Muscles within muscles: the neuromotor control of intra-muscular segments |
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Authors: | J B Wickham and J M M Brown |
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Institution: | (1) Electromyography Laboratory, Department of Biomedical Science, University of Wollongong, Northfields Avenue, Wollongong, 2522. New South Wales, Australia, AU |
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Abstract: | The aim of this investigation was to anatomically identify, and then determine the function of, individual segments within
the human deltoid muscle. The anatomical structure of the deltoid was determined through dissection and/or observation of
the shoulder girdles of 11 male cadavers (aged 65–84 years). These results indicate that the deltoid consists of seven anatomical
segments (D1–D7) based upon the distinctive arrangement of each segment's origin and insertion. Radiographic analysis of a
cadaveric shoulder joint suggested that only the postero-medial segment D7 has a line of action directed below the shoulder
joint's axis of rotation. The functional role of each individual segment was then determined utilising an electromyographic
(EMG) technique. Seven miniature (1 mm active plate; 7 mm interelectrode distance) bipolar surface electrodes were positioned
over the proximal portion of each segment's muscle belly in 18 male and female subjects (18–30 years). EMG waveforms were
then recorded during the production of rapid isometric shoulder abduction and adduction force impulses with the shoulder joint
in 40 degrees of abduction in the plane of the scapula. Each subject randomly performed 15 abduction and 15 adduction isometric
force impulses following a short familiarisation period. All subjects received visual feed back on the duration and amplitude
of each isometric force impulse produced via a visual force-time display which compared subject performance to a criterion
force-time curve. Movement time was 400 ms (time-to-peak isometric force) at an intensity level of 50% maximal voluntary contraction.
Temporal and intensity analyses of the EMG waveforms, as well as temporal analysis of the isometric force impulses, revealed
the neuromotor control strategies utilised by the CNS to control the activity of each muscle segment. The results showed that
segmental neuromotor control strategies differ across the breadth of the muscle and that individual segments of the deltoid
can be identified as having either “prime mover”, “synergist”, “stabiliser” or “antagonist” functions; functional classifications
normally associated with whole muscle function. Therefore, it was concluded that the CNS can “fine tune” the activity of at
least six discrete segments within the human deltoid muscle to efficiently meet the demands of the imposed motor task.
Accepted: 15 December 1997 |
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Keywords: | Functional differentiation Electromyography Deltoid Motor control Muscle function |
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