Introduction: Recently, scapular muscle fatigue during an elevation task was
found to alter scapular kinematics. Muscle fatigue may be a key factor in the
development of shoulder and neck discomfort and pain. Shoulder girdle muscle
fatigue has been shown to alter scapulothoracic kinematics. However it is unclear
whether muscle fatigue results in increased or decreased scapular upwards
rotation.
Aim: The focus of this study is to examine the effects of shoulder muscle fatigue
on the 3-dimensional scapular kinematics during arm elevation, using the
Qualisys motion capture system. We aim to examine whether an upwards
displacement of the humerus, increased anterior tilting of the scapula and
ncreased upwards rotation will result from fatigue, kinematic changes suspected
to cause a shortening of the subacromial space and leading to the development of
shoulder pain.
Methods: Nine healthy subjects took part in the study and data for six subjects
completing five repetitions of elevation and lowering in the scapular plane
abduction was obtained, before and after a fatigue protocol. Maximum voluntary
sometric contraction force which served as a measure of local muscle fatigue in
this study was measured with the subjects standing on a force plate performing
maximum voluntary isometric contractions of the shoulder while pushing up
against a strap attached to the floor besides the force platform. Realtime
3-dimensional kinematics was recorded during the repetitons of shoulder elevation
using a Qualisys Motion capture system.
Results: There was a significant drop in force between conditions indicating local
muscle fatigue. The results of this study indicate that overall muscular fatigue of
the shoulder affects scapulothoracic and glenohumeral kinematics. There was a
significant effect of fatigue for Anterior/posterior tilt of the scapula after the
fatiguing exercise, with a mean increase of 4.13°and a maximal increase of
7.73°anterior tilt however small, this represented a change in rotation of almost
45%.
Conclusion: This study showed that small but potentially clinically significant
changes in scapular kinematics were found after a shoulder fatigue protocol. An
increase in scapular anterior tilt, humeral internal rotation, flexion and abduction
in the early phases of shoulder elevation was found after fatigue