Study of scapular muscle latency, shoulder kinematics and muscle activity in people with and without shoulder impingement.

Abstract

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.