Background and significance: Shoulder impingement is a common shoulder pathology
which is associated with changes in kinematics and muscle activity around the shoulder
joint. The changes in muscle activity are theorized to be caused by changes in motor
program strategies controlling the smooth and coordinated movements at the joints.
Changes in muscle latencies, especially feed forward contractions, indicate alterations in
these motor control programs. The purpose of the study was to assess for differences in
the latencies and deactivation times of scapular muscles between subjects with and
without shoulder impingement. Research Methods: Twenty five healthy subjects and 24
subjects with impingement were recruited. Scapulothoracic and glenohumeral kinematic
data were collected using an electromagnetic system. Simultaneously myoelectric
activities using surface electrodes from upper trapezius, lower trapezius, serratus anterior
and anterior fibers of deltoid were collected as subjects raised and lowered their arm in
response to a light cue. Data was collected during unloaded, loaded and after performing
repetitive arm raising motion conditions. Analysis: The ratios of the number of feedforward
contractions during trials were compared by chi square analysis across groups
and conditions. The other variables were analyzed using 2 or 3 way mixed model
ANOVAs. Results: The percentage of trials showing feed forward contractions was
higher for upper trapezius and lower trapezius in the unloaded condition and lower for
serratus anterior in the condition after repetitive motion for the subjects with
impingement as compared to healthy subjects. Subjects with impingement also
demonstrated significantly earlier contraction of upper trapezius and an earlier
deactivation of serratus anterior during lowering of the arm as compared to the healthy subjects. All subjects exhibited an earlier activation and delayed deactivation of lower
trapezius and serratus anterior in conditions with a weight held in hand. The study found
decreased scapular upward rotation, decreased posterior tilt and a less anterior plane of
elevation in combination in subjects with impingement using logistic regression analysis.
No significant group differences were found for muscle activity as a percentage of the
reference contraction. Discussion and conclusions: The subjects with impingement
showed some significant differences for muscle activation and deactivation times to
indicate differences in motor control strategies. Rehabilitation measures should
incorporate appropriate training measures in tandem with strengthening and stretching
exercises to focus on improving movement patterns and muscle control.
University of Minnesota Ph.D. dissertation. August 2009. Major: Rehabilitation Science. Advisor: Paula M Ludewig, PT, PhD. 1 computer file (PDF); xiv, 250 pages, appendices 1-16. Ill. (some col.)
Study of scapular muscle latency, shoulder kinematics and muscle activity in people with and without shoulder impingement..
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