Joel R Martin

Luke A Wilhelm

Mark L Latash

Vladimir M Zatsiorsky

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FINGER ENSLAVING IN THE DOMINANT AND NON-DOMINANT HAND 
 
1Luke A. Wilhelm, 2Joel R. Martin, 1Mark L. Latash, and 1Vladimir M Zatsiorsky 
 
1The Pennsylvania State University, University Park, PA  USA 
2 George Mason University, Manassas, VA  
Email:  law326@psu.edu 
 
 
INTRODUCTION 
 
Patterns of finger force production and finger 
coordination have been well documented.  Force 
production patterns in multi-finger maximum 
voluntary contraction (MVC) tasks have been 
modeled with a neural network [1].  In particular, 
finger enslaving, the unintentional force production 
by non-task related fingers, has been consistently 
observed.  Enslaving has been suggested as an 
indirect measure of dexterity, since lower enslaving 
would imply a greater ability to move the fingers 
independently.  In addition, the dynamic-dominance 
theory states that the dominant (D) arm and hand 
are better suited for dynamic tasks, while the non-
dominant (ND) arm and hand are better suited for 
stabilization tasks [2].  The purpose of this study 
was to compare enslaving values between the D and 
ND hands using the neural network approach.  
Previous work suggested that there are significant, 
though small, differences in enslaving between the 
hands [3].  Given the background of the dynamic-
dominance theory, we hypothesized that the 
enslaving effects would be lower in the D hand. 
 
METHODS 
 
Twenty-two right-handed, young, healthy males 
were tested.  Handedness was assessed by the 
Edinburgh Handedness Inventory, the Grooved 
Pegboard test, and the Jebsen-Taylor hand function 
test. 
 
The MVC task was performed by 15 different finger 
combinations (I, M, R, L, IM, IR, IL, MR, ML, RL, 
IMR, IML, IRL, MRL, IMRL, where I designates 
the index finger, M – the middle finger, R – the ring 
finger, and L – the little finger).  Subjects were 
instructed to produce maximal force with the fingers 
of a given combination and to pay no attention to 
the non-instructed fingers. 
 
Neural network analysis resulted in the following 
equation: 
 
 
 =
1

 + 
 
(1) 
 
where [F] is a 4×1 matrix of individual finger 
forces, N is the number of fingers explicitly 
involved in the given task, [w] is a 4×4 finger 
connection weight matrix, [X] is a 4×1 matrix of 
neural command values ranging from zero (finger 
not explicitly involved) to one (finger maximally 
involved), and [v] is a 4×4 diagonal matrix of gain 
values.  For the case when all four fingers are 
explicitly instructed to press, the equation can be 
reduced to: 
 
  =  (2) 
 
where [IFC] is referred to as the interfinger 
connection matrix.  The diagonal elements of the 
IFC represent the amount of force produced by the 
fingers due to direct commands, and the off-
diagonal elements represent the amount of force 
produced due to enslaving effects.  The elements of 
the IFC were normalized by the total four finger 
force. The sum of normalized, off-diagonal 
elements was taken to be the enslaving index for a 
given subject.  Enslaving indices for each individual 
finger were calculated by summing the off-diagonal 
elements in the appropriate column of the IFC (first 
column for the index finger, second column for the 
middle finger, third column for the ring finger, and 
fourth column for the little finger). 
 
 
 
 
RESULTS AND DISCUSSION 
 
The results of a two-way ANOVA analysis (hand × 
finger) showed the index finger to have the lowest 
enslaving effects, while the ring finger showed the
highest enslaving.  This agrees with previous 
enslaving patterns found in the literature.  However, 
no significant difference in enslaving indices was 
found between D and ND hands (Fig. 1). 
 
 
Figure 1: Percent of total force produced during the 
four finger task due to enslaving effects.  Black bars 
are the D hand, and white bars are the ND hand. 
The handedness scores from the Edinburgh 
Handedness Inventory, the Grooved Pegboard test, 
and the Jebsen-Taylor hand function test were 
compared with the enslaving indices with Pearson’s 
r.  All three tests showed greater hand function in 
the D hand.  However, no significant correlations 
were found between any of the handedness tests and 
the enslaving indices. 
 
Some previous work suggested that there are 
significant differences in enslaving effects between 
the D and ND hand.  However, the reported 
differences were small, on the order of 2% [3].  In 
addition, these differences were only found in 
certain combinations of fingers, not all four fingers 
acting together.  This, in combination with the 
current findings, suggests that any differences in 
enslaving effects between the hands are small. 
 
Earlier studies have shown that although enslaving 
effects are a consequence of both peripheral 
mechanical coupling and central nervous system 
command coupling, neural factors play the largest 
role [4].  Furthermore, given appropriate practice 
and feedback, enslaving patterns have been shown 
to change, often in training sessions of no longer 
than one hour, suggesting that the neural 
mechanisms controlling enslaving are malleable [5]. 
Even though the D and ND hands are used 
differently in everyday tasks, the majority of tasks 
for both hands require multiple finger coordination 
as opposed to individual finger movement (e.g. 
grasping, object manipulation).  We hypothesize 
that this prevalence of multi-finger tasks for both 
hands leads to similar patterns of enslaving in the D 
and ND hands. 
 
CONCLUSIONS 
 
Enslaving effects were not found to differ between 
D and ND hands when using the neural network 
method of analysis.  We hypothesize that, although 
the D hand shows greater dexterity in functional 
tests, everyday use does not require extensive fingr 
individuation.  This creates similar patterns of 
enslaving between the D and ND hands. 
 
REFERENCES 
 
1. Zatsiorsky VM, et al. Biol. Cybern 79, 139-150, 
1998. 
2. Sainburg R, Exp Brain Res 142, 241-258, 2002. 
3. Li S, Human Move. Sci 19, 897-923, 2000 
4. Latash ML, et al. Brain Res 924, 198-208, 2002 
5. Wu Y, et al. J of Motor Behavior 44, 471-478, 
2012 
 
ACKNOWLEDGEMENTS 
 
The study was in part supported by NIH grants NS-
035032 and AR-048563.
 
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FINGER ENSLAVING IN THE DOMINANT AND NON-DOMINANT HAND

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FINGER ENSLAVING IN THE DOMINANT AND NON-DOMINANT HAND

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