A theoretical biomechanical hypothesis
Abstract
Broken-back hoof-pastern axis (BBHPA) is commonly associated with low palmar angle, caudal hoof collapse and altered dorsopalmar hoof proportions. These changes can alter the external moments acting around the distal interphalangeal joint (DIPJ), increasing mechanical demand on the deep digital flexor tendon (DDFT) and podotrochlear apparatus. However, some horses demonstrate BBHPA despite relatively healthy caudal hoof morphology, while others retain a degree of BBHPA despite appropriate trimming, caudal support, heel elevation or rocker-type shoeing.
The opposite condition, club foot, demonstrates that intrinsic DDF musculotendinous constraint can determine phalangeal alignment and subsequently hoof morphology. This raises the possibility that variation at the opposite end of the same mechanical spectrum may also exist. This paper proposes a theoretical Low Flexor Constraint Phenotype (LFCP) in which reduced passive or neuromuscular constraint within the DDF musculotendinous system allows the DIPJ to reach mechanical equilibrium at greater extension. Potential contributors include variation in musculotendinous slack length, tendon stiffness, accessory ligament mechanics, passive prestress and neuromuscular recruitment. Whole-limb posture represents an additional variable because postural adaptation can alter GRF orientation, joint moments and musculotendinous demand even when hoof-ground mechanics are otherwise appropriate.
LFCP is specifically proposed as a possible explanation for residual BBHPA in the absence of sufficient external hoof deformation to explain it. It is not proposed as an alternative explanation for BBHPA associated with caudal hoof collapse or prolapse, where restoration of healthy caudal hoof architecture remains mechanically indicated. Nor should unexplained BBHPA automatically be attributed to LFCP, because postural strategy may independently create or exaggerate the observed alignment. The hypothesis therefore predicts that similar radiographic alignment may represent different combinations of external hoof mechanics, intrinsic flexor constraint and posture, with potentially different DDFT and navicular loading between individuals.
1. Introduction
Hoof-pastern alignment is frequently interpreted predominantly through hoof geometry. Low heels, reduced palmar angle and increased dorsal lever arms alter the relationship between the ground reaction force (GRF), the DIPJ and the internal structures resisting joint extension. Consequently, deterioration of the caudal hoof can contribute directly to BBHPA and increased mechanical demand on the DDFT and podotrochlear apparatus.
The distal limb, however, is not a passive skeletal column positioned above the hoof. Joint position represents equilibrium between external moments generated principally by the GRF and internal moments generated through musculotendinous, ligamentous and articular structures. Hoof-pastern alignment should therefore be considered as the relationship between the three phalanges and the hoof-ground interface rather than as an isolated measurement of hoof-capsule geometry. The Point of Balance (PoB) model similarly describes sagittal hoof balance through the spatial relationship between the DIPJ centre of rotation, the GRF and the internal flexor system at midstance, providing a mechanical basis for considering phalangeal alignment as an expression of whole-system equilibrium rather than hoof shape alone. (Yxklinten & Sharp, PoB model)
Club foot provides evidence that this relationship can also operate from the inside outward. Persistent shortening or excessive constraint of the DDF musculotendinous unit can maintain the DIPJ in greater flexion, producing a broken-forward HPA and characteristic upright hoof morphology. Thus, internal soft-tissue mechanics can influence skeletal alignment and ultimately the hoof capsule that develops around that alignment. (O’Grady & Burns, 2026)
This establishes an important premise:
Increased flexor constraint -> increased DIPJ flexion -> altered phalangeal alignment -> altered hoof morphology.
The logical inverse is therefore worthy of investigation:
Could reduced flexor constraint allow the DIPJ to reach equilibrium at greater extension and contribute to persistent BBHPA in some horses?
2. Scope of the hypothesis: BBHPA without explanatory caudal hoof failure
A critical distinction is required from the outset.
LFCP is not proposed as an explanation for BBHPA associated with obvious structural failure of the caudal hoof.
A hoof displaying collapsed or underrun heels, caudal prolapse, loss of functional heel mass or other substantial dorsopalmar deformation already contains an external mechanical explanation for at least part of the altered phalangeal alignment. In such horses, the existence of hypothetical variation in flexor mechanics should not become justification for accepting unhealthy caudal hoof morphology.
Where BBHPA accompanies caudal hoof collapse, prolapse or loss of functional heel mass, restoration of appropriate caudal hoof architecture and hoof-ground mechanics remains the therapeutic objective. Established therapeutic farriery seeks to reduce inappropriate dorsal leverage, preserve or restore functional caudal support and establish appropriate palmar or plantar angle and spatial relationships around the distal digit. These principles are reflected in the therapeutic shoeing protocol described by Sharp and Tabor, in which correction of negative plantar angles and distorted caudal hoof morphology altered both phalangeal alignment and hindlimb posture, and in the subsequently described PoB approach to sagittal hoof balance. (Sharp & Tabor, 2022; Yxklinten & Sharp, PoB model)
The present hypothesis concerns a more specific clinical observation: BBHPA persisting despite a structurally competent caudal hoof, or residual BBHPA remaining after appropriate external hoof mechanics have been established.
LFCP should therefore be considered a hypothesis of residual or otherwise unexplained phalangeal alignment, not an alternative justification for pathological hoof morphology.
3. The biomechanical hypothesis
At its simplest, DIPJ equilibrium requires opposing external and internal moments to balance:
ΣM external + ΣM internal = 0
The external moment depends partly upon GRF magnitude and its moment arm around the DIPJ. The opposing flexor moment depends upon DDFT force and its moment arm:
M DDFT = F DDFT × r DDFT
Consequently, external flexor demand and internal flexor constraint are not the same variable.
A horse can experience substantial external flexor demand while possessing a DDF musculotendinous system that develops its required restoring force at a different length or joint position. This is why the term low flexor demand is potentially misleading. The proposed phenotype is better described as low flexor constraint.
Equine distal-limb models already incorporate tendon slack length, elastic properties and force-length behaviour when calculating musculotendinous forces. Brown et al., for example, modelled the distal forelimb using musculotendinous actuators incorporating tendon slack length and elastic modulus. (Brown et al., 2003)
Consider two horses exposed to comparable external DIPJ moments. In Horse A, the DDF system develops sufficient restoring force at joint position θA. In Horse B, because of a longer effective slack length, reduced stiffness or lower passive prestress, equivalent force may not develop until the DIPJ reaches greater extension:
θB > θA
The same external moment could therefore theoretically be satisfied at different joint positions.
This suggests the following provisional definition:
Low Flexor Constraint Phenotype describes a theoretical mechanical state in which the DDF musculotendinous system and associated passive or neuromuscular structures develop the restoring moment required for equilibrium at a more extended distal digital configuration than would be predicted from the external hoof mechanics alone.
Potential contributors include variation in DDF musculotendinous slack length, DDFT stiffness, accessory ligament properties, passive muscular tension and neuromuscular recruitment. These mechanisms are not mutually exclusive.
Posture represents an additional variable in this equilibrium. The external and internal moments acting around the DIPJ are not determined by hoof geometry and flexor properties alone, because the horse can alter the orientation and loading of the entire limb through postural adaptation. Changes in limb position alter the relationship between body mass, the GRF, joint centres and musculotendinous structures, thereby changing both external moment arms and internal tissue demand.
Consequently, the phalangeal alignment observed in a standing horse should not necessarily be interpreted as the passive equilibrium position of the distal limb. The horse may actively or subconsciously modify that equilibrium through neuromuscular control and whole-limb posture. A horse with relatively low passive flexor constraint might compensate through altered muscular recruitment or limb position, while another may permit the digit to remain at a more extended equilibrium. Conversely, a postural strategy adopted for reasons elsewhere in the kinetic chain could itself create or exaggerate BBHPA despite otherwise normal flexor properties.
The proposed model should therefore be considered as an interaction between external hoof mechanics, intrinsic flexor constraint and postural/neuromuscular strategy, rather than a simple relationship between hoof geometry and DDFT tension.
Human joint hypermobility provides a useful biomechanical analogy rather than a direct disease comparison. Rombaut et al. demonstrated lower passive muscle tension and reduced Achilles tendon stiffness in subjects with hypermobility-type Ehlers-Danlos syndrome. The relevance is the underlying mechanical principle: apparently similar musculoskeletal systems can possess different passive tissue properties and therefore reach equilibrium at different joint positions. (Rombaut et al., 2012)
4. Differential response to farriery may expose different equilibrium states
This hypothesis may help explain why horses with apparently similar BBHPA do not always respond identically to the same mechanical intervention.
Heel elevation changes hoof orientation and the moments acting around the DIPJ. Rocker or banana-type shoes can reduce the constraint imposed by a fixed ground-bearing orientation and allow greater sagittal rotation of the hoof relative to the ground. Within the PoB model, the resulting orientation can be considered an emergent equilibrium between external GRF and the internal force system rather than an angle imposed by the shoe itself. (Yxklinten & Sharp, PoB model)
If two horses with healthy caudal hoof structures and comparable BBHPA are exposed to similar changes in their external mechanical environment, one may approach straight phalangeal alignment while another reaches equilibrium while remaining moderately broken back.
Once caudal hoof integrity and appropriate external mechanics have been established, the second response need not automatically imply that progressively greater elevation or mechanical correction is required. It could indicate that the equilibrium position of the internal system differs between individuals. Equally, the horse may alter whole-limb posture in response to the intervention, changing the mechanical environment in which the distal digit is being assessed.
Individual responses to altered hoof angulation are already documented. Hagen et al. demonstrated variation in phalangeal response during progressive alteration of hoof angle, suggesting that the effect of a given mechanical intervention is influenced by the individual distal-limb system rather than hoof angle alone. (Hagen et al., 2018)
Their in-vivo observations are also important when considering load redistribution. The SDFT followed the response pattern of the DDFT rather than that of the suspensory ligament, contrary to the simple redistribution suggested by some earlier models. More recent in-vivo acoustoelastography likewise found decreasing stiffness-gradient indices in both DDFT and SDFT with progressive heel elevation. (Shaw & Brounts, 2025)
The biological response to changing hoof orientation is therefore more complex than simply transferring load from one distal structure to another.
5. Geometry is not necessarily force
The second major implication concerns pathology.
There is good evidence that hoof geometry influences DDFT and navicular loading. Eliashar, McGuigan and Wilson found navicular force to be negatively correlated with distal phalanx angle and heel-to-toe height ratio. Lower distal phalanx angles were therefore associated with greater calculated podotrochlear loading. (Eliashar, McGuigan & Wilson, 2004)
This remains highly relevant to horses in which BBHPA results from caudal hoof deterioration and provides another reason not to use the present hypothesis to normalise collapsed caudal hoof morphology.
However, geometry does not uniquely determine DDFT force.
Wilson et al. demonstrated substantially increased navicular force and stress during early stance in horses with navicular disease and attributed this principally to increased DDF muscle activity associated with unloading a painful heel. (Wilson et al., 2001)
McGuigan et al. subsequently demonstrated that palmar digital analgesia reduced navicular compressive force in affected horses, supporting the principle that neuromuscular behaviour can alter DDFT-mediated navicular loading independently of static hoof geometry. (McGuigan et al., 2001)
This distinction is central to the present hypothesis:
Similar phalangeal geometry does not necessarily imply identical internal tendon force.
A horse displaying BBHPA because caudal hoof collapse has altered the external DIPJ moment may therefore represent a different mechanical state from a horse with a structurally healthy hoof whose distal digit reaches equilibrium at greater extension because of lower flexor constraint. A third horse may display similar phalangeal geometry primarily as a consequence of its postural strategy. The same radiographic appearance could therefore potentially arise through different combinations of external hoof mechanics, intrinsic flexor constraint and whole-limb posture.
The low-flexor-constraint horse might theoretically experience lower DDFT and navicular loading than its radiographic alignment alone predicts. This remains hypothetical and does not imply that such a horse is mechanically unloaded or protected from injury. The external moments must still be resolved. Reduced passive flexor constraint could instead be associated with altered joint excursion, different muscular recruitment, altered contribution from other passive structures or compensatory changes elsewhere in the limb.
LFCP therefore proposes different distribution of mechanical demand, not absence of demand.
6. Testing the hypothesis
The hypothesis is experimentally testable.
The first requirement is appropriate phenotyping. Horses with BBHPA and obvious caudal hoof collapse should be distinguished from horses displaying BBHPA despite structurally appropriate caudal hoof architecture. Failure to make this distinction risks combining fundamentally different mechanical populations.
Horses with comparable residual BBHPA could then undergo a standardised mechanical perturbation, such as progressive controlled heel elevation or a rocker interface, while simultaneously measuring:
- P1-P2-P3 alignment
- DIPJ and PIPJ angles
- palmar or plantar angle
- centre-of-pressure behaviour
- DDFT and SDFT mechanical response
- whole-limb posture and proximal limb orientation
- and, ideally, DDF muscle activation
Because posture can itself alter distal-limb mechanics, limb configuration should be quantified concurrently rather than treated as experimental noise. Repeated measurements of proximal limb orientation, fetlock position and whole-limb placement relative to the body would help distinguish persistent phalangeal behaviour from transient postural variation.
Acoustoelastography has already been used to quantify in-vivo changes in digital-flexor mechanical behaviour during progressive heel elevation and may provide one method for investigating this question. (Shaw & Brounts, 2025)
The principal prediction is straightforward:
Horses with comparable external hoof morphology and BBHPA will demonstrate different phalangeal responses to identical external mechanical perturbation. These differences should be investigated in relation to both the mechanical behaviour of the DDF musculotendinous system and concurrent changes in whole-limb posture.
Longitudinal assessment provides a second test. If some horses repeatedly remodel the hoof towards the same phalangeal orientation despite restoration and maintenance of healthy external hoof parameters, this would support the possibility that internal mechanics and/or persistent postural strategy contribute to the morphology rather than morphology acting exclusively as the primary driver.
Conclusion
Club foot demonstrates that increased DDF musculotendinous constraint can alter phalangeal alignment and subsequently influence hoof morphology. It is therefore biomechanically reasonable to investigate whether variation at the opposite end of this mechanical spectrum also exists.
The proposed Low Flexor Constraint Phenotype describes a potential subset of horses in which the distal digit reaches equilibrium at greater extension because of individual variation in DDF musculotendinous or neuromuscular mechanics.
Crucially, LFCP applies to BBHPA that cannot be adequately explained by external hoof deformation. It does not redefine collapsed heels, caudal prolapse or loss of functional caudal hoof architecture as physiological variation, nor does it remove the requirement to establish healthy caudal hoof mechanics. Equally, persistent BBHPA after those variables have been addressed should not automatically be attributed to LFCP, because posture provides another means by which the horse can alter the observed mechanical state.
Instead, it raises a more specific question:
Once a structurally healthy caudal hoof and appropriate external mechanics have been established, why do some horses remain broken back while others readily restore phalangeal alignment?
One possibility is that BBHPA does not represent a single mechanical state.
External hoof geometry influences the mechanical demand imposed upon the distal limb, while individual tendon properties, passive prestress, neuromuscular behaviour and postural strategy may influence how that demand is satisfied and the joint position at which apparent equilibrium occurs.
If this hypothesis is correct, two horses with similar radiographic alignment may not possess identical internal forces, identical podotrochlear loading or identical responses to therapeutic farriery.
That proposition is experimentally testable and may provide a route towards understanding BBHPA not simply as an abnormal angle to be corrected, but as an observable output whose underlying mechanical cause must first be identified.
Credit to Ramon Batalla for the thought experiment, looking forward to writing this up as a paper together.
