One of the principle limitations of the human motor system is the ability to produce consistent motor responses. When asked to repeatedly make the same movement, performance outcomes are characterized by a considerable amount of variability. This occurs whether variability is expressed in terms of kinetics or kinematics. Variability in performance is of considerable importance because for tasks requiring accuracy it is a critical variable in determining the skill of the performer. What has long been sought is a description of the parameter or parameters that determine the degree of variability. Two general experimental protocals were used. One protocal is to use dynamic actions and record variability in kinematic parameters such as spatial or temporal error. A second strategy was to use isometric actions and record kinetic variables such as peak force produced. What might be the important force related factors affecting variability is examined and an experimental approach to examine the influence of each of these variables is provided

Carlton, L. G.

Newell, K. M.

English

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THE ROLE OF IMPULSE PARAMETERS 
I N  FORCE VARIABILITY 
L e s .  G. Car l ton 
Biomechanics Laboratory 
Freer  Hal l  
Univers i ty  of I l l i n o i s  
906 So. Goodwin Avenue 
Urbana, I L  61801 
K. M .  N e w e l 1  
I n s t i t u t e  f o r  Child 
Behavior and Development 
Universi ty  of Illinois 
51 Gerty Drive 
Champaign, I L  61820 
One of the  p r inc ip l e  l imitat ions of t he  human motor s y s t e m  i s  t h e  
a b i l i t y  t o  produce cons i s t en t  motor responses.  When asked t o  
repeatedly make the  same movement, performance outcomes a r e  
charac te r ized  by a considerable  amount of v a r i a b i l i t y .  This i s  
espec ia l ly  t r u e  f o r  r ap id  ac t ions  o r  when s a l i e n t  feedback cues a r e  
no t  ava i l ab le ,  requi r ing  the  performer or opera tor  t o  func t ion  i n  an 
open-loop manner. This occurs  whether v a r i a b i l i t y  i s  expressed i n  
terms of  k i n e t i c s  o r  kinematics.  V a r i a b i l i t y  i n  performance i s  of 
considerable  importance because fo r  t a s k s  r equ i r ing  accuracy it i s  a 
c r i t i c a l  var iab le  i n  determining the s k i l l  of t he  performer.  I n  
addi t ion ,  understanding the  f a c t o r s  a f f e c t i n g  response v a r i a b i l i t y  
w i l l  provide important i n s i g h t s  necessary- f o r  explaining F i t t ' s  Law 
( F i t t s ,  1954) and speed accuracy t r adeof f s  i n  genera l .  
What has long been sought i s  a desc r ip t ion  of t he  parameter o r  
parameters t h a t  determine t h e  degree of  v a r i a b i l i t y .  Two genera l  
experimental  p ro toca ls  have been used. One p ro toca l  i s  t o  use dynamic 
ac t ions  and record v a r i a b i l i t y  i n  kinematic parameters such a s  s p a t i a l  
o r  temporal e r r o r .  A second s t r a t e g y  has been t o  use i somet r i c  
ac t ions  and record k i n e t i c  va r i ab le s  such a s  peak fo rce  produced. 
While a number of hypotheses have been p u t  forward, t h e r e  a r e  two 
models which suggest t h a t  fo rce  parameters determine the  amount of 
v a r i a b i l i t y  i n  a va r i e ty  of t a sks .  
Most r ecen t ly ,  Schmidt, Zelaznik,  Hawkins, Frank E, Quinn (1979) 
presented a n  impulse v a r i a b i l i t y  model which p r e d i c t s  a l i n e a r  and 
propor t iona l  r e l a t ionsh ip  between the  impulse produced and impulse 
v a r i a b i l i t y .  A s  the  l e v e l  of force required t o  complete a response 
inc reases ,  the  v a r i a b i l i t y  i n  producing t h a t  force a l s o  inc reases .  
Based upon t h i s  r e l a t ionsh ip ,  Schmidt e t  a l .  demonstrated t h a t  speed- 
accuracy t r adeof f s  could be accounted f o r  by v a r i a b i l i t y  i n  f o r c e  
production. This work provided important advancements f o r  providing 
the l i n k  between v a r i a b i l i t y  a t  k i n e t i c  l e v e l s  and v a r i a b i l i t y  i n  
kinematic var iab les  cons i s t en t  with speez-accuracy r e l a t i o n s h i p s .  
21.1 
https://ntrs.nasa.gov/search.jsp?R=19860023525 2020-03-20T13:14:55+00:00Z
A second model, which we l a b e l  an impulse-ratio model, i s  an ex t r apo la t ion  
of t he  work by Bahrick, Bennet t ,  and F i t t s  i n  1955. They were i n t e r e s t e d  
i n  the  con t ro l  of a spr ing  l'oaded con t ro l  s t i c k  and how changes of  fo rce  
c h a r a c t e r i s t i c s  a f f ec t ed  t r ack ing  performance. The model proposed t h a t  
amplitude, terminal  to rque  and the  change of torque f r o m  i n i t i a l  t o  f i n a l  
torque l e v e l s  inf luenced accuracy. Extrapolat ing t o  i somet r ic  t a s k s ,  t h e  
impulse-ratio model would p r e d i c t  t h a t  force  v a r i a b i l i t y  i s  propor t iona l  t o  
the  r a t i o  of the change i n  fo rce  f r o m  i n i t i a l  fo rce  t o  peak fo rce ,  divided 
by peak force.  
Unfortunately,  t h e r e  has been l i t t l e  empirical support  f o r  e i t h e r  of t h e s e  
models. For example, t h e r e  i s  a l a r g e  body of evidence which supports  a 
non-proportional r e l a t i o n s h i p  between force and fo rce  v a r i a b i l i t y  i n  both 
i somet r ic  (Ful le r ton  & C a t t e l l ,  1892; Jenkins ,  1947; N e w e l l  & Car l ton ,  i n  
press; Noble & Bahrick, 1956) and f o r  dynamic movements ( N e w e l l ,  Car l ton,  & 
Carl ton,  1982).  I n  add i t ion ,  previous examinations of  fo rce  v a r i a b i l i t y  
have confounded a number of fo rce  va r i ab le s .  For example, v a r i a t i o n s  i n  
i somet r ic  peak fo rce  have co-varied wi th  changes i n  impulse and r a t e  of  
fo rce  production. 
The major purpose of t h i s  paper i s  t o  examine what might be t h e  important 
fo rce  r e l a t e d  f a c t o r s  a f f e c t i n g  v a r i a b i l i t y  and t o  provide an experimental  
approach t o  examine the  inf luence  of each of t hese  va r i ab le s .  The models 
prev ious ly  presented have impl ica ted  peak fo rce ,  impulse, and change of 
force .  B u t  when w e  consider  t h a t  a motor response r equ i r e s  t h e  generat ion 
of  fo rce  over time, it i s  noted t h a t  peak fo rce  i s  a func t ion  of the  rate 
of  fo rce  production and t h e  amount of  time t h a t  t h e  r a t e  is  generated.  
Thus, t h e  r a t e  of force  product ion and i t s  time of  app l i ca t ion  may be more 
fundamental than cons idera t ion  of peak fo rce  o r  impulse alone.  Each of  
t hese  va r i ab le s  are depic ted  on a t y p i c a l  force-time curve generated i n  an 
i sometr ic  force  production t a s k  (Figure 1). 
Figure 1. Typical i somet r ic  force-time curve.  
21.2 
Research Strategy 
We suggest that a reasonable strategy would be to conduct a series of 
experiments where each of the force parameters would be held constant while 
allowing others to vary systematically. It is anticipated that synthesis 
of the experimental findings would lead to an understanding of the 
contribution of each impulse parameter to response variability. A priori, 
it was reasoned that the impulse variability and impulse-ratio models had 
focused on the non-essential variables of force production rather than the 
essential variables. 
Six experiments examining isometric force production are suggested. In each 
study subjects are required to produce multiple discrete trials in order to 
evaluate response variability. The subjects are provided a force-time 
template which should be matched, and feedback after each trial regarding 
the discrepancy between the template and actual response, The first three 
experiments (Figure 2) manipulate the initial preload or steady force 
exerted before each trial. 
The Experiments 
Figure 1A represents four conditions which have equal peak force but allow 
for changes in the rate of force production as well as impulse size and 
change of force. 
to the force-time manipulations for each experiment. Thus, as preload 
increases the rate of force production and the change of force decreases. 
The triangulated force-time curves provide approximations 
The experiment outlined in Figure 1B keeps the change of force constant 
across 4 conditions but allows the impulse size and peak force to vary 
systematically. The rate of force production also remains constant. A test 
of the impulse-ratio model. is provided in Figure 1C. In each of the four 
conditions the ratio descibed by the change of force divided by peak force 
remains constant. The impulse size, rate of force production, and peak 
force varies with conditions. 
The second set of experiments (Figure 3) vary the time to peak force in 
order to manipulate the desired force parameters. A test of the impulse 
variability model is provided in Figure 3A.  The size of the impulse 
remains constant by increasing the time to peak force and reducing the 
peak force attained. As a result, the rate of force production changes €or 
each condition. As far as we know this is the first strong test of the 
impulse variability model. Figure 3B represents conditions with equal peak 
force and different rates of force production as well as different impulse. 
In Figure 3C the rate of force production is held constant while peak force 
and impulse vary. 
21.3 
Figure 2. Triangulated force-time curves for experiments 1-3. 
2 1 . 4  
Figure 3. Triangulated force-time curves for experiments 4-6. 
21.5 
DISCUSSION 
The p a t t e r n  of r e s u l t s  from the  s i x  experiments should provide an i n d i c a t i o n  
of  the  r e l a t i v e  importance of each of t he  fo rce  r e l a t e d  parameters to  f o r c e  
v a r i a b i l i t y .  The s implest  s o l u t i o n  would be provided if v a r i a b i l i t y  
remained cons tan t  a s  a funct ion of one of t h e  manipulations ou t l ined .  For 
example, i f  impulse v a r i a b i l i t y  remained cons tan t  ac ross  the  fou r  condi t ions  
oul ined i n  Figure 3A, evidence would support  t he  conten t ion  t h a t  impulse 
s i z e  determines v a r i a b i l i t y .  Changes i n  r a t e  of fo rce  product ion and peak 
force  would have no s i g n i f i c a n t  e f f e c t  on v a r i a b i l i t y .  Such a f ind ing  
would provide support  fo r  the  impulse v a r i a b i l i t y  model. 
W e  specula te ,  based on p i l o t  da t a  and t h e  na ture  of fo rce  product ion,  t h a t  
no s i n g l e  f a c t o r  w i l l  provide an accura te  accounting of t h e  fo rce  
v a r i a b i l i t y  funct ion.  However, we be l i eve  a phys ica l  d e s c r i p t i o n  i s  
poss ib l e  when mul t ip le  f a c t o r s  a r e  considered. Rate of  fo rce  product ion 
and the  time f o r  which t h a t  rate i s  developed would seem t o  be important  
f ea tu re s  with o ther  f a c t o r s  such as t h e  change of fo rce  from i n i t i a l  t o  
f i n a l  force  l e v e l s  playing some r o l e .  
While these  experiments have been ou t l ined  employing an i somet r i c  t a s k ,  t h e  
same manipulations can be produced i n  dynamic ac t ions .  
t a s k s  have d i f f e r i n g  cont ro l  problems, both r equ i r e  t h e  performer t o  
func t iona l ly  e x e r t  force  over t ime,  and hence, generate  an impulse ( t i m e  
i n t e r g r a l  of f o r c e ) .  Newtonian p r i n c i p l e s  of mechanics suggest  t h a t  
kinematic and k i n e t i c  approaches t o  response v a r i a b i l i t y  should be 
congruent and the re  have been r ecen t  a t tempts  a t  mapping t h i s  r e l a t i o n s h i p  
(Hancock & N e w e l l ,  i n  p re s s ;  Schmidt e t  a l . ,  1979) .  
Although these  
I n  summary, t h e  r e s u l t s  of t he  experiments should l ead  t o  an understanding 
of the  con t r ibu t ion  of each impulse parameter t o  response v a r i a b i l i t y .  
More important than the  r e l a t i v e  con t r ibu t ion  of t hese  f a c t o r s  i s  the  
development of a phys ica l  desc r ip t ion  l i n k i n g  impulse parameters t o  response 
v a r i a b i l i t y .  The ou t l ined  experiments provide a d i r e c t  t e s t  of t h e  
impulse-rat io  and impulse v a r i a b i l i t y  model, b u t  i n i t i a l  i n d i c a t i o n s  are 
t h a t  ne i the r  model accura te ly  accounts f o r  v a r i a b i l i t y  i n  performance. A 
model tak ing  i n t o  considerat ion more fundamental p r o p e r t i e s  of t h e  fo rce  
product ion mechanisms may provide a b e t t e r  desc r ip t ion  of response 
v a r i a b i l i t y  and assoc ia ted  phenomena such as F i t t ' s  Law and o t h e r  speed- 
accuracy t r adeof f s .  
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21.7 


The role of impulse parameters in force variability

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