Maximum muscle force measurement in infants and toddlers

BACKGROUND
There is no objective method to measure muscle force in young children. Such an
objective method will be beneficial for clinical practice. Objective muscle
force measurements provide the pediatric physical therapist with information
about the role of muscle force in functional motor performance in different
circumstances. In longitudinal studies and clinical trials such objective
measurements have to be comparable between different testers and are necessary
to document muscle force changes over time related to growth and development,
the effects of therapeutic interventions, or related to muscular diseases. In
young children, like in children with the Prader-Willi Syndrome (PWS), muscle
force impairments are strongly related to functional motor development and
limitations in activities of daily living. Because there is no standard method,
nowadays therapists establish muscle weakness on the base of systematically
assessment of motor skills. The problem with this method is that skill
acquisition is not only dependent of muscle force but also of age and growth.
Therefore an objective muscle force measurement is more accurately, to examine
the effect of muscle weakness on functional skill performance and to identify
how much force is necessary to perform specific age-related functional
activities. This information is necessary to get insight in the relationship
between muscle force and functional skills in healthy children and children
with neuromuscular problems.
The department of pediatric physiotherapy of the UMC St Radboud
Nijmegen has started, in addition to the Dutch National Growth Hormone Study
(DNGHS), the Motor Training in PWS (MoTraP) research project. The MoTraP study
investigates in an infancy group (0-3 years) if growth hormone treatment leads
to better profit of physiotherapy treatment. The intervention focuses on the
increment of muscle mass and muscle force related to motor skill acquisition.
Various motor tests are available and these are used to determine the motor
development of the child. However, these tests are a bit rough to test children
with PWS, because these tests only allow scoring if a child can or cannot do a
specific age-related skill. These tests do not give insight in the increment of
muscle force, the relationship between muscle mass and muscle force increment
and skill acquisition. Therefore it is necessary to look more accurately to
changes in muscle force, because of the expected relationship between muscle
mass, muscle force and skill acquisition.

Muscle force measurements
There is no standard method for young children to measure muscle force.
Only a few studies included children younger than the age of five, when
measuring muscle force (1-3). Previous investigators concluded that it is not
possible to measure very young children with the present measurement methods,
because of their activity level, shorter attention span, wariness of strangers,
and inconsistent performance in unfamiliar environments (3-5). Furthermore they
often have more inconvenience adjusting to a testing situation than older
children, and their normal, but challenging behaviors, can influence the test
results.
Muscle force can be estimated and measured using different methods,
ranging from observation without equipment, such as videotaping, to laboratory
examinations with isokinetic instruments. In clinical practice, the two most
common methods are manual muscle testing (MMT) scored by the Medical Research
Council (MRC) and hand-held dynamometry (HHD). Both methods are isometric
muscle strength testing methods. Standard MMT techniques are based on the
patients* ability to move against gravity and hold against the examiners
resistance. However, the sensitivity of detecting changes in muscle strength
with MMT is poor because it produces ordinal scale data, ranging from 0 (no
observable contraction) to 5 (normal movements possible against manual
resistance). Because MMT is unsuitable for detecting small changes in strength
and therefore in patient populations the HHD-method is recommended. The
HHD-method allows a simultaneous quantitative and objective assessment of the
strength of the muscular contraction through the use of a pressure sensor. An
examiner holds the dynamometer and applies tension against the subject (the
break test) or holds the dynamometer stationary while the subject exerts a
maximal force against it (the make test). The pressure sensor is positioned so
that the force exerted by the selected muscle during contraction is transmitted
indirectly to the pressure sensor. The HHD-method reproduces the muscle force
in Newton. There are studies available that establish reference values of
maximum isometric force obtained with a HHD in normal children aged 4-16 years
(6). However, no studies were found that measured young children from 0-3 years
old. This is probably because it is not possible to give instructions to young
children and it is not possible to provoke an isometric contraction in a
discrete movement direction in very young children.

Main objective
Because this study will include young children (0-3 years old) the
challenge is to measure isometric muscle force adjusted to their natural
behavior. From 4 months old a child can pull a toy towards himself. Therefore
we want to use this activity to provoke a maximum muscle force activity. At the
moment a child grabs a toy and pulls it towards himself the examiner will give
counterforce in the contrary direction. As a result of synchronizing the muscle
force between the child and the examiner, the child will reach his maximum
muscle force. The department of pediatric physiotherapy developed a prototype
of an Infant-Muscle-Force meter (IMF meter), which measures the muscle force
using a Load Cell placed under the chair of the child. In a pilot study with
this IMF meter it was found that it is possible to induce and register the
maximum muscle force of children and toddlers during such a pulling task.
This study will investigate whether it is possible to measure
objectively the muscle force in 0-3 years old children with the IMF-meter in
such a way that reference values can be used in clinical decision-making.
Therefore we will test the reliability and construct validity of the IMF meter
in different age groups (6, 9, 12, 18, 24, 30 and 36 months-old children). To
determine the reliability the intrarater en interrater reliability will be
tested and to determine the construct validity the IMF meter will be compared
to a standard motor task: the Alberta Infant Motor Scale (AIMS). We expect a
positive correlation between the IMF meter and the AIMS, because of the
expected relationship between muscle force and motor development. However it is
not possible to estimate beforehand the strength of correlation, because there
are no reference values available about this relationship. If the reliability
and construct validity of the IMF meter is positive, the IMF values obtained in
the different age group will be used as reference values for the IMF meter.

Instruments
Infant-Muscle-Force meter (IMF meter). The IMF meter takes use of a
Load Cell (LECB-50), with a high pass filter of 30 Hz, to register pull and
push forces. The load cell is part of a platform, above the platform a child
chair is installed (Figure 1). With the IMF meter it is possible to induce and
register the maximum muscle force of infants and toddlers during a pulling
task. The IMF meter takes use of the DAS-Wizard computer program. DAS-Wizard is
an Add-in for Excel that places measurements, with a sample frequency of 10 Hz,
directly into the cells of an Excel worksheet. With this information it is
possible to detect the maximum muscle force that the child can deliver. To
exclude other circumstances that can cause a peak value, for example chair
movement, the IMF data will be analyzed synchronous with a videotape of the
experiment.
Alberta Infant Motor Scale (AIMS). The AIMS is a standard reliable and
valid motor test (7) which describes in 58 developmental steps motor skill
acquisition in four different positions: prone, supine, sitting and standing.
Within every position the examiner scores the least and most mature
developmental motor step present in the motor repertoire of the child. The
range between the least and most mature motor step is called the motor-window.
The total score of the AIMS is the summation of all the motor steps preceding
the motor-window (the motor steps the child has outgrown) and all motor steps
present in the current motor repertoire within the motor-window.

Procedure
For an overview of the measurements see Table 1. Before starting with
the measurements height, weight, and age of each child will be recorded. First
we will measure the muscle force with the IMF-meter, followed by the AIMS. At
the end of the test muscle force will again be measured with the IMF meter.

Table 1: Overview of the measurments.
Time Measurements

5 min Introduction/Acquaintance/ length and weigth
5 min IMF-test-1*- Pulling three times
15 min AIMS- Prone- Supine- Sitting- Standing
5 min IMF-test-2*- Pulling three times

Total 30 min
* IMF-test-1 and IMF-test 2 will be performed counterbalanced by the same or
different tester to test intrarater en interrater reliability.


Two trained examiners will participate in this study. The subjects will
be randomly assigned to the examiners (Figure 2). For the intrarater
reliability subjects will be tested two times with the IMF meter, at the
beginning and at the end of the test, by the same tester. For the interrater
reliability subjects will be tested two times with the IMF meter by two
different examiners. One examiner will measure the muscle force at the
beginning of the test; the other examiner will measure the muscle force at the
end of the test. The examiner who will start with the first measurement is
counterbalanced.

During the measurements with the IMF meter the child will be seated on
a small chair, adapted for his size, with his feet several centimeters above
the floor. The upper body will be stabilized with a wide cloth strap that will
be wrapped around the child and chair. Children will be videotaped during the
session using a camera for a side view, so it is possible to analyze IMF data
synchronous with the videotape. The general idea of the IMF-meter is to provoke
a pulling activity from the child. To do so, the examiner, facing the child,
will try to find a toy the child likes. We expect the likelihood of the toy is
conditional to evoke the maximum muscle force. From that moment the game will
begin in which the child will pull the toy towards himself and the examiner
gives counterforce. As a result of synchronizing the muscle force between the
child and the examiner, the child will reach his maximum muscle force. This
procedure will be repeated three times, the highest value will be considered as
the maximum force. This value will be used, so the influence of practice and
learning during the actual data collection can be excluded. After the first IMF
measurements the child has to perform the AIMS, according to standardized
procedures. At the end of the test the muscle force will be measured for the
second time with the IMF meter. The total duration of the test will be around
half on hour.

The children will be measured during activities, which are part of their
natural behavior. There are no specific risk factors.
Extend of the burden for the child is minimal. Extend of the burden for the
parents is limited to the time investment. The parents are totally free to
choose for participation so the burden is low.