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Construct Validity of the Test of Infant Motor Performance

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Research Report
Construct Validity of the Test of Infant Motor
Performance
Background and Purpose. The purpose of this study was to assess the con-
struct validity of the Test of Infant Motor Performance (72MP), specijically the
test’s sensitivity for assessing age-related changes in motor skill and correlation
with risk for developmental abnormality. Subjects. Subjects were 13 7 term and
pretemz itzfants stratified by postconceptional age, medical complications score
on the Problem-Oriented Perinatal Risk Assessment System, and ethnicity and
race (non-Latino Caucasian, African-American, and Latino). Methods. Sub-
jects were tested on the TiMP at ages ranging from 32 weekspostconceptional
age to 3.5’ monthspast term-equivalent age. Scores (Rasch logit ability mea-
sures) were correlated with postconceptional age. A multiple regression analysis
was used to assess the contributions of age, risk, and etbnicity to the variance
in 7TMP scores. Results. The correlation between postconceptional age and
iTMPpe$brmance measures was .83. Risk and age together eaplained 72% of
the variance in TlMPperformance (R= .85, P< .00001). No dzfferences related
to ethnicity were found. Conclusion and LHscussbn The TlMP has validity
for assessing age-related development of functional motor skills in young in-
fants and is sensitive to risk for poor developmental outcome. [Campbell SK,
Kolobe WA, Osten El: et al. Construct validity of the Test of Infant Motor Per-
formance. Phys Ther. 1995; 75:5855%.1
Suzann K Campbell
Thubi HA Kolobe
Elizabeth T Osten
Maureen Lenke
Gay L Girolami
Key Words: Infant motor development; Motor skills; Pediatrics, deuelopment; Tests
and measurements, functional.
The evolution of high-technology
neonatal care units has resulted in the
SK Campbell, PhD, PT, FAPTA, is Professor and Director of Graduate Studies, Department of
Physical Therapy, University of Illinois at Chicago, M/C 898, 1919 W Taylor St, Chicago, IL 60612
presewation of life for a growing
(USA) (S.K.CA4MPBELL@UIC.EDU). Address all correspondence to Dr Campbell. number of infants at high risk for
THA Kolobe, PhD, PT, is Lecturer, Depanment of Physical Therapy, University of Illinois at
Chicaeo.
mortality, resulting in a large number
of children who have high risk for
-- --
rr-
ET Osten, OT, is in private practice in Skokie, Ill. She is also Clinical Instructor, Department of
Occupational Therapy, University of Illinois at Chicago.
mately 25% to 30% of at-risk Infants
exhibit some form of neuromotor
M Lenke, OT, is Clinical Supervisor Pediatrics, Department of Occupational Therapy, Lutheran
General Hospital, Park Ridge, IL 60068.
GL Girolami, PT, is Director, Pathways Center for Children, Glenview, IL 60025
This study was approved by the University of Illinois at Chicago (H-89-422) and Lutheran Gen-
eral Hospital (#888) institutional human subjects review boards.
This project was supported in part by a grant from the Foundation for Physical Therapy Inc. Dur-
ing the course of this project, Dr Campbell and Dr Kolobe were partially supported by Grant MCJ
IL 179590, Maternal and Child Health Bureau, US Public Health Service.
This work was previously presented at the 1993 Annual Meeting of the American Academy for
Cerebral Palsji and Developmental Medicine and received the Patricia Miller Award for research in
physical therapy; as a result, an expanded version of the paper was presented at the 1994 Annual
Meeting of the American Academy for Cerebral Palsy and Developmental Medicine.
disturbance early in As a result,
physical therapists and occupational
therapists have increasingly become
incorporated as regular members of
the neonatal special care team, provid-
ing assessment and treatment for ba-
bies at especially high risk for devel-
opmental dysfunction.* In addition,
therapists provide education to those
caring for these children.
Despite several decades of involve-
ment by therapists in special care
This article was submitted August 5, 1994, and was accepted March 2, 1995
Physical Therapy / Volume 75, Number 7 /July 1995

nurseries, little research exists to docu-
ment the effectiveness of their servic-
es.5 Studying the effects of these thera-
pies for hlgh-risk &ants is hindered
by the lack of functional motor out-
come measures appropriate for this
population. In addition, available tests
do not provide an adequate range of
items to allow for detailed quantitative
assessment of motor development,
and characterization of deviations from
the norm, in infants during the first
few months after birth. The purpose
of this research was to assess the sen-
sitivity to age-related change and to
risk for poor developmental outcome
of a new test, the Test of Infant Motor
Performance (TIMP). The TIMP was
developed for use by physical thera-
pists and occupational therapists in
assessing the components of func-
tional motor performance in high-risk
premature and very young term-born
infants.
A Theoretical Approach to
Developmental Motor
Assessment
Movement is a way that the infant
establishes control over the world,
communicates needs, and explores the
environment; movement is a basis for
early learning as well as being impor-
tant in its own right for developing
and maintaining musculoskeletal integ-
rit~.~ Infants use movement to orga-
nize their environment; to cornmuni-
cate and interact physically with
objects or people; to change postures
or to adjust positions; to respond to
environmental demands; and for self-
consolation, such as thumb-sucking.
Active movement has been shown to
be necessary for optimal perceptual
development? an area that is fre-
quently problematic in prematurely
born children.8 The goal of physical
therapy or occupational therapy’ for
young Infants with neurologic dys-
function is to assist them in develop-
ing these functional uses of movement
and to limit disability. No test has yet
been developed, however, that reflects
the movements that are basic to these
functional competencies in newborns.
Because poor postural control can
limit the rate at which functional skills,
such as manipulation and mobility, are
acquired during deveI0pment,9-~~
therapists often use assessment of
postural control as a basis for deter-
mining the need for intervention to
promote motor development.
Shumway-Cook and Wo~llacott~~
define postural control as control of
the body’s position in space for stabil-
ity (balance) and orientation. Stability
is the ability to maintain the center of
body mass within the base of sup-
port.12 Orientation is the alignment of
the body segments with respect to one
another as appropriate for the task.12
Thelen and colleagues13 place special
emphasis on the physical and social
contexts of actions in daily life as
aspects of the organization of postural
and movement responses. Interactions
among environmental and task charac-
teristics, physical and neurologic prop-
erties of the individual infant’s body,
and the inherent self-organizing prop-
erties of the human sensorimotor
system are the principal components
of their dynamical systems model of
infant motor development.l3 The two
related theoretical approaches, those
of Shumway-Cook and Woollacottl2
and Thelen and colleagues,~3 have
Influenced the structure and develop-
ment of the TIMP.
The systems model of motor develop-
ment described by Shumway-Cook
and Woollacott12 assumes that the
processes of postural control include
(1) development of sensory systems
and central neural processes that orga-
nize visual, vestibular, and somatosen-
sory inputs for orientation of the body;
(2) musculoskeletal change, such as
increasing strength and changing body
morphology; (3) development of neu-
romuscular synergies to maintain sta-
bility; (4) development of adaptive
mechanisms to modlPy control pro-
’Because intervention provided by physical therapists and occupational therapists in the very
early months of life is similar, no distinctions are made between them in this article. At later ages,
much more differentiation between therapeutic approaches is common.
cesses for posture in response to
changing environmental and task
demands; and (5) development of
anticipatory postural control processes
to prepare for destabilizing internal
and external forces. In their model,
motor behaviors are divided into four
functional categories: (1) sustaining
posture; (2) regaining posture; (3)
transitions between postures; and (4)
integration of posture into movements
such as locomotion, manipulation, and
exploration. The TLMP assesses behav-
iors in the first three categories that
we believe to be the basic sktlls lead-
ing to performance of activities at the
fourth level. Head control and other
functions are assessed in active and
alert states, for example, by testing the
ability of the child to maintain head
stability in a variety of spatial orienta-
tions, right the head when the body is
tilted, turn the head in various posi-
tions, and stabilize or orient the head
in response to interesting visual or
auditory events. A variety of position-
ing and stimulus materials (ie, toys,
examiner’s face) across items allows
infants to demonstrate their abilities to
perform in a number of environmental
and task contexts.
Comparison of the Test of
Infant Motor Performance WM
Available Infant Tests
Available tests primarily assess infant
neurobehavioral responses, social
interactions, and reflexes and muscle
t0ne."l*-~9 Each of these tests has one
or more problems that limits its use in
clinical practice, such as lack of sum-
mary or quantitative scores, excessive
length, or questionable or inade-
quately demonstrated reliability or
validity.20 In the tests developed by
Dubowitz and Dubowitz16 and by
Komer and colleague^,^^^^* the exam-
iner stabilizes the child’s head position
when testing many items, thus limiting
the infant’s ability to express indepen-
dent postural control capacities. Many
of these tests, however, do contain
small numbers of items that assess the
ability to control the head and trunk in
several spatial orientations and in
response to interesting stimuli in the
environment.
Physical Therapy / Volume 75, Number 7 /July 195

OBSERVED SCALE
1. Head in midline 2s
2/3. R/L head turn 180"
4. Hands together in midline
5/6. R/L mouths hand
7/8. R/L individual finger movements
9/10. R/L individual wrist movements
1 1/12. R/L financing objects
13. Pelvic lift
14. Bilateral hip and knee flexion
15/16. R/L individual ankle movements
17. Kicking
18/19. R/L head turn 180" in prone position
20. Head lift in prone position
23. Antigravity bilateral forearm movements
24. Antigravity bilateral arm movements
25. Ballistic movements
26. Oscillating movements
27. Reaching
ELICITED SCALE
1. Neck rotation with visual stimulation in semiupright position
2. Head control in seated position
3. Trunk extension in seated position
4. Head lifting from full neck flexion
5. Head lifting from full neck extension
6. Head control when lowered from sitting to supine position
7. Inhibition of trunk rotation with head turn
8/9. Midline head alignment withouvwith visual stimulation
10/11. R/L neck rotation with visual stimulation in supine position
12/13. Neck stretch/arm reactions to face covering
14. Antigravity hip/knee flexion in supine position
15. R/L rolling to prone position with leg adduction
16. R/L rolling to prone position with arm adduction
17. Neck flexion on pull-to-sit
18. Lateral head and trunk righting with elbow extension
19. Lateral hip abduction reaction
20. Neck and trunk extension in prone suspension
21. Crawling in prone position
22. Head lift in prone position
23. R/L head turning in prone position with auditory stimulation
24. Recovery of arm position after displacement in prone position
25. Standing
26. R/L head righting with tilt in upright suspension
Figure 1. List of items in the Test of Infant Motor Performance (Version 2.3). Miss-
ing items were deleted after psychometric analysis. (WL= rightAeJ.)
The TlMP is designed for prematurely
born infants from 32 weeks’ gesta-
tional age up to about 4 months after
term-equivalent age or for full-term
mfants up to 4 months of age.21 Some
items in the test were selected from
those in the previously mentioned
infant tests.15J6 These include items
developed by Brazelton15 involving
orientation in response to visual and
auditory stimuli and reactions to a
cloth over the eyes; items from the test
by Dubowitz and Dubowitz16 involv-
ing control of the head in the upright
seated position and when sus-
pended in the prone position, and the
ability to flex the arms from an ex-
tended position in a prone posture;
and items developed by Arniel-Tison
and GrenieF for eliciting evidence of
developing postural control in sitting
and side-lying positions. In each case,
however, new scoring descriptors
were developed to reflect the goals of
the TLMP.
Movements reflecting the qualitative
changes in coordination (oscillating
movements and ballistic [ie, rapid,
forceful] movements) recently identi-
fied by Hadders-Algra and Prechtl22
and Cioni and Precht123 are also
scored when they occur spontane-
ously. The presence of ballistic move-
ments (called "swipes" and "swats") at
7 to 10 weeks postterm is thought to
reflect increasing amounts of recipro-
cal innervation in muscular coordina-
tion, which makes these rapid, forceful
movements possible.24 Because these
qualitative developments do not ap-
pear in children with spastic cerebral
palsy, their absence at an appropriate
age may have diagnostic signifi-
~ance.~5 All other items in the TIMP
are original in conception and scor-
ing26727; a complete list is presented in
Figure 1.
The TIMP requires an average of 36
minutes to administer and to score,
and it consists of two scales, one for
rating the presence of spontaneous
motor behaviors and the other for
rating the infant’s responses to being
positioned and handled in a variety of
spatial orientations and to interesting
types of visual or auditory inputs.
Items on both scales are scored only
when infants are in quiet alert or ac-
tive alert behavioral states, as defined
by Brazelton.l5
The Observed Scale on TIMP Version
2.3 (items denoted by 0-1 consists of
27 dichotomously scored behaviors
reflecting the infant’s spontaneous
attempts to change positions or to
orient the body in various ways, to
selectively move individual body seg-
ments, and to perform the qualitative
types of movements mentioned earli-
er.22-24 Examples include the ability to
Physical Therapy / Volume 75, Number 7 /July 1995

delivery of effective interventions to
improve motor development.
The analyses presented in this article
address the first two of these issues, as
well as possible ddterences related to
ethnicity of the infant. Our hypotheses
were that ability on the TIMP would
be positively correlated with postcon-
ceptional age and that ability on the
TIMP would be negatively correlated
with degree of perinatal medical com-
plications related to increased mortal-
ity and developmental deviance. The
literature on motor performance in
children of ddterent racial and ethc
backgrounds also suggests that Cauca-
sian children might be expected to
have lower scores than African-
American children.2B-3O
Method
Subjects
The subjects in this study were 137
Infants born prematurely or after a
full-term pregnancy who were tested
one to three times (with the exception
of 1 child who was tested four times),
Figure 2. Three-month-old infant demonstrating spontaneous pet$omance of
yielding 174 measures of performance
head lifting in prone position (Obsewed Scale item 20).
on the TIMP. Subject selection was
center the head in line with the mid-
line of the body in the supine posi-
tion, bring the hand to the mouth,
produce ballistic movements of the
arms or legs, and llft the head in the
prone position (Fig. 2).
The second part of the test is the Elic-
ited Scale (items denoted by E-), made
up of 26 items (6 of which are scored
separately for each side of the body),
each rated on five- or six-point scales.
Performance on these items reflects
the infant’s ability to solve movement
"problems" posed to elicit evidence of
developing postural control in a vari-
ety of spatial orientations. Examples
include rolling to a prone position
with head righting when the leg is
rotated across the body (Fig. 3); inhib-
iting rolling when the head is turned
to one side; turning the head to follow
a visually interesting object or to
search for a sound in a prone, supine,
or supported sitting position; and
aligning the legs, trunk, and head in a
made in accordance with a preestab-
vertical orientation when placed in a
lished plan for stratifying mfants by
standing position.
postconceptional age, medical risk,
Purpose and Hypotheses
The purpose of this article is to
present preliminary evidence of the
construct validity of the TIMP. If the
test is truly a measure of the postural
and selective control underlying func-
tional motor performance, it should
possess certain characteristics. Perfor-
mance measures (or total scores)
should vary as a function of (1) age-
related changes in motor performance;
(2) the presence of risk factors for
developmental disability, such as ex-
treme prematurity or perinatal as-
phyxia; (3) medical diagnosis, such as
chronic lung disease or cerebral palsy;
and race or ethnicity. Based on this
plan, we attempted to recruit 3 mfants
of each racial or ethc group at each
of three levels of risk for developmen-
tal disability for a total of 27 mfants in
each of seven predefined postconcep-
tional age ranges (Tab. 1). Most of the
sample was cross-sectional (ie, tested
once); however, 9 infants who were
tested three or four times constituted a
sample of convenience for longitudi-
nal assessment. Children were tested
in their homes or in one of two inpa-
tient special care nurseries (the Uni-
versity of Illinois Hospital, Chicago, Ill,
or Lutheran General Hospital, Park
Ridge, Ill).
(4) the presence of developmental
Age was calculated from the expected
deviance in any of the subsystems that
date of confinement for each infant’s
contribute to the organization of func-
mother. For all subjects, the average
tional movement (eg, visual or audi-
gestational age at birth was 33 weeks
tow impairment, altered reflex activity
or defects in motor control); or (5)
(SD=5); 29 of the subjects were full
term at birth (38-42 weeks), and the
12 / 588
Physical Therapy / Volume 75, Number 7 / July 1995

Figure 3. Tbree-month-old infant demonstrating ability to roll to prone position
with head righting ajfer right leg adduction across body in supine position (Elicited
Scale item 15L). Scale scores: @pelvis lrjfs passively off surface; l=peluis lifts from
support su~ace and head turns, but trunk and am do not respond; .?=pelvis and
trunk lift fmm suppott surface and head turns to side, but am remains behind trunk;
3=pelvis, trunk, and am lift from support surface, head turns to side, and infant rolls
part way but not onto side; 4=peluis, trunk, and am lift from support surface, head
turns, and infant rolls onto side or ouer without head righting; 5= when traction is
applied to leg at the end of the maneuver, infant rolls to prone position with head
righting.
rest were born prematurely. Subjects postconceptional age into seven
were selected for stratification by groups: three groups of prematurely
Table 1. Number of Tests by Postconceptional Age Group, Medical Risk, and
Average Gestational Age at Birtha
Gestational Age
Risk (wk)
-
Age Groupb Low Moderate High Total X SD Range
1 (12%)
2 (1 8%)
3 (1 6%)
4 (1 7%)
5 (1 0%)
6 (1 3%)
7 (1 4%)
Column total
Total percentage
"Percentage of total sample shown in parentheses for age and risk groups
’~ge group 1.=32-33 weeks postconceptional age; age group 2=34-35 weeks postconceptional
age; age group 3=36-37 weeks postconceptional age; age group 4=38-41 weeks postconcep-
tional age; age group 5=2-5 weeks postterm; age group 6=6-9 weeks postterm; age group
7=10-13 weeks postterm.
born mfants at postconceptional ages
32 weeks to 37 weeks; a term age
group consisting of 10 full-term infants
and 19 prematurely born infants who
had reached term-equivalent age; and
three groups of infants who were 2 to
13 weeks past term-equivalent age,
defined as 40 weeks postconception
(Tab. 1). The three oldest age groups
consisted of 9, 11, and 11 full-term
infants. The average gestational age at
birth (last column of Tab. 1) of infants
tested within each of the seven
postconceptional age groups varied
from 30 to 35 weeks.
Risk assignment (high, medium, low)
was made based on scores on the
Newborn form of the Problem-
Oriented Perinatal Risk Assessment
System (POPRAS),s1,s2 derived from
reviews of medical records. Scores of
2 to 60 were considered low risk,
scores of 61 to 90 were considered
medium risk, and scores over 90 were
considered to be high risk.
Ethnicity/race was taken from each
infant’s medical record. Only children
who were African-American, Latino, or
non-Latino Caucasian were included
in the sample. Thirty-five percent of
the infants were non-Latino Caucasian,
36.5% were African-American, and
28.5% were Latino, distributed across
all categories of age and risk.
Testing
Informed consent was obtained from
the parents of each mfant, and permis-
sion to test was obtained from the
mfant’s physician or nurse before
assessment began. Children who were
observed repeatedly were never tested
more than once while they were in
the same age category. Chronologic
age at the time of testing of the chil-
dren below term-equivalent postcon-
ceptional age averaged 20 days
(SD = 18, range=>68) for the 32- to
33-week-old mfants, 26 days (SD=20,
range= 1-81) for the 34- to 35-week-
olds, and 29 days (SD=27, range=
1-77) for the 36- to 37-week-olds.
Five therapists (three physical thera-
pists, two occupational therapists)
tested all infants in the study. Their
Physical Therapy / Volume 75, umber 7 /July 1995

intrarater and interrater reliability on
14 videotaped tests of ~nfants from all
three ethnic and racial groups and
with a variety of ages and degrees of
medical risk was analyzed with the
FACETS computer program.33~34 The
analysis allows the investigator to
assess whether raters use items sys-
tematically across subjects and
whether different raters use item rat-
ings similarly. All testers met the pre-
determined criterion35 of fewer than
5% misfittingt ratings.
Data Analysis
Test performance for all children on
the TIMP was subjected to psychomet-
ric analysis for fit to a theoretical Ra-
sch measurement modeP7-39 using the
BIGSTEPS computer program.@ In this
model, raw ordinal-level scores for
success in passing items, or levels
within items, are transformed into a
linear scale of mea~urement.~’ The
number of successes (S) indicates the
infant’s level of function, and the num-
ber of levels the infant failed to pass
(F) is the dficulty of the item. Taking
the log(S) and the log(F) can yield B
(the ability of individuals) and D (the
dficulty of items), which, if plotted
for the group, would be linear. The
measurement model focuses on the
difference between ability and dfi-
culty; thus, B - D = log(S) - log@)
= 10g(S/F).~l This statement is refor-
mulated into a more general statement
about probabilities (ie, the chances of
success on items by infants), so that
the model for dichotomously scored
items is described as log[P,/P,] = B
- D. The model has an additional
term for the ddficulty of the various
steps in the item when several levels
of ratings are possible within each
item. Analysis of the raw ordinal-level
scores by imposing this measurement
model definition on the observed
responses of tested individuals evalu-
ates the extent to which the data fit
the model. Those parts of the data that
do not fit the model are identified
with misfit statistics. If the data pro-
vide an overall fit to the measurement
model, a scale has been formed that is
assumed to be based on a unidimen-
sional construct (all items measure the
same thing) and to approximate an
interval-level scale (ie, measurement
units are the same size over the whole
range of the scale), a property that
generally does not hold true for the
original raw scores on items. Data
from a scale with these properties can
be used to order individuals by ability
and can be analyzed with parametric
statistics.*l
Items on the TIMP were calibrated
according to dilficulty level and as-
sessed for fit to the measurement
model. Both item dficulties and per-
son ability measures are expressed in
logits, or logarithmically transformed
probabilities of success given a partic-
ular level of ability (log-odds probabil-
ity scaling), ranging from about - 5.0
to +4.0 for the TIMP and forming a
hierarchical linear scale with equal
inter~als.3~ The internal consistency
coefficient was .98, indicating that the
items on the TIMP form a coherent
scale. Clarity of the measure, as re-
flected by the item separation index,
was 7.38 (root mean square er-
ror=0.19). The practical meaning of
this measure is that the TIMP items
can be separated into more than seven
ddferent average levels of dficulty
across the 5-month age span of the
children tested.
+The computer program is designed to identify as a misfit any observed item rating that differs
with high probability from the rating that was expected based on the psychometric model of
unidimensionality of item structure. Fit to the model requires that higher item ratings are achieved
by persons with higher ability (greater total raw score) and that persons with a given functional
level have a greater probability of scoring higher on easier items than on more difficult items; the
mean square fit statistic is used to identify ratings that deviate from the e~pectations.3~ Ratings can
misfit because items are not well defined, because raters apply the item definitions inconsistently,
or because infants perform in unexpected ways (eg, failing easy items relative to their overall
ability). Misfitting item scorings were deleted from the reliability analysis when unusual child
performance appeared to cause misfit to the model but examiners’ ratings were in close agree-
ment. A 5% occurrence of misfitting ratlngs could be expected by chance alone, so this criterion
was used to establish an acceptable level for rater reliability, both within and across raters.
Item calibrations are shown in Figure
4, in which every dichotomous item in
the test and each level within multi-
level items is placed at its median
dilf~culty calibration in logits. The
easiest item (0-14) is observed sponta-
neous hip and knee flexion or being
able to flex the hips and knees in the
supine position so that the legs are off
the supporting surface at least mo-
mentarily; the most dficult item (0-
26) is spontaneous demonstration of
an oscillating quality of arm or leg
movement.22 Other dficult skills are
reaching (0-27), and Arniel-Tison and
Grenier19 items for evolung evidence
of the development of lateral control
of the head and body in space (E-18
and E-19). Items demonstrating sdar
levels of dficulty, such as items
0-2/3, E-10/11, and E-16 R/L, are
primarily items that rate the same
activity but on different sides of the
body. A few items of similar degree of
dficulty involve use of different body
parts (eg, 0-2/3 involve spontaneously
turning the head a full 180 , 0-13
involves pelvic control, and 0-23
involves antignvity forearm move-
ments with elbows on the support
surface). These abilities, therefore,
seem to appear at about the same
time during the course of
development.
Floor or ceiling effece have not been
noted (ie, item difficulties extend well
below the lowest perfomance mea-
sures obtained for the subjects tested),
and the highest or lowest ratings pos-
sible for a few item have not been
attained by any subjects. We therefore
suspect that the test could be sensitive
to reflecting the ability of children
who are younger and older than those
in our sample. Before the TIMP could
be used on older or younger children,
however, further research will be
needed. Items 0-21 and 0-22, sponta-
neous rolling to side-lying R/L, seri-
ously misfit the Rasch model and have
been eliminated from the test because
they did not show consistent differ-
ences with increasing age. After con-
sultation among the raters, several
additional misfitting item definitions
were subsequently revised slightly to
eliminate ambiguities in descriptors we
were able to identdy as potentially
14 / 590 Physical Therapy / Volume 75, Number 7 /July 1995

LOGITS: -6 -4 - 2 0 2 4 6
E-SCALE ITEM
E26 TILT L 0 1 1 4
E26 TILT R 0 EASY 1 2 3 .4 HARD 4
E25 STAND 0 STEP 1.2 34 5 STEP 5
E24 ARM REL 0 1. 23 45. 5
E23 TURN L 1 234 56 . 6
E23 TURN R 1 2.34 5 6 . 6
E22 PRONE 0 1. 2.3 45. 5
E21 CRAWL 0 1 2. 3 4 4
EZO SUSPEND 0 1 .2 3 4. 4
El9 LAT TRUN 0 12 .3 3
El8 ARM PROP 0 12.34 4
El7 PTS 0 .1 2.3 4 .5 5
El6 ROLL L/A 2 3. 4. .5 5
El6 ROLL R/A 2 3. 4. .5 5
El5 ROLL L/L 2 .3 4. 5. 5
El5 ROLL R/L 2 .3 4. 5 5
El4 HIP/KN F 0 123. 4
El3 OEFEN AR 0 1. .2 3 5. 5
El2 OEFEN HD 0 1234 . 4
Ell L ROTATE 0 1 2.3 4 4
El0 R ROTATE 0 1 23 4 4
E9 HO MIO/V 0 1.2 3 45 5
E8 HO MIO/SU 0 .1 2 3.4 4
E7 INHIB NCK 0 1 2 .345 5
E7 INHIB NCK 0 .1 2 .345 5
E6 HEAD LOVE 0 12 3 4. 4
E5 LIFT FLEX 1 .2 3 4 4
E4 LIFT EXT 0 12 .3 4 5 5
E3 TRUNK BEN 0 1 2 .3 4. 4
E2 HEAD CTL/ 0 12. 34 5. 5
El TRN HEAD/ 0 1 23 4 . 4
t
I I I I I
-6
i
- 4 - 2 0 2 4 6
DISTRIBUTION 1211121111
OF INFANTS: 1 1 1 14133356389401120636543214112 1 1 1
LOGITS: -6 - 4 - 2 0 2 4 6
0-SCALE ITEM
026 OSCILLAT O" .1 HARD 1
027 REACH 0 1. ITEM 1
025 BALLISTI 0 1. 1
0 9 R WRIST 0 1 1
010 L WRIST 0 1 1
07 R FING MV 0 1 1
020 PRONE LF 0 1 1
015RANK 0 1 1
016 L ANK 0 1 1
0 8 L FING M 0 1 1
04 HANDS TOG 0 .1 1
018 R TRN PR 0 1 1
019 L TRN PR 0 1 1
012 FINGOBJ 0 1 1
011 FINGOBJ 0 1. 1
06 L HAND MO 0 1. 1
05 R HAND MO 0 1 1
023 FOREARM 0 1 1
02 TURN L 0 1 1
013 PELV LIF 0 1 1
03 TURN R 0 1 1
024 ARMS OFF 0 1 1
01 MI0 HEAD 0 .1 EASY 1
017 KICK 0 1. ITEM 1
014 HIP/KN F 0 1 1
I
I I I I I
I I I
-6 -4 - 2 0 2 4
Figure 4. Calibration ~urstone threshold median dzjiculty lew0 of Test of Infant
Motor Pe$brmance items (in logits) based on Rasch psychometric analysis (after dele-
tion of misfitting items). Top: Elicited Scale items in test-item order illustrating dzficulty
of each step within the item rating scale, with easiest step to the left and hardest step to
the rrght. tlottom: Observed Scale items arranged in order of dzficulty from items easy
to pass by less skilled, younger infants to hard items passed only by oldest and most
able infants. The measurement ruler is shown at the top and bottom of each section.
Physical Therapy / Volume 75, Number 7 /July 1995
responsible for misfit or to better re-
flect the developmental sequence
observed in the Rasch analysis.
Descriptive statistics were calculated
for each age group and risk group,
and correlation and multiple regres-
sion analyses were used to assess the
sensitivity of the test to reflect age-
related changes in motor develop-
ment, ethrucity and race, and the risk
for developmental deviance.
The calibration of chld performance
measures on the TIMP is given in
logits in Table 2, which shows the
means, standard deviations, and
ranges for each age group and risk
group in the stratified sample. The
mean score progressively increased
with age, reflecting the sensitivity of
the test to development of motor
slulls. The Pearson Product-Moment
Correlation Coefficient between
postconceptional age in days and the
performance measure in logits on the
TIMP was .83. The Rasch analysis
derived a person separation index of
6.02 (root mean square error=0.21),
indicating that the children tested can
be separated into at least six different
levels of ability across the 5-month age
range tested.
Initial demonstration of the sensitivity
of the TLMP to age-related change in
individual children is shown in Figures
5 and 6 for nine children who were
assessed on three to four different
occasions at least 12 days apart (with
one exception when two tests were
separated by only 6 days). Figure 5
shows the performance of five infants
The frequency distribution of child abil-
ity measures is located on the same logit
scale and is shown in the middle of the
figure. Average performance is located at
0 logits (obtained by 11 infants) on the
scale and indicates that the average
child in our sample would have about .5
probability of passing Observed Scale
item 4 (hands together in midline) and
a high probability of passing Observed
Scale items with dzjiculty calibrations to
the left of item 4, and a .5probability of
receiving scores of 3 on Elicited Scale
items 3, 7, 11, and 23, and so on.

Table 2. Test of Infant Motor PerJomzance Logit Ability Measures by Age Group
and by Medical Risk Within Age Group
Logit Scoreb
-
Age Groupa Risk X SD Range N
Total sample
1
Low
Moderate
High
Low
Moderate
High
Low
Moderate
High
Low
Moderate
H~gh
Low
Moderate
High
Low
Moderate
High
Low
Moderate
High
"Age group 1=32-33 weeks postconceptional age; age group 2=34-35 weeks postconceptional
age; age group 3=36-37 weeks postconceptional age; age group 4=38-41 weeks postconcep-
tional age; age group 5=2-5 weeks postterm; age group 6=6-9 weeks posttem, age group
7= 10-13 weeks postterm.
?he mean logit score is greater than zero because scores were anchored after about 60% of the
data were collected.
at low or moderate risk for poor mo-
tor outcome; Figure 6 shows the data
for four infants at high risk for poor
motor outcome. Based on the data
points from all infants assessed repeat-
edly, the average change in scores
over a 2-week period would be ex-
pected to be about 0.48 logit, greater
than twice the average error value
(0.21). Three children demonstrated
essentially no change in scores
decreases of 0.04, 0.04, and 0.12-
over periods of 12, 14, and 24 days,
respectively. Two infants were at high
risk and one Infant was at low risk for
poor motor outcome. Each was a
young premature infant who subse-
quently showed gains between the
second and third tests of 0.93, 1.09,
and 1.35 logits, respectively. The num-
bers of days between these later tests
ranged from 16 to 47 days.
Multiple regression analysis demon-
strated the joint and unique contribu-
tions to explaining variance in TIMP
performance measures of postconcep-
tional age, medical risk, and 0/1 di-
chotomous variables for ethmcity/race.
The combination of these independent
variables in predicting TIMP logit
measures resulted in a multiple R of
.85 (K .00001). Seventy-two percent
of the variance in TIMP scores was
explained by the combination of these
three variables. Postconceptional age
was a sigmficant variable (beta= .80;
K .00001). Medical risk, as reflected
in the infant’s total Newborn POPRAS
score, was also significant at K.OOO1
(beta= - 3.92). The negative coefficient
indicates that ability decreased with
increasing degree of medical risk; the
simple correlation between TIMP and
POPRAS scores was - .29. Differences
in performance based on race/ethnic-
ity did not reach the .05 level of prob-
ability. Figure 7 shows the relationship
among postconceptional age, POPRAS
risk score, and TIMP performance
measures in a three-dimensional plot.
Discussion
The results of these analyses demon-
strate that the TIMP meets two impor-
tant criteria necessary for potential
usefulness as a tool for assessing in-
fants at risk for poor motor outcome.
Scores increase systematically with
increasing postconceptional age, and
children with greater numbers of med-
ical complications do less well than
same-age peers. In infants assessed
over time, the average change in TIMP
scores over 2 weeks was more than
twice the error of measurement, and
rates of change were approximately
linear. These findings suggest that it
will be possible to develop norms for
performance of premature and young
full-term infants that will be useful in
identlrying those chlldren whose mo-
tor performance is poorer than
expected.
Of other tests for newborns that have
been published, only the test devel-
oped by Komer and colleague~~7~~~
was designed for the purpose of mea-
suring age-related change in behavior.
Their test has been shown to be sensi-
16 / 592 Physical Therapy / Volume 75, Number 7 /July 1995

passively positioned in a midline
orientation.
200 220 240 260 280 300 320 340 360 380 400
Postconceptional Age (d)
Figure 5. Longitudinalperformance (in logits) on the Test of Infant Motor Perfor-
mance offive infants at low or moderate risk forpoor motor outcome.
tive in preterm infants to weekly
changes in alertness and orientation to
sensory stimulation, irritability, and
muscle extensibility (degree to whch
a muscle can be passively extended or
elongated). Despite including a di-
mension called "motor development,"
however, the test of Korner and col-
leagues is not useful for therapists
interested in the development of pos-
tural control because many of the
items require that the child’s head be
200 220 240 260 280 300 320 340 360 380 400
Postconceptional Age (d)
Figure 6. Longitudinal performance (in logits) on the Test of Infant Motor Perfor-
mance of four infants at high risk for poor motor outcome.
The test developed by Dubowitz and
Dubowitz16 similarly requires hation
of the mfant’s head in midline when
most items are tested. Testing the
infant with a passively positioned
head may explain the insensitivity of
the test in Darrah and colleagues’
research on the effects of waterbed
use to promote more flexed postures
in premature infants,42 which we
would have expected to result in
improved ability to independently
center the head. Because head control
is an important aspect of postural
development in the early months of
life and is frequently impaired in chil-
dren with cerebral palsy, the infant’s
ability to independently control head
position in a variety of spatial orienta-
tions and in response to a variety of
sensory and social stimuli is a major
construct assessed in the TIMP.
Further evidence of the construct
validity of the TIMP is found in the
hierarchy of average item difficulty
identdied by the Rasch analysis. For
example, oscillating (0-26) and ballis-
tic (0-25) movements are ranked as
very dficult (ie, passed only by in-
fants with overall high scores), com-
mensurate with Hadders-Algra and
Prechtl’sZ2 and Cioni and Prechtl’sa
identification of the initial appearance
of these aspects of movement at about
7 to 10 weeks of age postterm. Other
dficult items are those involving lat-
eral righting of the head and trunk
(E-18, E-19, E-26). Diagonal and rota-
tional components of movement re-
quire advanced levels of skill relative
to sagittal-plane movements of flexion
and extension, as clearly demonstrated
by the developmental sequence of
behaviors on the Alberta Infant Motor
S~ale.~3
To be useful for the purpose of identi-
fying children who might benefit from
physical therapy or occupational ther-
apy in the period of early infancy, the
TIMP should demonstrate (1) accept-
able rater reliability among therapists
who were not involved in test devel-
opment and (2) test score stability
across short time periods. Other as-
Physical Therapy / Volume 75, Number 7 / July 1995

Figure 7. nree-dimensional representation of individual Test of Infant Motor
Performance (TIMP) iogit measures by postconceptional age and Pwblem-Oriented
Perinatal Risk Assessment System (POPRAS) risk score. Each peak represents an indi-
vidual’s TIMP score (in logits). Infants’per$ormance increased with age (right arrow)
and decreased with degree of medical complications on the POPRAS (lej arrow).
pects of construct validity also remain
to be assessed, such as responsiveness
to change produced by effective inter-
ventions and sensitivity to important
deviations from the norm, including
the presence of signs of motor perfor-
mance deficits or delayed develop-
ment. For example, we are currently
planning a research project to assess
the discriminative validity of the TIMP
in identifying differences in develop-
mental growth curves of defined
groups of infants. These groups will
include infants with documented brain
insults, chronic lung disease, and
extreme prematurity at birth without
brain or severe lung impairments, as
well as both premature and full-term
infants at low risk for poor motor
outcome. Developmental outcome at
1 year will be assessed with the Al-
berta Infant Motor S~ale.~3 This will be
done to assess the predictive validity
of the TIMP using a test with a similar
theoretical basis emphasizing postural
control during functional activities.
Because the regression coefficients for
ethcity/race were close to signifi-
cance (P= .lo, non-Iatino Caucasian
children’s ability was lower than
African-American children’s ability;
African-American and Latino children
performed s~milarly), differences in
performance related to this variable
also warrant further attention with
larger samples before the test is
normed for diagnostic use.
Future research might also assess the
TIMP from the perspective of dynami-
cal systems theory.44 For example, if
this theory holds, children with visual
or auditory impairment should dem-
onstrate poorer performance on those
head control items in which stimuli
are used to which they are unable to
respond when compared with perfor-
mance on head control items that do
not require use of their impaired sub-
system. If task characteristics are deter-
minants of motor responses, altering
the toys used to elicit responses might
also reveal variations in age-related
performance.
Dynamical systems theory also sug-
gests that children might use different
developmental paths to achieving
sirmlar motor milest0nes.~5 The overall
item sequencing and individual item
scaling on the TLMP, therefore, should
be evaluated to determine whether the
sequence of item difficulty indeed
forms a hierarchy of postural control
skills that all children need to achieve
to reach the functional motor out-
comes of sitting alone and beginning
to grasp objects, which are expected
of children at 4 to 6 months of age, or
whether other paths to achievement of
these skills are possible. Longitudinal
research would be necessary to inves-
tigate this question and related ques-
tions that could be posed based on
Shumway-Cook and Woollacott’s
model. l2
Although item scaling is ordered hier-
archically, the Rasch model does not
assume strict adherence to an order
on the part of an infant when deriving
a logit ability mea~ure.39.~~ Perfor-
mance on items that is strictly hierar-
chical generates a misfit statistic be-
cause such regularity is considered
suspect. The statistical model is instead
a probability model that generates an
expectation of obtaining certain per-
formances, given the difficulty calibra-
tion of each item and the child’s over-
all ability as estimated from the total
raw score. Because of this characteris-
tic, one advantage of the Rasch model
for clinical use is that small numbers
of item ratings can be missing from a
child’s test without compromising the
ability to derive an overall ability mea-
sure based on the item performances
available. The presence of multiple
items (or steps wihn items) with
similar difficulty calibrations also
means that not all items may
need to be used in any given test
administration.
One aspect of our research plan in-
volves attempting to develop individu-
ally tailored testing. With this ap-
proach, the therapist would assess the
child on a small number of items that
cover a wide age range (eg, items E-3,
E-13, E-14, and E-17 in Fig. 4) to de-
rive an initial estimate of the child’s
ability. Then only items with difficulty
calibrations slightly above and below
the child’s estimated ability would be
used for further testing, and items
expected to be too hard or too easy
for the child could be omitted. Al-
though Infants have tolerated the time
currently needed for testing with no
adverse effects, tailored testing would
be useful to reduce the time required
of the therapist and the demand on
the infant.
Physical The] rapy / Volume 75, Number 7 /July 1995

Two other major issues of construct
validity currently being addressed in
our research on the TIMP are (1) the
relationship between postnatal age
and TIMP performance and (2) the
ecological validity of TIMP items. A
group of Infants born at a variety of
gestational ages is being assessed at 36
weeks postconceptional age to evalu-
ate whether earlier exposure to the
extrauterine environment affects TIMP
performance and, if so, which items
vary by postnatal age. Ecological valid-
ity is being studied by comparison of
the items involving use of handling
during test administration (E-scale)
with actual demands for movement
provided by parents’ handling of their
Acknowledgments
Appreciation is expressed to Mary
Murney, PT, Joyce Laskey, RN, David
Sheftel, MD, Lucky Jain, MD, Dharma-
puri Vidyasagar, MD, and the nursing
staff of the special care nurseries at
Lutheran General Hospital and Univer-
sity of Illinois Hospital for assistance in
subject recruitment. We also thank the
parents of our subjects for their will-
ingness to allow their babies to be
tested and videotaped.
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