This informal CPD article ‘From Muscular Chains to Complex Systems’, was provided by Dr. Mauro Lastrico, Physiotherapist at AIFiMM Formazione, an organisation recognised by the Italian Ministry of Health as an authorised CME provider. They offer organised training courses in the Mézières Method, a rehabilitative and postural approach.
Physical foundations of musculoskeletal system organisation
Previous contributions in this series have applied the physical principles of muscle shortening [1], body equilibrium [2] and vector analysis [3,4] to various anatomical districts, demonstrating how muscular dominances determine specific alterations of the physiological joint sequence. The preceding contribution [25] identified three systems — neurophysiological, biomechanical and psychosomatic — that converge on the muscle as final effector. The present article addresses the conceptual leap from the linear view of muscular chains to the understanding of the musculoskeletal system as a complex system, governed by precise and predictable physical laws.
1. Historical Evolution
The concept of the muscular chain has historical roots that precede its modern clinical systematisation by many decades. Reuleaux in 1875 introduced the concept of the kinetic chain as a mechanical system in which the movement of each segment is linked to that of every other [5,6]. Subsequently, Baeyer in 1924 defined the kinematic articular system, transforming the idea of isolated anatomical segments into components of a broader system [5]. From the 1940s, several researchers observed clinically that groups of polyarticular muscles appeared to behave as integrated functional systems: Mézières in France proposed the idea of muscles overlapping like roof tiles, forming myofascial chains [6,7]. In the following years, Souchard, Busquet and Myers developed different classifications, united by the empirical observation that muscles operate in interconnected systems [8,9,10].
These clinical observations, whilst initially lacking rigorous mathematical models, highlighted real phenomena that the physics of complex systems can now formally explain [5,11]. Non-linear mathematics and the theory of complex systems account for phenomena that the traditional view could not interpret: how small dysfunctions can generate diffuse symptomatology, why compensations are often unpredictable, and how the body manages to develop adaptive strategies [5,11,12,16].
2. The Musculoskeletal System as a Complex System
A complex system may be defined as any entity composed of more than one interacting element [5,11]. This definition encapsulates an essential characteristic: the scalar and infinitely divisible nature of complexity. Each individual represents a complex system with respect to their own systems — visceral, neurological, musculoskeletal — and each of these is in turn decomposable into subsystems: the musculoskeletal system divides into regions, each region into joints, each joint into specific components [5,11,16]. The same logic applies in the opposite direction: the individual becomes a subsystem when one wishes to study the behaviour of a population [5]. What is commonly called a muscular chain in reality represents an example of a complex system, governed by four fundamental characteristics [5,11].
3. First Characteristic: Interdependence and Interaction
In a complex system, all the elements that compose it are interdependent and interacting [5,11,27]. It is the classical example of the spider’s web: acting on one portion, it is the entire web that must adapt to the modification. Applying this principle to the musculoskeletal system, any segmental action localised to a body region will determine adaptations in adjacent regions [3,5]. Such adaptations may be corrective or of negligible magnitude, but they may equally be aggravating [5,22].
If the regional action occurs in the corrective direction of the skeletal axes but determines an increase in the energy of the system — understood as an increase in muscular tone — systemic deteriorations greater than the correction obtained will occur [5]. The mechanism is as follows: the local correction requires the high-intensity activation of specific muscle groups; this activation, in a system in which all elements are interconnected, propagates to adjacent regions, compelling muscles not directly involved in the correction to raise their tone in order to maintain overall equilibrium [5,11,20].
The phenomenon is clinically observable: when a patient in the standing position is asked to actively correct internal rotation of the lower limbs, there may manifest, as systemic adaptations, anterior flexion of the trunk, upper limb abduction and widening of the support base, with consequent greater difficulty in maintaining the standing position and a systemic increase in muscular tone [5,17]. Similarly, the simple request for external derotation of the humeri may activate in co-contraction, in addition to the external rotators, the scapular adductors, the upper fibres of the trapezius and even the humeral internal rotators, producing compression of the thoracic vertebrae and an increase in tone [5]. In both cases, the systemic deteriorating components are of greater magnitude than the positive effect of the correction itself. This observation carries direct therapeutic implications: even corrective interventions must be modulated to avoid the onset of mathematically predictable systemic problems [5,21,26].
4. Second Characteristic: Only Systemic Understanding
A complex system can only be understood by considering it as a whole [5,11]. To interpret the significance of the segmental strategies, both static and dynamic, that the system implements, one must observe the connections between the altered regional pattern and the overall patterns [5]. The symptom may be an expression of local suffering, referred suffering or systemic discomfort [5,17,20]. To identify the central origin of peripheral symptoms, a comprehensive musculoskeletal analysis is required, together with the use of dermatomal charts and those of peripheral innervation; the execution of analytical and systemic tests will allow the detection of the dominances that interfere with muscular balancing and the distinction between primary and secondary shortenings [5,17,18,22].
Latissimus dorsi opposes resistance
5. Third Characteristic: Emergent Abilities and Substitutive Moments
A complex system, in the pursuit of its objectives, is capable of generating solutions that are not predictable from the examination of individual elements [5,11,13]. This characteristic means that in performing an action, not necessarily only the muscles anatomically designated will be employed: such muscles may be replaced or supplemented by muscles that, according to a linear vectorial analysis, should not or could not come into play [5,14,28]. That is, substitutive moments are produced, which manifest in two principal contexts [5,30].
In the first context, substitution occurs for energy optimisation [5,19]. If the antagonists to the target movement are in excess of resistive force — that is, in shortening — the isolated muscles find themselves in vectorial subdominance and unable to execute the action. Since the motor objective takes priority over the mode of execution, these muscles will be supplemented by others through emergent coordinative patterns [5,13,15,25]. An example clarifies the principle: in physiological resting inspiration, the diaphragm, after descending, should allow its costal insertions to increase the lateral volume of the rib cage. If the latissimus dorsi opposes resistance to lateral thoracic expansion, since respiratory function is prioritised, the diaphragm through its vertebral insertions and together with the psoas, with which it forms a force couple, will proceed to lift the thorax by increasing the lordosis [5,19,29]. The muscles that tend to be substituted are not random but predictable according to the mathematical logic of force couples: among these, the infrahyoid muscles, serratus anterior, rectus abdominis, triceps brachii, quadriceps femoris and monoarticular muscles in general [5,29].
In the second context, substitutive moments constitute protective strategies: articular blocks that are not mechanical but muscular, aimed at the protection of potential or latent mechanical conflicts [5,25]. The absence of pain is not necessarily synonymous with the absence of latent pathologies [5,17].
A frequent clinical example is observed during gait: in the phase in which both feet are in contact with the ground, the patient presents limited hip extension. Once a reduced extension angle is reached, flexion of the lower limb and anterior projection of the pelvis begin prematurely [5,24]. The limitation is not articular but muscular in origin: hip extension would place the iliopsoas under tension which, through its vertebral insertions, would produce an increase in lumbar lordosis with potential mechanical conflict in the lumbosacral region [5,24,25]. If greater hip extension is requested whilst maintaining the foot in contact with the ground, pelvic anteversion, anterior projection of the lumbar spine and onset of lumbar pain not perceived during spontaneous gait are observed [5,17]. The limitation of hip range of motion thus appears functional to the safeguarding of latent conflicts in the lumbar region [5,25].
The therapeutic priority is to detect the substitutive strategies in operation and to create the conditions for the isolated muscles to resume their anatomical function, through the recovery of length of the braking muscles — those in excess of resistive force [1,5,26].
6. Fourth Characteristic: Equilibrium at the Edge of Chaos
The concept of equilibrium at the edge of chaos is borrowed from non-linear dynamics and deterministic chaos theory, where it represents the condition in which a complex system operates with maximum efficiency and adaptability [5,11,12,16]. Biomechanically, this state is identified by the dominance of Working Force over Resistive Force: WF >> RF [1,5]. In this region, the system responds optimally to small signals, modifying its state with minimum energy expenditure [5,11,12]. A concrete example: the transition from standing to walking requires, in optimal conditions, only a small displacement of a body segment to create the gravity/ground reaction force couple that favours progression. If the system is at the edge of chaos, this small signal is sufficient; if the system is rigid, a muscular activation of much greater intensity will be necessary to achieve the same result [2,5,19].
The physiological vertebral sinusoidal curve and the systemic joint sequence are possible only if no specific structural alterations are present and if all muscles work at ideal length [2,5]. If the muscular system is in increased resistive force due to excess basal tone, and if this persists over time, involvement of the connective tissue portion of the fibre with residual shortening is produced [1,5,23]. The system enters a self-perpetuating circuit: the misalignment of individual centres of gravity requires greater basal tone for the maintenance of the standing position; the increase in tone produces shortening with further misalignment [2,5,20]. The system moves away from the edge of chaos and becomes rigid, losing dynamic capacity [5,11,12].
Conclusions
The conceptual transformation from empirical muscular chains to physically demonstrable complex systems represents the natural development of brilliant intuitions toward a complete scientific understanding of the human musculoskeletal system [5,6,11]. The four characteristics of complex systems — interdependence of elements, only systemic understanding, emergent abilities, equilibrium at the edge of chaos — offer an interpretive key for phenomena previously observed in clinical reality but unexplainable, opening new diagnostic and therapeutic possibilities based on the principles of physics [5,11,12,16].
As with all the districts analysed in the series, the coherent therapeutic sequence involves the reduction of Resistive Force in shortened dominant muscles, followed by strengthening to consolidate the correction obtained [1,3,26]. The application of the theory of complex systems transforms empirical observations into quantifiable analyses: the four characteristics provide interpretive tools based on verifiable physical principles, allowing the prediction and understanding of behaviours that the linear approach could not explain [5,11].
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References
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