This informal CPD article ‘Biomechanical Analysis of the Lower Limb - Part 2: Vectorial Dominances of the Foot’, 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.
This article completes the analysis of the lower limb begun in the previous contribution, which addressed the vectorial dominances of the hip and knee [1]. The theoretical framework remains that presented in the foundational contributions of the series: the model of muscle shortening as viscoelastic deformation [2,11], body equilibrium as alignment between the weight force (G) and the ground reaction force (R) [3], and vector analysis as a tool for identifying muscular dominances [4,5]. Subsequent contributions have applied these principles to the vertebral column in the frontal [16,17] and sagittal [18] planes, to the hyoid bone [19], to the temporomandibular joint [20,21] and to the shoulder complex [22,23]. The foot presents a biomechanical particularity unique in the human body: at the medial plantar arch, all muscular vectors act in summation, without antagonism — an engineering principle consistent with the need to support the entire body weight.
1. The Ankle Joint
1.1 Flexion–Extension and Pronation–Supination Dominances
The vectorial dominance at the ankle, both in terms of the number of acting muscles and the expressible work force, is in the direction of plantar flexion and supination [6,7,9,13].
The dorsiflexors — tibialis anterior, extensor digitorum longus and extensor hallucis longus — are numerically and vectorially inferior to the plantar flexors: triceps surae, peroneus longus and brevis, flexor digitorum longus and tibialis posterior [6,7,30]. Similarly, the pronators — peroneus longus and brevis, extensor digitorum longus — are subdominant with respect to the supinators: triceps surae, tibialis posterior, flexor hallucis longus, flexor digitorum longus and tibialis anterior [6,7].
1.2 Adaptive Strategies of the Ankle
When the plantar flexors and supinators enter excessive shortening, the vectorial dominances are such that they cannot be counterbalanced by the direct antagonists [6,7]. To allow the sole of the foot to rest on the ground in the standing position, the system must employ adaptive strategies by modifying the femorotibial–fibular articular sequence [6,28].
In the presence of shortening of the plantar flexors and supinators, ground contact of the sole becomes possible fundamentally through hyperextension in internal rotation of the knee [1,6,10]. This mechanical link demonstrates the bidirectionality of influences within the kinematic chain of the lower limb: knee alterations may determine distal adaptations, just as foot alterations may oblige the knee to rotatory compensations [1,6,14].
2. The Medial Plantar Arch
2.1 Muscles Acting on the Medial Arch
The muscles capable of modifying the medial plantar arch are divided into three groups [6,7,13,30]: anterior muscles of the leg (tibialis anterior, extensor digitorum longus, peroneus longus), posterior muscles of the leg (triceps surae, tibialis posterior, flexor hallucis longus, flexor digitorum longus) and muscles of the sole of the foot (abductor hallucis, flexor hallucis brevis, adductor hallucis, quadratus plantae).
Tibialis anterior and tibialis posterior act as supinators, increasing the arch height. Peroneus longus, with its course beneath the foot and insertion on the first metatarsal and medial cuneiform, behaves like the string of a bow which, upon shortening, increases and stabilises the plantar vault whilst pronating the ankle [6,7,15]. The triceps surae, a strong heel supinator, produces greater arch height as a mechanical resultant of calcaneal supination [6,10]. Abductor hallucis, spanning from the calcaneus to the great toe, acts analogously as the bowstring of the arch [6,7].
The medial plantar
2.2 The Biomechanical Particularity: All Vectors in Summation
In all the joints analysed throughout the series, although in vectorial imbalance, the acting forces are in antagonism with each other [4,5]. The medial plantar arch presents the particularity of having all muscular vectors in summation of action: every acting muscle contributes to the support of the arch, none to its depression [6,7].
From an engineering standpoint, this is unsurprising: since the medial arch is responsible for supporting the entire body weight — in the standing position the body’s centre of gravity is discharged at the apex of the medial arch — it is consistent that all muscular tie rods act in support of the ligaments and the arch-shaped conformation of the bones [6,15,28].
Architecturally, an arch is capable of sustaining great vertical loads provided the bases are stable; otherwise the arch collapses [15]. In the plantar vault, rigid bases are replaced by muscular tie rods better suited to withstand dynamic stresses. Muscular action is therefore directed at supporting the bases [6,7]. Furthermore, since the medial plantar arch is not composed of a single bone — which would render it stable but rigid and inadequate for dynamic stresses — the musculoligamentous tension assumes additional importance, dynamically stabilising the plantar vault when expressed at the necessary minimum [6,7,11].
3. Flat Foot
3.1 Differential Diagnosis
Since the action of all the acting muscles is directed toward supporting the medial arch, in the absence of specific pathologies the collapse of the plantar vault must be caused by a structural bone deformation of sufficient magnitude to prevent the muscular tie rods from forming the arch [6,7,12]. When peripheral neurological paralysis with consequent muscular inactivity is present, the collapse obviously has a different origin [6].
It is therefore necessary to differentiate, in the presence of reduced medial arch height, whether it is caused by a true structural collapse of the vault, or by increased volume of the plantar musculature as an expression of an adaptive mechanism for problems originating elsewhere [6,12,25].
3.2 The Adaptive Mechanism of the Compensated Flat Foot
Clinical assessment should determine whether the bony arch has genuinely collapsed or whether the reduced arch height is due to soft-tissue volume beneath an intact skeletal structure [6,12]. This distinction is critical, as it determines whether the presentation is a primary structural problem or a secondary adaptation.
To identify a possible adaptive mechanism, the femoral derotation test under load is indicated [6,12]. The patient stands in a spontaneous posture. The clinician assesses medial arch height and the presence of femoral rotations. The patient is then asked to actively derotate the femurs, or the clinician passively guides the movement, whilst the feet remain in contact with the ground [6]. If, once the femoral internal rotation is eliminated, the medial arch height increases and weight-bearing shifts to the lateral border of the foot, this finding is indicative of a cavus foot masked by lower limb rotation [6,7].
The pronator muscles of the ankle, being vectorially subdominant with respect to the supinators, cannot counterbalance them and allow physiological contact of the sole with the ground [6,7]. Furthermore, the principal pronator, peroneus longus, itself contributes to the excessive arch height [6]. To allow the foot more functional contact, the femoral internal rotators intervene to compensate for the insufficient pronators, and the calcaneus will display a non-primary valgus as a consequence of lower limb internal rotation [6,8,28].
The apparently flat foot is therefore caused by a cavus foot compensated proximally through femoral and tibial rotation, employing muscles that do not act directly on the foot [6,29]. This finding carries a direct clinical implication: correcting the apparent arch reduction with orthotic devices or surgical intervention without first identifying the primary cause risks disrupting the compensatory mechanism and producing deterioration in other body regions [6,12,27]. Any intervention on the foot must therefore be evaluated within the context of the entire lower limb kinematic chain, not at the skeletal region of application alone [6,27,29].
First and fifth metatarsals
4. The Anterior Plantar Arch
4.1 Mechanism of Collapse
At the anterior arch, the vectorial dominances are expressed differently from the medial arch [6,7]. The bases of the arch (first and fifth metatarsals), in addition to the ligaments, are supported solely by the oblique head of the adductor hallucis [6].
All other muscles act on the toes: the shortening of the flexors and extensors produces a vectorial dominance in dorsiflexion of the first phalanx and plantar flexion of the second and third [6,7,13]. Dorsiflexion of the first phalanx of the toes produces a mechanical thrust on the metatarsals, projecting them toward the ground. Under load, the portion of the overall force G discharged onto the anterior arch contributes to this thrust [6,15]. The force supporting the bases of the arch — the transverse portion of the adductor hallucis — is subdominant: the first and fifth metatarsals move apart and the anterior arch flattens [6,7].
5. Hallux Valgus
5.1 Mechanism of Muscular Dominances
The lateral stability of the great toe is under the control of the adductor and abductor hallucis, extensor hallucis longus and brevis, and flexor hallucis longus and brevis [6,7,30]. Taking the axis of the foot as reference — from the great toe to the second toe — the systemic shortening of these muscles produces a resultant and a vectorial dominance expressed as abduction of the first metatarsal and adduction of the distal phalanx of the great toe [6,7,26].
Collapse of the anterior plantar arch, by producing relative abduction of the first and fifth metatarsals with respect to the second toe, accentuates the angular deviation of the great toe. The two patterns frequently present in association [6,7].
5.2 Anterior Projection of the Centre of Gravity
In the assessment it is important to determine whether the associated pattern of hallux valgus and anterior arch collapse is primarily determined by selective shortening of the muscles acting in these regions, or is rather a consequence of the anterior projection of the overall force G applied to the body’s centre of gravity [2,6,15]. This force, unlike what occurs at the medial arch, is opposed solely by the adductor hallucis in its transverse portion, whose expressible potential is insufficient to counteract the force of the entire body weight [6,7]. Consequently, the metatarsals from the second to the fourth may, in extreme cases, come into contact with the ground, and the arch collapses [6].
Conclusions
Analysis of the foot reveals a biomechanical specificity that distinguishes it from all other districts analysed in the series [1,4,5]. At the ankle, the dominances in plantar flexion and supination impose adaptive strategies that propagate along the proximal kinematic chain, confirming the bidirectionality of mechanical influences in the lower limb [6,10,14].
The medial plantar arch, with all muscular vectors in summation of action, represents an engineering principle unique in the human body, consistent with the need to support the entire body weight [6,7,15]. The apparently flat foot as an adaptive mechanism of a cavus foot compensated through femoral rotation demonstrates the importance of a systemic assessment [6,8,29].
As with all the districts analysed, the correct identification of primary causes — whether proximal or distal — guides the intervention toward stable resolution of the alterations [2,4,24,27]. The coherent therapeutic sequence involves first the reduction of Resistant Force in shortened dominant muscles, followed by strengthening to consolidate the correction obtained [2,4,25,27]. The muscular shortenings described in the lower limb are potentially reversible through appropriate therapeutic techniques that act upon the mechanisms of tissue remodelling and proprioceptive recalibration [2,11,27].
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References
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