This informal CPD article ‘Clinical Reasoning in Musculoskeletal Biomechanics - Part 1: The Four Constants and Differential Diagnosis’, 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.
The previous contributions in this series have developed a physical model of the musculoskeletal system: the mechanics of the fibre [1], body equilibrium between the weight force and the ground reaction force [2], the vector analysis of dominances [3,4] and the reading of the musculoskeletal system as a complex system [6].
This article translates that knowledge into tools for everyday clinical reasoning. The approach does not replace consolidated diagnostic competencies but integrates them with a systemic perspective grounded in physical principles [6,8]. Since a symptom may express local suffering, referred suffering or systemic discomfort, only the observation of the system as a whole, in addition to the analysis of the individual element, allows these possibilities to be interpreted correctly [6,24].
1. The Four Constants (4K)
When a symptom of mechanical conflict manifests, in the absence of congenital or acquired conditions, the musculoskeletal system presents four recurring constants [8,12].
1.1 First Constant (1K): The Intra-Articular Mechanical Conflict
The symptom and the functional impairment are determined by the intra-articular mechanical conflict. When forces concentrate in restricted areas instead of distributing over the articular surfaces, localised overloads are created [12,13]. The forces G and R, instead of distributing over the entire support surface, condense in limited zones, generating the conflict [2,27].
1.2 Second Constant (2K): The Asymmetric Vectorial Shortening
The muscular vectors that should ensure the correct articular axis are in asymmetric shortening along dominant lines, and this produces the intra-articular misalignment [1,3]. The loss of the axis is always a consequence of muscular dominances: no joint moves on its own; it is the asymmetry of the muscular traction forces that determines the alteration of the physiological articular sequence [3,9].
1.3 Third Constant (3K): The Systemic Misalignment
In the presence of a symptom, the misalignment concerns not only the symptomatic joint but, to a micro or macro degree, all the body’s joints; often the most misaligned are not the symptomatic ones [6,25]. The interdependence and interaction of all the elements cause a local perturbation to propagate systemically: the symptomatic joint may represent the breaking point of a system already compromised as a whole, not necessarily the site of the greatest misalignment [6,24].
1.4 Fourth Constant (4K): The Systemic Distribution of Shortenings
Like the misalignment, shortening too is not confined to a single joint: it involves the entire myofascial apparatus, with shortenings distributed throughout the system [1,14].
1.5 The Constants in the Absence of Symptoms
The third and fourth constants are present even in the absence of symptoms. The mechanisms that sustain the rise in tone and the subsequent shortening of the connective portions are always active, through the neurophysiological, biomechanical and psychosomatic systems [5,28,29]. The onset of the symptom is linked to the time factor, by summation of shortenings: the patient is often surprised by it, unable to relate it to a triggering event [5]. This occurs because the appearance of the symptomatology represents the exhaustion of the protective mechanisms of the pre-emptive antalgic reflexes [5,28].
2. Differential Diagnosis: Local or Referred Symptom
The symptom — pain, functional impairment or their summation — may be of local origin or the expression of suffering originating from another district. Causal differentiation is decisive for orienting the intervention [8,10].
2.1 Symptom of Local Origin
Three conditions may sustain it. The first is the articular mechanical conflict: in the district concerned an alteration of the articular sequence sufficient to justify the symptom is detectable, with condensation of the loads G and R in restricted areas [2,12]. The second is the alteration of muscular dynamics with substitutive mechanisms: the static configuration may appear physiological, but in movement strategies emerge in which muscles not designated for the action are recruited through emergent synergistic coordinations [6,21,23]. The joint moves as if it were in conflict, generating non-physiological patterns; the same mechanisms manifest as protective limitations of the range of motion or as co-contractions that alter the biomechanics [11,22]. The third is the intrinsic muscular problem: localised contractures, fascial knots or trigger points that have not yet produced significant axial alterations and which, being limited to the muscular component alone, are of simpler resolution [14,16].
Muscular Shortening
2.2 Referred Symptom
When locally no alterations sufficient to account for the symptomatology are evident, the symptom may be sustained by three causes. The first is an axial alteration in another district: the loss of articular sequence in one region may generate symptoms at a distance through the functional connections, because systemic interdependence propagates the perturbation along extended chains [6,27]. The second is vertebral origin: a vertebral problem may manifest in the periphery following the dermatomal projections of the corresponding roots [19,30]. Knee pain without local articular or ligamentous alterations may recognise its cause in the L3-L4 region, which will appear altered; similarly, elbow pain may express a vertebral suffering at T1-T2 [18,30]. The third is the secondary muscular shortening, triggered by problems of other systems: a mechanism that will be examined in Part 2.
3. The Tools of Differential Diagnosis
The distinction between local and referred symptom rests on clinical tools, integrated by instrumental investigations.
The static physical examination reveals the principal misalignments and the mechanical conflicts; the body’s asymmetries are interpreted according to the principles of vectorial dominances [3,4,9].
The dynamic physical examination identifies the patterns of altered activation: the system may generate substitutive strategies and emergent recruitments, and may present joints incarcerated by muscles that limit their range of motion as if they were skeletally locked, in order to prevent latent conflicts from manifesting [10,20,21]. Such protective limitations may escape the static assessment and reveal themselves only in movement [10].
The dermatomal and peripheral innervation charts show the distal projections of vertebral suffering and allow one to trace the peripheral manifestation back to the spinal segment involved [18,19,30].
The identification of alterations of other systems — stomatognathic, visual, visceral — through targeted tests recognises the primary causes capable of determining secondary muscular shortenings, the subject of Part 2 [8].
Instrumental investigations complete the clinical assessment. Radiography, ultrasonography, magnetic resonance imaging and laboratory tests document congenital or acquired structural alterations and pathologies of other systems, making it possible to confirm or exclude a primary cause and to distinguish the mechanical conflict of functional nature from the structural lesion [8,13].
Conclusions
The four constants provide the interpretive grid for the musculoskeletal symptom; the distinction between local and referred symptom, supported by the tools, identifies its origin [8,10]. The biomechanical diagnosis precedes and guides every therapeutic choice: without the identification of the responsible dominances and their systemic distribution, the intervention risks segmental technicism or genericity [15,17]. This approach represents the evolution of empirical intuitions toward an understanding grounded in verifiable physical principles [7,26]. Part 2 will address the distinction between primary and secondary shortenings, the viscero-somatic correlations and the two equations that derive from them [24].
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