Degree Name

Doctor of Philosophy


Department of Biomedical Science


Recent research has suggested that segments of a whole muscle that have a line of action closely approximating the direction of intended joint movement (prime mover segments) are activatedfirst and display the highest amount of myoelectric activity compared to segments with more divergent lines of action (synergist segments). However, how individual segments of muscle groups are selectively activated by the central nervous system (CNS) to effectively co-ordinate both static and dynamic tasks about a joint remains unresolved. Therefore, the purpose of the present study was to investigate the activation patterns of intramuscular segments within a group of three shoulder muscles during the performance of static and dynamic shoulder joint contractions. Three experiments were performed. The first experiment involveda cadaveric dissection in which the degree of anatomical segmentation, nerve branch patterning and segmental geometry was determined. The second and third experiments then examined the function of the identified anatomical segments in both static and dynamic contractions around the shoulder joint. Miniature surface electrode pairs (interelectrode distance of 7mm) were positioned over the predetermined anatomical segments of the pectoralis major, latissimus dorsi and the deltoid in 16-20 healthy male volunteers (aged 18-30 years) while they were seated in an experimental chair. Electromyographic (EMG) data was sampled at 1000 Hz whilst the subjects performed either static (second experiment) or dynamic (third experiment) shoulder joint tasks. The static protocol utilised shoulder joint tasks such as abduction, adduction, flexion and extension tasks at various intensities of contraction (%MVC) and at varying shoulder joint angles whilst matching an idealised force-time curve displayed on an oscilloscope. The dynamic protocol utilised rapid shoulder joint adduction and extension movements in which some of these movements were unexpectedly increased or decreased in load. EMG waveforms from the two EMG experiments representing each muscular segment were rectified and low passfiltered (20Hz) and their temporal characteristics determined relative to the force-time record (static experiment) or electrogoniometer (dynamic experiment). Various intensity measures were also used to determine the extent of functional differentiation. Results indicated that timing and intensity of individual muscle segments were highly co-ordinated both within the one muscle and between different muscles. Consistent with previous research (Paton & Brown, 1994; Paton & Brown, 1995), those segments with the most conducive line of action for a particular task were usually activatedfirst and displayed the highest levels of myoelectric activity. It Was concluded that the notion of individual muscle segments acting asmuscles within muscles to allow the CNS tofinetune the activity of skeletal muscles to efficiently meet demands of an imposed motor task was confirmed. Furthermore, the study provided insight into neuromotor strategies utilised by the central nervous system to effectively co-ordinate the activation patterns of intramuscular segments within groups of muscles surrounding the shoulder.