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A. Chalard et al.
(1). Most importantly, in agreement with a suggestion
made in a previous report (10), these results unequivo-
cally establish that spasticity and spastic co-contraction
have different functional repercussions with regards to
impaired motor function in adults with brain injury.
It is well known that selective muscle activation
is necessary for skilled and coordinated upper limb
movements, which implies concomitant activation of
agonists and relaxation of antagonists. Lesions that
damage the corticospinal pathways, such as stroke or
traumatic brain injury, cause long-lasting impairment
of the ability to produce selective patterns of EMG
activity (11). However, it has been shown that, during
rehabilitation, the decrease in co-contraction index is
correlated with the improvement in Fugl-Meyer score
among post-stroke subjects (12). In agreement with a
previous study (4), our finding of a strong association
between the co-contraction index and Fugl-Meyer
Assessment score thus highlights the importance of
impaired motor selectivity as a mechanism that con-
tributes to greater spastic co-contraction.
The adverse consequence of the presence of spastic
co-contraction on upper limb motor impairment is
further supported by the significant association bet-
ween the co-contraction index and the score on Action
Research Arm Test, taken to reflect upper limb functio-
nal limitation. Furthermore, by analysing EMG-based
assessment of spastic co-contraction in patients with a
wide range of motor impairment, our results highlight
that the more severe the motor impairment, the greater
the co-contraction index.
These results also failed to show an association bet-
ween the net elbow extension torque during maximal
voluntary contraction, taken as a functional marker
of triceps brachii paresis, and either the limitation of
active elbow extension, the score of Fugl-Meyer As-
sessment or of the Action Research Arm Test. These
findings indicate that motor weakness is not a primary
factor limiting active elbow extension, and lead to the
conclusion that non-selective motricity and function
impairment are not directly linked to motor weakness.
Taken together, the above results demonstrate the
detrimental impact of spastic co-contraction on upper
limb motor function, leading to abnormal restricting
arm movement patterns, especially in subjects with
more severe motor impairment.
Despite strong evidence of improvement in spasti-
city induced by botulinum toxin treatment, few studies
have shown effectiveness in improving active function
and active movement (13, 14). A limitation that may
explain this lack of efficacy in active function is the
use of spasticity, as assessed during passive stretching,
as a marker of muscle overactivity during movement
(15). In contrast, the change in spastic co-contraction
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after injection of botulinum toxin has been poorly
studied. From a clinical perspective, these results are
in line with the Subcommittee of the American Aca-
demy of Neurology (13), which recommend the use
and development of a method and outcome regarding
motor function and active movement. We support the
requirement to report active range of motion as an out-
come of muscle overactivity treatments (7), and the use
of EMG-based quantification of spastic co-contraction
as a relevant tool for providing effective interventions
related to altered recruitment of antagonist muscles and
limitation of active movement.
Study limitations
Although a limitation of this pilot study is the small
sample size, significant findings were made relative
to the link between spastic co-contraction and clinical
scores. Any generalization of these results, however,
should be viewed with caution, especially because
hemiparetic subjects had different aetiologies of brain
injury (i.e. stroke and traumatic brain injury) and that
little is known about the influence of the type of brain
injury on spastic co-contraction.
This pilot study assessed the co-contraction index
during submaximal isometric contraction, while spastic
co-contraction is sensitive to stretch (1). Thus, future
research should investigate co-contraction during ac-
tive elbow extension.
Conclusion and clinical implications
The results of this pilot study suggest that spastic co-
contraction alters upper limb function in subjects with
hemiparetic brain injury. These findings may partially
explain the lack of data concerning the efficacy of
treatments, such as botulinum toxin to improve up-
per limb function (6, 13, 14). Although measurement
of spasticity is usually performed to assess muscle
overactivity, these results highlight the importance
of considering spastic co-contraction to assess active
motor function, and support further studies on changes
in spastic co-contraction after injection of botulinum
toxin, in connection with the improvement in active
function. Practical applications arising from this work
are to improve the assessment of factors that restrict
movement and implementation of treatments aimed at
improving motor function in subjects with brain injury.
ACKNOWLEDGEMENT
Alexandre Chalard is an employee of Ipsen Innovation within
the framework of a Conventions Industrielles de Formation par
la REcherche (CIFRE) PhD fellowship. Others authors in this
study declare that they have no conflict of interest in relation
to this study.