Muscles within muscles: Coordination of 19 muscle segments within three shoulder muscles during isometric motor tasks

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Abstract

The aim of the present study was to determine how the intra-muscular segments of three shoulder muscles were coordinated to produce isometric force impulses around the shoulder joint and how muscle segment coordination was influenced by changes in movement direction, mechanical line of action and moment arm (ma).

Twenty male subjects (mean age 22 years; range 18–30 years) with no known history of shoulder pathologies, volunteered to participate in this experiment. Utilising an electromyographic technique, the timing and intensity of contraction within 19 muscle segments of three superficial shoulder muscles (Pectoralis Major, Deltoid and Latissimus Dorsi) were studied and compared during the production of rapid (e.g. approximately 400 ms time to peak) isometric force impulses in four different movement directions of the shoulder joint (flexion, extension, abduction and adduction).

The results of this investigation have suggested that the timing and intensity of each muscle segment’s activation was coordinated across muscles and influenced by the muscle segment’s moment arm and its mechanical line of action in relation to the intended direction of shoulder movement (e.g. flexion, extension, abduction or adduction). There was also evidence that motor unit task groups were formed for individual motor tasks which comprise motor units from both adjacent and distant muscles.

It was also confirmed that for any particular motor task, individual muscle segments can be functionally classified as prime mover, synergist or antagonist – classifications which are flexible from one movement to the next.

Introduction

It is now well accepted that motor units, within individual muscle segments of single skeletal muscles, can be independently controlled by the central nervous system (CNS) to produce particular motor outcomes [3], [21]. This phenomenon, which may be termed ‘functional differentiation’ [18], has been described within a number of individual skeletal muscles, including the Tensor Fascia Latae [18], the Gluteus Maximus and Medius [16], the Triceps Surae [3], the Biceps Brachii [2] and the Pectoralis Major [19]. Ettema et al. [5] has suggested that large absolute differences in moment arms between different segments of a single muscle partially explain this phenomenon.

More recent studies on the Deltoid [24] have shown that skeletal muscle has a greater potential for functional differentiation than hitherto accepted. Wickham and Brown [24] have determined that the Deltoid muscle, commonly described as having only three (anterior, middle and posterior heads) functional muscle segments [10], is composed of at least seven muscle segments which all have the potential to be independently coordinated by the CNS. Functionally independent muscle segments provided the CNS greater flexibility to “fine tune” the activity of its Deltoid motor units when controlling movements of the shoulder joint.

Furthermore, Wickham and Brown [24] have suggested that the seven individual muscle segments of the Deltoid may be functionally classified as either prime mover, synergist or antagonist muscle segments based upon their mechanical lines of action (joint movement most likely from approximation of a segment’s origin and insertion) and periods of activation. Attempts to assign functional classifications to individual muscle segments, classifications hitherto used to describe whole muscle function [1] are useful in characterising the activity of each individual muscle segment. For example, the prime mover segments of the Deltoid were found to have the most efficient mechanical lines of action, the earliest and longest durations of activation and the highest amplitudes of myoelectric activity. Deltoid muscle segments with increasingly divergent mechanical lines of action (synergist segments) were found to have increasingly later onsets, shorter periods of activation and lower intensities of myoelectric activity. Finally, antagonist segments, with mechanical lines of action opposing the movement, were activated last, had the shortest periods of activation and variable amplitudes of myoelectric activity [24].

Applying functional classifications to muscle segments within a single muscle is useful to provide a clearer understanding of how the CNS controls motor unit activity within a muscle during a range of motor tasks. However, there have been few studies into how the CNS controls the muscle segments of individual muscles constituting a muscle group around a particular joint. We do not as yet understand how the muscle segments of individual muscles, within a muscle group, are coordinated together to produce motor tasks or how that coordination is affected by factors such as movement direction and moment arm.

Therefore, the aim of the present study was to determine how the muscle segments of individual shoulder muscles were coordinated together to produce isometric force impulses around the shoulder joint and how that coordination was influenced by movement direction, mechanical line of action and moment arm. This investigation compared the activation of 19 muscle segments within three superficial shoulder muscles (Pectoralis Major, Deltoid and Latissimus Dorsi) during the execution of rapid (e.g. 400 ms time to peak) isometric force impulses in four different movement directions (shoulder-flexion, -extension, -abduction and -adduction). Our intention was to understand how individual muscle segments were coordinated not only within a single muscle, but across a group of muscles that controlled a single joint.

Section snippets

Procedures

Twenty male subjects (mean age 22 years; range 18–30 years) with no known history of shoulder pathologies, volunteered to participate in this experiment.

The subjects sat in an adjustable dental chair within a wire cage with their extended right upper limb positioned snugly within an arm cast (Fig. 1). The arm cast was immobilised by attachment to the wire cage to permit the subjects to perform a series of isometric shoulder joint tasks. The arm cast, so fitted, ensured that the right upper limb

Biomechanical data

Table 1 provides a biomechanical analysis of each of the 19 muscle segments derived from one representative cadaver. Note that the largest muscle segment cross-sectional areas were found in segments P1 and P2 (clavicular head) of the Pectoralis Major, segment D3 (middle head) of the Deltoid and segment L2 (upper fibres) of Latissimus Dorsi. Within both the Deltoid and the Latissimus Dorsi, the “stronger” muscle segments were found towards the middle of the muscle. In contrast, the upper

Overview

The aim of the present study was to determine how 19 muscle segments within three adjacent superficial shoulder muscles, the Pectoralis Major, the Deltoid and the Latissimus Dorsi, were controlled by the CNS during the production of four rapid isometric shoulder tasks (abduction, adduction, flexion and extension).

Of particular interest was to understand how the timing and intensity of muscle segment activation within and across the three superficial shoulder muscles was influenced by the muscle

Conclusion

The aim of this study was to determine how 19 muscle segments within the Latissimus Dorsi, Deltoid and Pectoralis Major, were controlled by the CNS to produce four isometric shoulder motor tasks.

The results of this investigation have suggested that the timing and intensity of each muscle segment’s activation were coordinated across muscles and influenced by the muscle segment’s moment arm and its mechanical line of action in relation to the intended shoulder movement direction (e.g. flexion,

Acknowledgements

The authors wish to thank the University of Wollongong for technical and financial support to Dr. Wickham during his doctoral studies and to Dr. Brown during his sabbatical leave in Germany. Thank you also to Mr. Luke Bones and Mr. Daniel Wickham for assistance with preparation of the illustrations and to the reviewers for the excellent comments and suggestions. The work described in this study was approved by the Human Ethics Committee of the University of Wollongong.

Mark Brown has a Doctoral qualification from the University of Queensland and is currently a Senior Lecturer and Assistant Dean at the University of Wollongong. He was Head of the Department of Biomedical Science between 1997 and 2001 and is the immediate past Vice President (exercise science) of Australia’s professional Exercise Science association (AAESS). His research interests include functional segmentation of skeletal muscle and sustainable transport.

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    Mark Brown has a Doctoral qualification from the University of Queensland and is currently a Senior Lecturer and Assistant Dean at the University of Wollongong. He was Head of the Department of Biomedical Science between 1997 and 2001 and is the immediate past Vice President (exercise science) of Australia’s professional Exercise Science association (AAESS). His research interests include functional segmentation of skeletal muscle and sustainable transport.

    James Wickham received a Bachelor of Sports Science (exercise science) from the University of New South Wales in 1992 and a Diploma of Education (physical education) in 1993 from the same institution. In 1995 he received an Honours Degree from the University of Wollongong and a PhD in 2002. Since 1999 James has been employed as an Anatomy Lecturer at La Trobe University in Melbourne. His research interests include using electromyography to quantify shoulder muscle activation patterns and functional differentiation within skeletal muscles.

    Darryl McAndrew received a Bachelor of Science (Human Movement Science) from the University of Wollongong in 1993 and is currently an Associate Lecturer within the Department of Biomedical Science, UoW. He is concurrently completing a PhD focusing on the fibre type characteristics of segmental muscle and a M.Sc. in OH & S, specialising in Occupational Hygeine. His research interests include identifying muscle fibre type characteristics via mechanomyography and occupational exposure to industrial noise.

    Xu-Feng Huang received his PhD degree from the University of New South Wales in 1992. He is currently an Associate Professor in the Department of Biomedical Science and the Director of Neurobiology Research Centre for Metabolic and Psychiatric Disorders in the University of Wollongong. His research interest includes the central regulation of energy balance.

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