe.cients of friction are 9[72 for dry brass and 9[16 for talc
covered brass[ Dynamic and kinematic data are plotted from initial touch of the test object with the _ngers until touchdown of the object on the
second table[ The traces are synchronized at the lift!o} "dashed lines# of the object from the _rst table[
friction which ranged from 9[41Ð0[07 for dry brass and
9[13Ð9[23 for talc!coated brass[
A Pearson product moment correlation was calculated
between the peak load force rate measured during the
loading phase and the subsequent peak vertical accel!
eration of the wrist measured during the aerial phase
for both object weights[ For the seven subjects and two
friction conditions the correlation was 9[42 "n 69\
P ³ 9[990# for the 307 g weight and 9[47 "n 69\
P ³ 9[990# for the 0969 g weight[ The peak load force
rate occurred before object lift!o} whereas the peak accel!
eration of the wrist occurred after object lift!o} and the
latency between these two was not _xed[
3[ Discussion
In this study we examined the e}ect of surface slip!
periness on the coordination between grip force\ load
force and the kinematics of a hand!held object[ The
results show that the strategy adopted to prevent slip of
a grasped object during transport consists not only in
644
P[ Saels et al[ : Neuropsycholo`ia 26 "0888# 640Ð645
Table 0
Results of the measurements made with the dry brass and talc covering surfaces for both object weights
Mean "S[D[#
Weight "307 g#
Variables
Dry brass
Talc
Peak vertical acceleration 2[2 "29[47# 1[2 "29[66#
"m s−1#
Peak grip force "N#
19 "21[7#
14 "24[9#
Peak grip force "N s−0#
011 "224[4# 013 "247[8#
Peak grip force:MVC ")# 25 "209[5# 33 "209[1#
Peak load force rate "N s−0# 53 "208[5# 34 "208[2#
Two!way!repeated measures ANOVA
Weight "0969 g#
E}ect of weight
Dry brass
Talc
df
F
1[2 "29[21#
0[2 "29[25#
0
07[78
25 "24[4#
31 "24[2#
050 "247[9# 042 "245[4#
52 "205[1# 64 "208[9#
70 "204[4# 44 "26[7#
P
E}ect of friction
df
F
P
Interaction
df
F
P
9[994
0 01[58
9[901
0 9[91
9[772
0 166[92 ³9[990
0
5[32 9[933
0 86[56 ³9[990
0
2[05 9[015
0 05[60
0 9[97
0 04[92
0 22[38
9[995
9[677
9[997
9[990
0
0
0
0
9[326
9[206
9[172
9[269
9[58
9[21
0[28
9[83
Statistically signi_cant
regulating the grip force but also involves adjusting the
load force by modulating the acceleration of the hand[
Grasp stability during manipulative actions is assured
by maintaining a fairly constant ratio between the grip
force and the load force ð4\ 8Ł[ The ratio at which slipping
occurs\ denoted as the slip ratio\ depends on the
coe.cient of friction of the gripping surfaces[ Johansson
and Westling ð4Ł reported that the more slippery the
object\ the higher the force ratio employed mainly by
producing higher rates of grip force increases during the
preload and loading phase whereas the load force was
una}ected[ In Johansson and Westling|s ð4Ł experiments\
the subjects lifted the test object only one or two cen!
timeters above the table surface at very low speeds "about
1 cm s−0#[ The load force was therefore not greatly in~u!
enced by the acceleration of the object and did not appear
to greatly exceed the weight of the object[ In contrast\
Flanagan and Wing ð1Ł showed that higher acceleration
produces a proportional increment in the load force
which increases the probability of object slip[ In their
study the object acceleration and deceleration was
accompanied by a signi_cant grip force modulation[ The
present study adds to these _ndings by demonstrating
that the relationship is reciprocal[ That is the friction
between the object and the hand signi_cantly in~uences
the subsequent acceleration of that object[
The subjects in the present experiment grasped the
object and carried it at their preferred speed 29 cm higher
and 14 cm forward[ In the event of slip during the aerial
transport phase\ the appropriate ratio between the grip
force and the load force can be restored either by increas!
ing the grip force or by decreasing the acceleration of the
hand and thus reducing the load force[ In the present
study\ subjects used both strategies simultaneously when
displacing the surfaces covered with talc as shown by an
increase in the peak grip force and a decrease in the
vertical acceleration[ The dynamics of prehension and
the kinematics of the upper limb are bound to each other
to insure that the safety margin is adequate and the hand!
held object does not slip out of _ngers[ An important
implication of this study is that the nervous system seems
to adopt this strategy over a range of grip forces extending
from 10Ð87) of the MVC[ A further implication is that
even when grip force reserves are available the nervous
system may nevertheless lessen the acceleration[
Although Table 0 shows an identical numerical
decrease in vertical acceleration and increase in grip force
when shifting from one surface to the other for the two
weights the relative or percentage changes are di}erent[
E}ectively a 4[9 N increase in the peak grip force for the
307 g weight represents a 14) change\ whereas the same
increase corresponds to only a 06) increase in grip force
with the 0969 g weight[ This relative di}erence is even
greater with respect to the peak acceleration[ The same
0[9 m s−1 decrease in acceleration corresponded to a 29)
decrease for the 307 g weight and a 32) decrease for the
0969 g weight[ These results would seem to suggest both
the increased grip strategy and the decreased acceleration
strategy depend on a quantitative internal estimate of
friction and inertial load[
It may also be pointed out that the increase in the
peak grip force was not invariably accompanied by a
signi_cant increase in the peak grip force rate such as
previously reported ð4Ł[ In contrast\ we noted that sub!
jects signi_cantly decreased both the load force rate and
the vertical acceleration[ This suggests that the modu!
lation of the grip and load forces may be controlled
independently according to speci_c task requirements or
movements[ That is\ either strategy could be adopted for
preventing slip of a grasped object[
We would also like to emphasize the correlation
between the rate of loading during the loading phase and
the acceleration of the object during the aerial phase over
a range of frictional conditions[ This correlation suggests
that before object lift!o}\ and as a function of friction
the nervous system plans and controls the peak load force
rate and the subsequent peak vertical acceleration of the
wrist together[ Our results support the hypothesis that
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P[ Saels et al[ : Neuropsycholo`ia 26 "0888# 640Ð645
the vertical acceleration of the object after lift!o} could
be adjusted in a feedforward manner using a}erent infor!
mation in relation with skin deformations occurring dur!
ing the preload phase when the contact between the digits
and the object is being established ð09Ł[ In a previous
study\ Kinoshita et al[ ð6Ł observed that the peak of the
grip and load force rates changed linearly with the
increasing speed in a lifting task using the pinch grip[
They concluded that the grip force as well the load force
which was generated prior to the event of object lift!o}
were programmed in advance with fast lifting speeds\ and
could be considered as preparatory motor actions[ The
_nding of our study is in accordance with the results of
Kinoshita et al[ ð6Ł and emphasizes the role played by
predictive mechanism in planning and controlling arm
movements involving hand!held objects ð3Ł[
Taken together\ the data from the present study show
that the friction between the object surface and the _ngers
contributes to an internal model which not only a}ects
the dynamics of prehensile force\ but which also has a
signi_cant impact on the control of more proximal elbow
and shoulder muscles accelerating the hand[
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