Tactile feedback plays a critical role in maximum finger force production.

Abstract

This study investigates the role of cutaneous feedback on maximum voluntary force (MVF), finger force deficit (FD) and finger independence (FI). FD was calculated as the difference between the sum of maximal individual finger forces during single-finger pressing tasks and the maximal force produced by those fingers during an all-finger pressing task. FI was calculated as the average non-task finger forces normalized by the task-finger forces and subtracted from 100 percent. Twenty young healthy right-handed males participated in the study. Cutaneous feedback was removed by administering ring block digital anesthesia on the 2nd, 3rd, 4th and 5th digits of the right hands. Subjects were asked to press force sensors with maximal effort using individual digits as well as all four digits together, with and without cutaneous feedback. Results from the study showed a 25% decrease in MVF for the individual fingers as well as all the four fingers pressing together after the removal of cutaneous feedback. Additionally, more than 100% increase in FD after the removal of cutaneous feedback was observed in the middle and ring fingers. No changes in FI values were observed between the two conditions. Results of this study suggest that the central nervous system utilizes cutaneous feedback and the feedback mechanism plays a critical role in maximal voluntary force production by the hand digits.

Full text

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Tactile feedback plays a critical role in maximum finger force production
Jae Kun Shima,b,n, Sohit Karola, You-Sin Kimc, Na Jin Seod, Yoon Hyuk Kimb, YuShin Kime,
Bum Chul Yoone,nn
aDepartment of Kinesiology, University of Maryland, College Park, MD, USA
bDepartment of Mechanical Engineering, Kyung Hee University, Global Campus, Korea
cDepartment of Leisure Sports, Jungwon University, Chungcheongbuk-do, 367-805, Korea
dDepartment of Industrial Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI, USA
eDepartment of Physical Therapy, College of Health Science, Korea University, San 1, Jeongreung-dong, Seongbuk-gu, Seoul 135-778, Korea
a r t i c l e i n f o
Article history:
Accepted 2 December 2011
Keywords:
Hand
Finger
Anesthesia
Cutaneous feedback
Maximum force
a b s t r a c t
This study investigates the role of cutaneous feedback on maximum voluntary force (MVF), finger force
deficit (FD) and finger independence (FI). FD was calculated as the difference between the sum of
maximal individual finger forces during single-finger pressing tasks and the maximal force produced by
those fingers during an all-finger pressing task. FI was calculated as the average non-task finger forces
normalized by the task-finger forces and subtracted from 100 percent. Twenty young healthy right-
handed males participated in the study. Cutaneous feedback was removed by administering ring block
digital anesthesia on the 2nd, 3rd, 4th and 5th digits of the right hands. Subjects were asked to press
force sensors with maximal effort using individual digits as well as all four digits together, with and
without cutaneous feedback. Results from the study showed a 25% decrease in MVF for the individual
fingers as well as all the four fingers pressing together after the removal of cutaneous feedback.
Additionally, more than 100% increase in FD after the removal of cutaneous feedback was observed in
the middle and ring fingers. No changes in FI values were observed between the two conditions. Results
of this study suggest that the central nervous system utilizes cutaneous feedback and the feedback
mechanism plays a critical role in maximal voluntary force production by the hand digits.
Published by Elsevier Ltd.
1. Introduction
The human hand is one of the most versatile organs of the
body and is the primary tool by which humans manipulate
objects. One of the most important sensory feedback mechanisms
for dexterous manipulations is the cutaneous feedback from the
digits (Johansson and Flanagan, 2009). Cutaneous afferent feed-
back provides critical information regarding the current state of
the system and enables the central nervous system (CNS) to
modify the motor plan accordingly (Gandevia and McCloskey,
1978; Johansson, 1998; Nowak et al., 2002). Different types of
cutaneous receptors provide feedback about the shape, texture,
pressure, temperature and other physical properties of objects to
the CNS during manipulation (McGlone and Reilly; Johnson et al.,
2000; Johnson, 2001; Johansson and Flanagan, 2009).
Previous studies have administered digital anesthesia as a
primary modality to remove cutaneous feedback from the digits
and investigated the role of cutaneous feedback on CNS control
over hand and finger actions (Gandevia and McCloskey, 1977;
Kilbreath et al., 1997; Reilly et al., 2008; Schieber et al., 2009).
Studies on sub-maximal force production tasks such as grasping
and pinching have shown that effective removal of cutaneous
feedback leads to an increase in errors during motor performance
(Nowak et al., 2001; 2004; Monzee et al., 2003). The extent of
cutaneous feedback is known to change with the digits involved
in the task and also affects the forces being perceived by the digits
during weight matching tasks (Gandevia and McCloskey, 1978;
Gandevia et al., 1980; Kilbreath and Gandevia, 1991; Kilbreath
et al., 1997). Further, patients who have lost cutaneous feedback
as a result of sensory neuropathy have also been reported to
produce inaccurate as well as lower amount of sub-maximal
forces (Forget and Lamarre, 1987; Cole and Sedgwick, 1992).
Maximum voluntary force (MVF) production by the digits is an
essential part of performing day to day activities, and its impor-
tance has been well documented in the literature (Duinen et al.,
2010; Slobounov et al., 2002; Goodman et al., 2004; Lang and
Schieber, 2004; Yu et al., 2010). As opposed to sub-maximal force
production tasks such as everyday pressing and grasping, where
fine motor control with effective cutaneous feedback is vital for
successful motor performance, maximal force production may
utilize different control mechanisms. However, there is little
Contents lists available at SciVerse ScienceDirect
journal homepage: www.elsevier.com/locate/jbiomech
www.JBiomech.com
Journal of Biomechanics
0021-9290/$-see front matter Published by Elsevier Ltd.
doi:10.1016/j.jbiomech.2011.12.001
nCorresponding author at: Department of Kinesiology, University of Maryland,
College Park, MD 20742, USA. Tel.: þ1 301 405 9240.
nnCorresponding author. Tel.: þ82 2 940 2833.
E-mail addresses: jkshim@umd.edu (J.K. Shim), yoonbc@korea.ac.kr (B.C. Yoon).
Journal of Biomechanics 45 (2012) 415–420

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information available on the role of cutaneous feedback during
MVF production, and it is unclear if and how cutaneous feedback
plays a role in the maximal force producing capacity of the hands.
A previous study on the effect of anesthesia during a pinching
task showed that cutaneous feedback might play a role in MVF in
pinching (Augurelle et al., 2003).
Further, several behavioral aspects of pressing have been well
documented in the literature (Li et al., 1998; Shim et al., 2007). It
has been shown that when a single digit produces a voluntary
force, the other fingers produce an unintended force. This
phenomenon has been called finger enslaving (FE) or finger
independence (FI) in the literature (Li et al., 1998; Lang and
Schieber, 2004). It has also been shown that during multi-finger
pressing, the sum of maximal forces produced by the individual
fingers is greater than the sum of forces produced by all the
fingers together. This has been referred to as finger force deficit
(FD) in the literature (Li et al., 1998). Both FI and FD are thought
to depend on the central as well as biomechanical factors (Li et al.,
1998; Shinohara et al., 2003; Shim et al., 2008). Studies have
shown changes in FI and FD with age, gender, strength training
and the phalanges of the fingers being used while pressing
(Shinohara et al., 2003; Shim et al., 2007; Shim et al., 2008).
However, little is known about the role of cutaneous feedback on
MVF, FD or FI.
The purpose of this study was to investigate the role of
cutaneous feedback in maximum force production by single digits
as well as multiple digits of the hand during pressing tasks. It was
hypothesized that MVF values would decrease, FD values would
increase and FI values would decrease during MVF production by
fingers after the removal of cutaneous feedback.
2. Methods
2.1.Subjects
Twenty healthy volunteers (sex: males, age: 23.9571.00 years, body mass:
68.0075.21 kg, height: 174.6775.59 cm) with no history of neurological
disorders participated in the experiments. All the participants were right handed
according to the Edinburgh handedness test criteria. The hand length measured
from the middle finger tip to the lunate of the wrist was 17.1871.02 cm, and the
hand width measured across the metacarpophalangeal (MCP) joints of the index
and little fingers was 9.487.60 cm. All the participants gave informed consent
based on the procedures approved by the Internal Review Board.
2.2. Experimental setup
The experimental setup included four two-directional (tension and compres-
sion) force sensors for four fingers (2nd–5th fingers) with amplifiers (Models 208
M182 and 484B, Piezotronics, Inc.). The sensors were mounted on a customized
aluminum frame (14.0?9.0?1.0 cm) and had parallel slits aligned vertically
(14.0 cm). These slits enabled adjustments to the sensors according to the hand
and finger sizes of the subjects. A C-shaped aluminum thimble was attached at the
bottom of each sensor. Thimbles were placed at a fixed distance of 3 cm in the
mediolateral direction, such that the distal phalange of each finger could
comfortably rest on an individual sensor. Signals from the sensors were condi-
tioned, amplified and digitized at 1000 Hz with a 16-bit A/D board (PCI 6034E,
National Instruments Corp.) and a custom software program made in LabVIEW
(LabVIEW 7.1, National Instruments Corp.) A desktop computer (Dimension 4700,
Dell Inc.) with a 19 in. monitor was used for data acquisition. Offline data
processing was done using customized programs written in MATLAB (MATLAB
7, MathWorks, Inc.).
2.3. Procedure
Subjects were asked to rest the distal phalanges of each of the four fingers of
the right hand in the C-shaped thimbles, such that all joints were slightly flexed
and formed a dome shape with the hand (Fig. 1a). The MCP joints were flexed at
about 251. The palmar surface of the palm and the fingers was not restrained
physically in order to prevent any tactile feedback from the cutaneous receptors in
the palm. Subjects were instructed to sit on a chair facing a computer screen, with
the shoulder abducted 351 in the frontal plane and elbow flexed 451 in the sagittal
plane. The forearm rested on a customized wrist-forearm brace (comprised of a
piece of foam that was attached to a semi-circular plastic cylinder) fixed to a
wooden panel (29.8?8.8?3.6 cm). Two Velcro straps, one near the wrist and the
other near the elbow, were used to prevent any wrist joint or forearm movements.
In order to remove the gravitational effects of the fingers, the force signals for the
initial .5 s were averaged for each finger and subtracted from the later signals.
Thus, only the force signals due to active force production were shown on the
computer monitor in real-time to subjects (Fig. 1b).
Five different finger combinations (four single-finger tasks and one four-finger
task) for the MVF task were presented to the subjects. Subjects were shown the
force being produced by the task finger or fingers on the screen. One trial was
performed for each condition. The five conditions were presented to the subjects
in a randomized order, with an interval of 3 min between consecutive pressing
conditions. Once the subjects had comfortably positioned their fingers on their
sensors, the investigators started the data collection program, which generated a
‘‘get ready’’ sound. This was followed by a ‘‘ding’’ sound after 2 s. Subjects were
instructed to ‘‘press as hard as possible with the task finger and relax the fingers
once they feel they cannot press any harder’’. After 7 s, another audio cue in the
form of a ‘‘ding’’ sound was presented to indicate the end of the trial. Throughout
the trial, subjects were given real time visual feedback of their force performance
in the form of a horizontal bar on the computer screen. The horizontal bar moved
in the downward direction as the subjects pressed on the sensors with the task
finger. The vertical axis on the visual feedback being presented was labeled with
the task finger force in Newton. No subject reported fatigue.
This process was repeated for two different experimental conditions. In one
condition, subjects were asked to press the sensors and follow the experimental
protocols in the normal condition. In the other condition, 5 min after a topical
anesthesia (Dermacain Cream 5%, Hana Pharm Co., Ltd., Seoul, Korea) was applied
to the digits, local anesthesia (Lidocaine HCI 1%, DaiHan Pharm. Co., Ltd., Seoul,
Korea) was injected at four sites around the middle phalanges of all four fingers
(35 cc. for index, middle and ring fingers; 25 cc. for little finger). This was followed
by a stroking massage in the direction of distal phalanges. Von Frey tactile hair
stimulation was used to assess the threshold of tactile sensation in the normal and
anesthesia condition (Voller et al., 2006). Cutaneous anesthesia was defined as the
inability of the subjects to detect the application of a Von Frey filament exerting a
pressing force on the distal pad of the digits. In the normal condition, subjects
were able to detect the filament size of (2,44), while in the anesthesia condition,
they were unable to detect the filament with the maximum diameter (size 6,65).
2.4. Data processing
Maximum voluntary force (MVF) was measured as the peak forces produced
by task finger or fingers during single-finger tasks and a four-finger task. The force
deficit (FD) for each finger, FDi, was calculated as the difference of the maximum
force produced by an individual finger in a single finger task, Fi,i, and the force
produced by the same finger in the four finger task, Fi, IMRL(Eq. (1)).
FDi¼Fi,i?Fi,IMRL:
The overall FD was calculated by taking the average FD values across all single-
finger tasks (FDi). Further, FD values were normalized by the respective MVF of the
task-fingers in both the conditions.
The average value of FI across all four fingers was calculated as shown below
(Eq. (2)).
ð1Þ
FI ¼ 100?
Pn
j?1½100Pn
i?1ðFij=Fi
n
maxÞ=ðn?1Þ?
ð2Þ
where, iaj, n¼4, Fi
involuntary force produced by the non-task finger i during the j finger MVF task.
maxis the maximal force produced by the finger i, and Fijis the
2.5.Statistics
One-way repeated-measures ANOVAs were conducted to compare the MVF, FD
and FI values between the two conditions, with and without anesthesia. The level
of statistical significance was set at p¼.05.
3. Results
3.1. Maximum voluntary force (MVF)
The MVF values during single-finger tasks decreased signifi-
cantly after the administration of local anesthesia for all the finger
combinations (Fig. 2a). Specifically, the average values of MVF
decreased from 42.1571.84 N to 34.0071.90 N in the index
finger task, from 36.0571.63 N to 30.2271.42 N in the middle
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finger task, from 26.0171.54 N to 21.1071.34 N in the ring
finger task, from 21.8171.19 N to 17.517.95 N in the little
finger task and 113.8475.51 N to 79.0974.91 N in all the four
fingers pressing together (Fig. 2b). Decrease in MVF values after
the administration of anesthesia was statistically significant for the
individual fingers (Index: F1,19¼25.63, po001; Middle: F1,19¼19.14,
po001, Ring: F1,19¼18.15, po001, Little: F1,19¼12.18, po05) as
well as the four-finger task (F1,19¼46.09, po001).
3.2. Force deficit (FD)
Force deficit (FD) generally increased after the administration
of anesthesia (Fig. 3a). The increases in FD values were significant
for the middle, ring and little fingers (Middle: F1,19¼15.25, po05;
Ring: F1,19¼6.85, po05, Little: F1,19¼3.92, po05). FD value
changed from .477.30 N to 3.1371.28 N for the middle finger,
from .557.48 N to 3.9671.20 N for the ring finger and
5.587.88 N to 7.9271.02 N for the little finger. FD value for
the index finger in the normal condition was 7.1371.72 N as
compared to 7.9171.57 N in the anesthesia condition, although
this change was not statistically significant. The average FD values
for all four fingers together also increased significantly from
4.257.97 N to 6.0071.32 N after the administration of local
anesthesia (Fig. 3b). The results were supported by ANOVA
(F1,19¼46.09, po.001). On average, normalized FD values in the
normal condition were 50% of those in the anesthesia condition
and followed similar trends as the absolute FD values (Fig. 3c and
d). The increases in normalized FD values were significant for the
middle, ring and little fingers (Middle: F1,19¼19.32, po05; Ring:
F1,19¼8.73, po05, Little: F1,19¼2.85, po05).
3.3. Finger independence (FI)
No significant changes in the FI values were observed. The
average FI value in the normal condition was 77.0575.23%
while that after the administration of digital anesthesia was
80.9176.79% (F1,19¼3.07, p405).
4. Discussion
Decrease in MVF values after the removal of cutaneous feed-
back is consistent with findings from a previous study on deaf-
feranated hand (Reilly et al., 2008). Previous studies have shown
that the peripheral afferents have a net facillitatory effect on
motoneurons (Gandevia and McCloskey, 1978; Gandevia et al.,
1993). One of such studies used ulnar nerve block to deaffernate
the intrinsic muscles as well as the cutaneous receptors of the
hand. It was reported that during attempted maximal voluntary
efforts the mean discharge rate of single motor axons was
significantly lower than those of normally-innervated motor
units (Reilly et al., 2008). Another study using ring block anesthesia
Fig. 1. (a) Subjects were instructed to insert the distal phalanges of the fingers in the thimbles attached to a customized aluminum frame. (b) A real-time visual feedback of
the force production was presented on a computer monitor to the subjects.
Fig. 2. (a) Maximum voluntary force (MVF) in N, produced by the individual fingers of the subjects in the normal condition (outer black lines) and after the administration
of anesthesia of the distal phalanges of the fingers (inner gray lines). Radii of the data-point circles represent the standard errors of the mean. I, M, R and L as shown in each
axis represent the index, middle, ring and little finger tasks, respectively. The data are showed in the scale between 0 N and 50 N. (b) MVF values for four-finger tasks with
and without anesthesia administration. Means and standard errors are shown across the subjects (npo001;nnpo05).
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also reported a decrease in the maximum generated forces in a
pinch grip task, although the study did not discuss this finding
in detail (Augurelle et al., 2003). It has been reported that the
maximum flexion forces generated at the distal phalanges are
primarily produced by the extrinsic hand muscles, the flexor
digitorum profundus and flexor digitorum superficialis, which
are multi-tendoned, with each muscle having an insertion into
several digits (Li et al., 2001; Valero-Cuevas, 2005). Changes in
MVF and FD after the removal of cutaneous feedback could be
attributed to changes in the magnitude of motor commands from
the CNS.
Previous studies employing a similar task in fatigued fingers as
well as in elderly populations have also reported a reduction in
MVF comparable to those observed in this study (Danion et al.,
2000, 2001; Shinohara et al., 2003). It has been suggested that the
reduction in MVF during fatigue or aging could result from the
drop in maximal discharge rate of the motoneurons. This drop in
discharge rate could result either from the prolonged refractory
period after hyperpolarization or from changes in the central
commands to accommodate the slowed contractile properties of
the muscles (Shinohara et al., 2003). Unlike fatigue or aging, it is
unlikely that the contractile properties of the muscles change
after the application of ring block anesthesia. However, it is to be
noted that the administration of anesthesia not only affects the
cutaneous receptors, but also could affect tendon organs, thus
removing feedback related to muscle contraction (Roland and
Ladegaard-Pedersen, 1977).
If the drop in the MVF observed in this study occurs because of
a decrease in the discharge rate of motor neurons, this would
suggest a dependency of the maximum motor output on the
existing cutaneous feedback available to the CNS. Previous studies
have shown that the presence of visual feedback could improve
the sub-maximal voluntary force production performance during
isometric tasks, although the role of visual feedback during MVF
production during isometric pressing is a matter of further
research (Cole and Sedgwick, 1992). Another mechanism that
may have contributed to the changes in the MVF found in our
study is a change in the mechanical properties of the fingertips
with the injection of anesthesia, thus affecting the friction
coefficient between the finger coefficients and the sensors could
have affected MVF. A recent study on maximum pinch force
production has shown significant effects of surface friction on
force magnitude (Seo et al., 2011). However, it is unlikely that a
25% decrease in the MVF could occur by slight changes in the
frictional properties of the glabrous skin alone.
The neurons in the primary motor cortex have receptive fields
in the periphery that receive inputs from the primary somato-
sensory cortex (Johansson, 1998). Since the cutaneous feedback in
the present study was removed before the MVF production task
was performed, it is possible that the absence of cutaneous
feedback might have affected the primary somatosensory cortex,
thus resulting in a decreased motor output. If this hypothesis is
accepted MVF should increase when the cutaneous feedback is
increased, by either changing the surface condition of the force
sensors or by stimulating the digits with a mild electric current.
However, this remains a matter of further investigation. The
primary motor cortex also receives inputs from the posterior
parietal areas, which integrate multiple sensory modalities for
motor planning. While cutaneous feedback is thought to be the
primary source of sensory inputs, visual feedback has also been
known to provide afferent inputs in the absence of tactile feed-
back (Cole and Sedgwick, 1992; Collins et al., 1999).
Fig. 3. (a) FD in N, calculated from single-finger tasks in the normal condition (inner black lines), and after the administration of anesthesia (outer gray lines). Radii of the
data-point circles represent the standard errors of the mean. I, M, R and L as shown in each axis represent the index, middle, ring and little finger tasks, respectively. The
data are showed in the scale between 0 N and 8 N. (b) Average FD over all four fingers. (c) FD values normalized with task-finger MVF. Radii of the data-point circles
represent the standard errors of the mean. I, M, R and L as shown in each axis represent the index, middle, ring and little finger tasks, respectively. The data are showed in
the scale between 0% and 50%. (d) Average of normalized FD over all the four fingers. Means and standard errors are shown across the subjects (npo001;nnpo05).
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FD values observed in the study showed an increase with the
loss of cutaneous feedback, in comparison to MVF, which
decreased with the administration of digital anesthesia. A pre-
vious study has shown that FD remains constant across children
of different ages, while the MVF increases significantly, suggest-
ing a fixed distribution of motor commands across different
muscles involved in pressing (Shim et al., 2007). Patients with
Down’s Syndrome are also known to exhibit lower FD values,
suggesting that changes in FD are a result of changes in central
mechanisms of motor command production (Latash et al., 2002).
The changes in FD with the administration of anesthesia observed
in this study could be a result of decreased amount of motor
commands flowing to the extrinsic hand muscles. Previous
studies have suggested that FD could change due to several
central factors such as an increased innervation ratio, reduced
recruitment of motor units involved or reduced maximal
discharge rate (Shinohara et al., 2003). It has been suggested
that the motor commands to the instructed digits result in a
‘‘spillover’’ to the antagonist muscles of the neighboring digits,
thus resulting in an increase in FD (Shinohara et al., 2003). No
matter what the mechanism for the reduction in MVF after the
removal of cutaneous feedback is, changes in FD suggest that not
only the magnitude of motor commands change, but their
distribution to the muscles might change as well after the
cutaneous feedback is removed from the digits.
Results from this study show that the values of FI remain
unaltered after the administration of digital anesthesia. This
suggests that the distribution of motor commands among differ-
ent digits of the hand remains unaltered, irrespective of the
sensory feedback and results in a proportional decrease of
enslaved finger forces. Since it is reasonable to assume that the
biomechanical factors in this study were not changed with
anesthesia, the unchanged FI values may be assigned to the
unchanged central factors associated with the independent
actions of fingers.
Differential increase in FD values with the removal of cuta-
neous feedback across the four digits suggest that the magnitude
of MVF reduction, and hence the amount of motor discharge,
varies for different fingers. Reduction in MVF values as well as
increase in FD, along with no changes in FI suggest that cutaneous
feedback could play a major role in regulating the magnitude of
motor command from the brain, but not the distribution of those
commands to different fingers.
In conclusion, this study reports a decrease in the MVF and
increase in the FD with the removal of cutaneous feedback from
the digits through anesthesia. While the neurophysiological
mechanism for this finding needs further investigation, it is
proposed that similar to sub-maximal force production tasks,
cutaneous input is an important requirement to achieve max-
imum force production by the hand digits.
Conflict of interest statement
No author has any financial or personal relationship that could
inappropriately influence the work submitted for publication.
Acknowledgments
This study was supported in part by Seoul Olympic Sports
Promotion Foundation of the Ministry of Culture, Sports and
Tourism of Korea, Kyung Hee University International Scholars
Program, and Korea University Grant (K0823281). Authors
appreciate Hyo Young Pyeon, PT, MHSc, and Young Ki Hong,
MD, PhD, for their advice and help during experiments.
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