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cohort of children with OI type I, III and IV.
9,13
Muscle
strength was described as normal in children with OI
type I but was decreased in types III and IV. In order
to provide information regarding the natural course of
developmental outcome parameters, including muscle
strength, the same authors found no significant changes
in total muscle strength (upper or lower extremities)
during a 4-year follow-up study of 49 children (mean
age 11, 3 ± 3.8).
10
The absence of significant changes in
muscle strength found in this study could be explained
by the method used for measuring muscle strength.
Manual muscle testing is an incredibly subjective mea-
surement and might just not be sensitive enough to
detect changes over time. To our knowledge, the first
study in which sensitive dynamometers were used
is the study of Takken et al., in which muscle strength
was measured in 17 children (mean age 13.3 ± 3.9 years)
with OI type I using a handheld dynamometer.
14
Muscle
strength was measured in shoulder abductors, hip flex-
ors, ankle dorsal flexors and grip strength. The results
of this study indicated that muscle strength was signifi-
cantly reduced in children with OI type I, ranging from
−1.24 (±1.4) to −2.88 (±2.67) standard deviations lower
compared with those of their healthy peers. The authors
speculated that the reduced muscle strength may be
caused by a decrease in muscle mass or by a diminished
neuromuscular coordination.
14
Many patients with OI
are not involved in regular exercise with sufficient inten-
sity. Patients might avoid exercise because of the risk
of fractures and therefore could induce hypoactivity-
related muscle atrophy and muscle weakness. However,
in the latter study it was unclear whether the reduced
muscle force was a consequence of a hypoactive lifestyle
or a specific consequence of the impaired muscle colla-
gen synthesis.
The most recent study concerning muscular strength
is the study of Caudill et al. in which the authors indi-
cated that ankle strength might be associated with skills
that affect activities of daily living, such as standing on
one foot.
15
Plantar flexor weakness was described in
a cohort of 20 children and adolescents with OI type I
(age 6-18 years).
Several studies have shown the general benefi-
cial effects of medication; however, only a few studies
address the effects of these medications on muscular
strength. The study of Montpetit et al. examined changes
in maximal grip strength (measured with a dynamom-
eter) during pamidronate therapy in 42 children (mean
age 11.4 ± 2.6 years) and adolescents with OI type I, III
and IV.
16
At the start of pamidronate therapy, maximal
grip strength was decreased compared with age-specific
reference data (−2.7 SD ± 2.1); however, it was normal
for weight (−0.1 ± 1.8). At 4 months' follow-up, grip
strength was significantly increased, whether related to
age or weight (−2.0 (±1.8) and 0.6 (±1.5), respectively).
16
This gain in grip strength was maintained in the 2-year
follow-up measurement. Although the etiology of the
low grip strength at baseline is not clear, a rapid increase
in grip force after a single infusion cycle (measure-
ment after 4 months) was clearly described, could not
be reflected by natural maturation or evolution and is
probably a direct (or indirect) effect of the medication.
16
The study of Land et al. assessed the effects of cyclic
intravenous pamidronate during 3 years in 59 pediat-
ric patients (mean age: 6.1 years) with OI type I, III and
I V.
17
They also showed a significant increase of maxi-
mal grip strength (63%;
p
< 0.001). Strikingly, the fast-
est increase was observed in the first 12 months (+35%)
of the treatment and all the described increases were
still significant when corrected for weight and height.
17
In 2007 the same authors assessed the effects of cyclic
intravenous pamidronate in a cohort of 10 children and
adolescents (mean age: 7.1 years) with OI type VI.
18
In
this study each patient had completed at least 3 years of
pamidronate therapy. Grip strength was measured in the
non-dominant hand. The authors stated a low maximal
isometric grip force at baseline measurement (z-score,
mean ± SD: −5.0 ± 2.3) which increased significantly
during treatment (+1.4 ± 1.2). Although a low number
of participants had some methodological quality issues,
they showed possible pharmacological influences upon
muscle strength.
18
Knowledge regarding plasticity and trainability of
skeletal muscles are of great interest for future thera-
peutic possibilities in the pediatric OI population. To
our knowledge, the only randomized controlled study
which addresses trainability, and therefore possible
plasticity, in children with OI is the intervention study
of Van Brussel et al. which included (next to aerobic
training) strength training for children with moderate
types of OI, consisting of OI adapted exercises without
heavy weights.
19
Although the intervention consisted
only of low-resistance strength training without heavy
weights, there was a 12% improvement in muscle force
in the children in the intervention group during the
training period compared with the children in the con-
trol group.
19,20
This improvement is less than has been
reported for healthy children after an 8-week resistance
training program, for which improvements in muscle
force between 5% and 40% were reported.
21
The smaller
improvement in this study was expected because
only very light resistance was utilized. However, the
improvement in muscle strength is clinically relevant
for patients with OI because muscle force and the
strength of bones are strongly associated.
22
Mobility and Gait Pattern
The present level of mobility in children with OI can
be classified in different levels of ambulation according
