The Equine Foot, Biomechanical and Anatomical Form Function. Hoof
Balance and Symmetry, a Review of Current Theory.
M. N. Caldwell FWCF¹* & D. Duckett FWCF²
¹The School of Veterinary Nursing & Farriery Science, Myerscough College,
Myerscough Hall, Bilsborrow, Preston, Lancashire, PR3 0RY
². 709 Tennis Avenue, Ambler. PA. 19002. U.S.A.
*¹ Tel: 01995 642000 ext; 2057 Mob: 07792374551 emails;
markncaldwell@btinternet.com or mcaldwell@myerscouch.ac.uk
Word count does not include figures and references.
3758
Key words: Farriery, anatomy, biomechanical, conformation, hoof trimming, foot
balance, hoof capsule, pathology.
Summary
Farriery attempts to maintain equilibrium within the foot by trimming to achieve an illdefined and subjective empirical interpretation of “Static Foot Balance” (Hickman &
Humphrey 1987; Stashak 2002). This interpretation of foot balance is mostly derived
from historical texts (Dollar & Wheatley 1898; Russell 1897; & Lungwitz 1891;
1897).
In nature shape and form of a structure relate directly to function. The hoof capsule
is such an example of this concept. The primary functions of the hoof capsule and
its associated structures are to absorb impact shock during locomotion, assist in the
transfer of weight from the skeletal column and protect the underlying structures
whilst providing grip during locomotion (Butler 2005).
This paper reviews the anatomical and biomechanical considerations of the current
foot balance model. It explores the relationship between the anatomical structures
and the mechanical function within the foot and reviews the farriery considerations
of our understanding of both static and dynamic foot balance models.
It appears that the physics of load distribution casts doubt on the validity of
theoretical foot balance model given the range of biomechanical variations within
the population. Further investigation needs to be undertaken into the common
morphometrics that might constitute a range of normal.
1
Introduction
A fundamental principle of farriery is to achieve equilibrium of static and dynamic
forces acting on internal structures within the foot and lower limb. External
influences should be neutralised by internal forces with the form of the foot
maintained so as not to inhibit its natural function. Farriery attempts to maintain
equilibrium within the foot by trimming to achieve an ill-defined and subjective
empirical interpretation of “Static Foot Balance” (Hickman & Humphrey 1987;
Stashak 2002). This interpretation of foot balance is mostly derived from historical
texts (Dollar & Wheatley 1898; Russell 1897; & Lungwitz 1891). Today’s modern
domesticated horse is far from the forces of nature that historically shaped and
controlled their development. Horses are more reliant than ever on the knowledge
and skill of the hoof care professional with an understanding of the anatomy,
physiology and function of the lower limb, foot and all its component parts. The
composition, position and functional relationship of the component parts of the foot
are so densely compacted that within the hoof capsule there is little room for
manoeuvre.
In nature shape and form of a structure relate directly to function. The hoof capsule
is such an example of this concept. A horse’s natural protection against predators is
“fright and flight” the hind quarters of the horse are a compact mass of large loco
motor muscles which are designed primarily to propel the horse quickly forward. The
front limbs are designed to primarily support the horse through impact, deceleration
and loading during locomotion (Back & Clayton 2001). Movement is effected via
neurological impulses causing the limbs to retract or protract. To minimise the
energy required for this movement the limbs are as light weight as practical and are
manoeuvred via a series of levers and pulleys. The pulleys are the joints, which are
2
held together with a complex array of ligaments. The levers are the tendons which,
together with the ligaments, act like a coil spring releasing stored energy when
movement is necessitated. In simple terms every time the limb is loaded the
elasticated collagen fibres that make up the tendons and ligaments store energy
which is released when the limb needs to be lifted and propelled forward.
Anatomical Considerations
The gross anatomy of the foot and limb is well documented. In farriery the foot of the
horse is referred to as the keratinized hoof capsule and its contents. The hoof
capsule is a continuation of modified epidermal tissue which forms a firm yet flexible
protective layer surrounding the skeletal components at the distal extremity of the
limb (Kempson. 1987). The primary functions of the hoof capsule and its associated
structures are to absorb impact shock during locomotion, assist in the transfer of
weight from the skeletal column and protect the underlying structures whilst
providing grip during locomotion (Butler 2005).
Each epidermal structure has a different density and hardness dependent upon its
primary function and is produced by its own corresponding dermal layer and gains
its strength and flexibility from its chemical composition and moisture content. The
outer layer of hoof wall is rich in disulphides giving additional strength. The sole and
frog are rich in sulfhydryl a group which gives these structures greater elasticity
(Pollitt 1988) which gives flexibility. The average growth rate of 6-8mm per month
(Stashak 2002) is said to be equal to the wear of the foot at the bearing border
ground interface (Back and Clayton. 2001).
The horn itself is composed of densely packed, longitudinally aligned, horn tubules
which are generated from tiny projections of the dermis known as papillae (Fig 3).
3
These tubules are cemented together by intertubular horn cells which proliferate
from germinative cells of the dermis between the papillae. The intertubular horn is
formed at right angles to the tubular horn and gives the hoof wall a mechanically
stable, multidirectional, fibre-reinforced composite (Bertram and Gosline, 1987).
Interestingly hoof wall is stiffer and stronger at right angles to the direction of the
tubules. This contradicts the usual assumption that the ground reaction force is
transmitted proximally up the hoof wall parallel to the tubules. The hoof wall appears
to be reinforced by the tubules but it is the intertubular material that accounts for
most of its mechanical strength stiffness and fracture toughness. The tubules are
three times more likely to fracture than intertubular horn (Leach, 1980; Bertram and
Gosline 1986).
Contrary to common misconception these tubules are not aligned randomly but
appear to be arranged in 4 distinct zones and differing in “tubule density” from the
inner most layers to the outer layer (Reilly et al. 1996). The highest tubular density is
at the outermost layer (fig 4). Since the ground reaction force is transmitted
proximally up the wall (Thomason et al 1992) the construction of the epidermal
structures appears to be part of the mechanism that allows the transfer of impact
load across the hoof wall from the rigid, high tubule density, outer wall to the more
plastic, low tubule density, inner wall. The material properties of hoof are said to
have plastic elastic characteristics (Reilly et al; 1996). These characteristics allow
for deformation under stress and then return to original form following a limited
period of strain. Maximum stress for prolonged periods of strain reduces the
elastomeric property and the structures ability to return to its original form.
Projecting from the inner wall are some 600-900 primary laminae each with 100-200
secondary epidermal laminae (Pollitt. 1988). The primary epidermal laminae are
4
produced by the dermis of the coronary corium. The secondary epidermal laminae
are produced by a germinative layer of the epidermis of the laminar corium. The
primary and secondary epidermal laminae interdigitate, dovetail, with the
corresponding dermal lamellae. The dermal lamellae originate from the laminar
corium surrounding the parietal surface of P3 and the lower border of the ungual
cartilages. This relationship of interdigitation of laminae is thought to allow for the
partial suspension of P3 within the hoof capsule and the transference of tensile
forces radially from P3 (Reilly 2006) (Fig 5).
As hoof wall continues to grow distally the primary epidermal laminae are allowed to
slide past the secondary epidermal laminae. The process involves the remodelling
of the epidermis around the proliferation of new cells. The basement membrane
which surrounds the secondary dermal laminae is thought to release itself via tissue
inhibiting desmosomes and demisomes (Pollitt.1998).
At the solar border the sole and wall are separated by the white line. The white line is
produced by epidermal primary lamellae and the terminal papillae at the distal fringe
of P3. The white line is a flexible junction between both horny structures allowing for
movement between the two structures during load bearing.
The soles concavity combined with the shape of the frog are implicated not only as
an anti-slip device (Butler 2005; Stashak 2002) but are said to play a major role in
the anti-concussive mechanism. It is generally accepted that the horses mass is
distributed through the limbs with 60% being supported by the fore limbs and 40%
through the hind limbs (Butler, 2005:, Back & Clayton, 2001; Williams & Deacon
1999). Both hind and front limbs play a role in support and propulsion however the
primary role of each differs. Front feet are generally larger and rounder with less concavity
5
to the soles to provide a greater surface to dissipate impact shock. Hind feet have a less
acute dorsal hoof wall angle and a more concave sole. This configuration is best suited for
grip and rapid propulsion, pushing as they dig into the ground.
The frog is a wedged shaped mass of elasticated horn. Located at the basal surface
of the solar margin it is occupies the space between the sole and bars uniting either
side its longitudinal axis to form the collateral sulci. It extends palmadorsal from the
heel bulbs approximately 2/3rd the length of the bearing surface (Ovnicek 1995). The
so called true point of frog, where solar horn and frog horn merge, lies distal to the
internal insertion of the deep digital flexor tendon (DDFT). Given its close
relationship to the bars, and the composition and topography of the internal frog
stay, it is more likely to act as an expansion mechanism for the heels during impact
(Emery et al. 1977) whilst serving to protect the DDFT, digital cushion, distal
sesamoidean bone and its associated bursae.
Occupying the space between the large flexible ungual cartilages, which are attached
to the palmar processes of P3 and distal to the DDFT, lies the digital cushion and
venous plexus (Fig 2). The digital cushion is a large mass of highly elastic fibro fatty
cartilage forming the heel bulbs. It is said that venous blood is flow is assisted by
compression of the cartilages through the interactive mechanism of the frog and
digital cushion during the impact and loading phases of the stride (Hickman &
Humphreys 1987, Pollitt, 1988 and Butler, 2005). Bowker (1988) referred to this
hydraulic compression of the vascular bundles as hemodynamic flow. Ratzlaff et al;
(1985) and Bowker (1988) suggested that this action assists in the absorption of
impact shock. Bowker argued that there is a complex inter-relationship between the
ungual cartilages and the digital cushion. The medial projection on each ungual
6
cartilage is thrust upward by the bars on contact causing being transferred through
the venous plexus dissipates high frequency energy waves reducing impact shock on
the bone and ligaments of the foot.
Bowker noted that horses with good foot
conformation had more blood vessels in caudal aspect their feet than those with a
history of foot problems. Bowker (1988) also noted that the digital cushion was made
of fat and an elasticated fibro cartilaginous material. Fibro cartilage is made up, in
part, by a protein called collagen. Collagen makes up the fibres found within tendons
and ligaments and whose mechanical properties allow for the storage of energy.
Physiological Considerations
The relationship between the anatomical and mechanical function of the structures
within the foot is integral to our understanding of static and dynamic foot balance.
The biomechanical function of the individual anatomical structures within the foot
are dependent on the ability of all of them to work in harmony for the good of the
whole. This relationship is called a “mechanism”. The mechanisms within the foot
enable the horse to travel at great speed over long distances and across varied
terrain. As the foot makes contact with the ground up to 90% of the external shock
is dissipated before it reaches P1 (Douglas et al 1998). A pluralistic approach to
understanding the anti-concussive mechanism of the hoof recognises more than
one ultimate principle.
The foot’s internal and external structures combined exhibit three functional
characteristics that allow for the dissipation of forces during operation of the foot
mechanism described. (Barrey, 1990)
7
1.
The absorption of impact and rapid deceleration forces through
friction, resistance, and the elastic properties of structures that deform under
load.
2.
The dissipation of ground reaction force via pressure resistance to
force and stress resistance through the material properties of the wall
3.
Plastic structural characteristics that allow for the return to original
form following deformation.
During the stance phase the horse’s weight is transferred through the limb to the
distal interphalangeal articulation. Because of the distal phalanx’s (P3) unique
attachment to the hoof capsule via the laminar interdigitation, this weight is
transferred to the wall radially as tensile forces (Reilly 2006). The dermal epidermal
laminar interface acts partially as a suspensory apparatus for P3. The tensile forces
are transferred distally along the hoof wall (Fig 5) and converted into pressure at the
ground bearing border by ground reaction force vectors (GRFV). Both the sole and
frog have a supporting role following initial contact and impact phases of the stride.
At the first point of impact, this is usually heel first or flat the hoof is rapidly
decelerated by the vertical landing forces. Heel first landing is accentuated at faster
gaits (Back, et al, 1995). Horizontal movement is rapidly reduced to zero within
milliseconds of impact. The shape and geometry of the foot changes as it comes
under load. Following initial impact Bowker (1988) suggested a hemo-dynamic flow
theory. He argued that there is a complex inter-relationship between the ungual
cartilages and the digital cushion. At the point of impact the collateral cartilages are
thrust upward by the heel buttresses and bars, causing negative pressure in the
digital cushion draining blood from the front to the back of the foot, creating
8
hydrostatic cushioning. Simultaneously tendons and ligaments under load store
excess energy from the vertical loading and fetlock extension. During the loading
phases the proximal dorsal wall flattens and moves palmar distal, the heels expand
(Fig 6) whilst the sole and frog flatten under load following the outward movement of
the quarters (Douglas. et al. 1998, Lungwitz 1891). The relatively flexible laminar
attachment at the heels compared with the toe allows greater expansion caudally.
The frog descends with the sole until it contacts the ground. Known as the pressure
theory (Hickman & Humphrey 1987; Butler 2005) this movement simultaneously
compresses the digital cushion. Colles (1989) noted that in some horses with
reduced frog pressure, such as those recently shod, the heel actually contracted
under load. The Depression theory suggests that under the influence of weight
during impact and loading that P3 is rotated palmardistally. This is more likely to be
linked to hyperextension of the distal interphalangeal joint (DIPJ) caused by GRF
vectors following heel first landing (Rooney 1969). This rapid deceleration of the foot
by vertical landing forces and horizontal braking forces immediately post impact
have been associated with the pathology of disease (Raddin, et al; 1972; Viitanen,
et al; 2003). As the fetlock reaches the maximum extension required for the force
applied the stored energy is used to accelerate retraction of the fetlock through to
mid stance. Foot geometry and bearing borders are returned to their original
position (Back & Clayton 2001).
Theoretical Considerations in Static Foot Balance
The English dictionary defines Balance: as ‘the harmony of design and proportion”
or “stability produced by even distribution of weight on each side of the vertical axis”.
Both definitions could easily be applied to the equine foot. Trimming and shoeing
9
are said to have marked effects on the performance and soundness of the equine
athlete. Ideally, trimming optimizes the interaction between the hoof and ground
during locomotion (Balch, et al 1998).
Since the hoof is a three-dimensional structure, it should be balanced in both the (X)
mediolateral and (Y) dorsopalmar planes whilst maintaining proportions through the
proximodistal axis (Z) with its centre of mass (COM) remaining in equilibrium (Fig 8).
For the purposes of biomechanical study the COM for the foot has been calculated
from previous studies of cadavers (Springs & Leach; 1986). Body segments were
weighed and calculated as a proportion of the overall body weight and then
suspended at a point of balance. These points were then referenced to anatomical
landmarks identifiable on the live horse for analysis purposes (Back & Clayton,
2001).
Static foot balance refers to the alignment and spatial orientation of the foot and limb
in the static mid stance position as a three dimensional object. All three dimensional
objects have three axes and six co-ordinates. The dimensions of the foot are
measured along these axes to the point where the X and Y axis intersect each other
at along the longitudinal Z axis. This is an important concept in retaining static
balance as it assumes equal weight / force at both coordinates of the axis.
Dorsopalmar (lateral projection)
The phalangeal axis of the adult horse is a natural conformation that can only be
altered either by injury or surgery. However, the hoof axis in relation to the pastern
axis can and often is, altered either by neglect on the part of the owner, or worse
10
still, by inadequate farriery practice. Textbooks tend to be misleading, all too often
quoting the ideal front foot angle as being between 45º and 50º (Hickman &
Humphrey 1987). (Turner 1988; 1992; Stashak 2002; and Butler 2005). stated that
the heels should be parallel to the toe angle. More importantly both should present
parallel to the internal dynamic structures the mid-line of the phalanges. A
perpendicular line dropped from the centre of rotation of the DIPJ should bisect the
ground bearing border equally (Colles 1983; 1989; Balch et al, 1997; O’ Grady &
Poupard 2003).
Dorsopalmar (Solar View)
When viewed at its solar margin the following should be noted: The hoof wall is
thickest and is most dense around the toe region, thinning as it wraps around the
quarters arriving at the buttress to become the last weight bearing point of the foot.
This is roughly adjacent to the widest point of the frog, before inverting on itself to
form the bars which then terminate at the approximate centre of rotation of the coffin
joint (Colles 1983; 1989; Balch et al, 1991; 1997; O’ Grady & Poupard 2003). The
centre of weight distribution through the hoof capsule is said to be approximately
1cm palmar of the trimmed point of frog (Ovnicek 2003). A perpendicular line drawn
from the buttress to the toe will intersect the medial and lateral optimum points of
break-over (Duckett 1990; Caldwell 2001).
Mediolateral
The height of wall from coronary band to ground bearing border should be the same
at any 2 opposite points (Russell 1897; Stashak 2002 and Butler 2005) with both the
bearing border and coronary borders perpendicular to the longitudinal axis of the
limb.
11
Discussion
Farriery is an empirical craft with much of its knowledge and tradition handed down
from one generation to the next. Much of what is quoted in standard farriery text
regarding the hoof represent the mean approximation and should at best be
regarded as “guidelines” to work within.
It has long been accepted practice in farriery to relate the morphology of the hoof
and its bearing border (BB) shape to an ideal form which is symmetrical around its
longitudinal axis (Stashak, 2002; Butler, 2005). The reality of biological systems is
that small-to-moderate asymmetries are present in the majority of structures. The
BB shape is no exception. It has been well documented that external influences
(Thomason 1998) can have an effect on the hoof’s shape. Leach and Zoerb (1983)
noted that the percentage of total body weight acting through the forelimbs of adult
horses is approximately 60% and as a result front feet are reportedly larger and
rounder with less concavity to the soles to provide a greater surface to dissipate
weight.
According to Wolfe’s Law (1986) biologic systems such as hard and soft tissues
become distorted in direct correlation to the amount of stress imposed upon them.
Horn is said to posses both “elastic” and “plastic” deformation characteristics. The
compressive forces (C) and the tensile components (T) in reaction with ground
reaction force vectors (GRF) stress the horn over time until it reaches its yield
strength (Hall 1953; 2003). At this point plastic deformation begins, for example
collapsed heels. Once the process has begun, plastic deformation is not generally
reversible. Hoof wall is thickest at the toe, the region of greatest friction, and thins
gradually towards the heels with the medial quarter thinner than lateral (Pollitt,
12
1998). The wall is oblique to the direction of the GRF. The centre of pressure (COP)
location and point of force (POF) trajectory (Wilson et al; 1988) are said to be
influenced by external factors, such as conformation. By extending the moment arm
the stress time line will influence the magnitude of force on the axial coordinates is
likely to be a significant factor in both structural orientation and integrity and lead to
permanent plastic deformation.
The resultant increase in torque application from an increase in moment induces
stress by moving the pressure towards the limbs longitudinal axis (Hall 1953). The
resulting stress from pressure being applied externally to the cross section of the
limb would seem to cause distortion of those soft tissue structures within the foot
compromising both their integrity and biomechanical function. The physics of weight
distribution and support suggest that each hoof capsule would take not only a
different form around the bearing border but throughout its structure dependant on
the forces applied to it. It would further indicate that pressure applied outside the
hoof’s ability to absorb might restrict the dermal structures responsible for
regeneration.
13
Conclusion
It appears that the physics of load distribution cast doubt on the validity of
theoretical foot balance model given the range of biomechanical variations within
the population. Further investigation needs to be undertaken into the common
morphometrics that might constitute a range of normal. The affects of farriery
protocols on the structure, function, loading and movement of the equine foot are
not fully understood. If we are to fully understand farriery’s role in lameness
resolution clearly a more appropriate definition of the term foot balance needs to be
recorded.
14
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differences between the distal portions of the forelimb and hindlimbs of horses at
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Balch, O.K., Ratzlaff, M., Hyde, M., Grant, B. and White, K. (1988) Correlation of
equine forelimb hoof impact patterns to changes in the medio-lateral balance of the
hoof. Anat. Histol. Embryol. 17, 361-362.
Balch, O., White, K. and Butler, D. (1991) Factors involved in the balancing of
Equine Hooves. J. AM. Vet. Med. Ass 198, 1980-1989
Balch, O.K., Butler, D. and Collier, A. (1997) Balancing the normal foot: hoof
preparation, shoe fit and shoe modification in the performance horse. Equine Vet.
Educ. 9, 143-154.
Barrey, E. (1990) Investigation of the vertical force distribution in the equine forelimb
with an instrumented horseboot. Equine vet. J., Suppl. 9, 35-38.
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130,121-136.
Butler K.D. (2005) The Principles of Horseshoeing 3. Butler Publishing. Maryville,
Missouri
Bowker, R. M., Van Wulfen, K. K., Springer, S. E., & Linder, K. E. (1998),
Functional anatomy of the cartilage of the distal phalanx and digital cushion in the
equine foot and a hemodynamic flow hypothesis of energy dissipation,
Am.J.Vet.Res., vol. 59, no. 8, pp. 961-968.
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UK farriers Convention, Equine Vet. Jour. Publishing, 28-33
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4, pp. 297-303.
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Emery, L. (1977) Horseshoeing Theory and Hoof Care, Lea & Febiger, Philadelphia,
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17
Illustrations
Figure 1 to Figure 8
PROXIMAL
PHALANX
P1
MIDDLE
PHALANX
SDFT
DDFT
FIG. 2
P2
CDET
CSL
DIGITAL
CUSHION
PHALANX
DS
CORIUM
OF FROG
FRONTAL
PLANE at
section1
(Denoix.
2000),
FROG
CORONARY
CORIUM DISTAL
DERMAL
LAMELLA
P3
EPIDERMAL
LAMELLA
SOLAR
CORIUM
WALL
SOLE
WHITE LINE
FIG.1. Sagittal section of the foot illustrating some of the important anatomical
structures contained within the hoof capsule. The red dotted line is the frontal plane at
section 1 (Denoix. 2000).
18
Coronary Coria
P2
Collateral
cartilages
Coronary venous
plexus
Digital Cushion
Frog stay
Bars
Frog
PLANTER PALMER DISTAL OBLIQUE VIEW
FIG.2 Transverse section through heel bulbs illustrates some of the important
anatomical structures contained within the hoof capsule. In this plane the collateral
cartilages, coronary plexus, coronary corium, digital cushion, frog stay and solar
corium are all visible within the hoof capsule whilst externally the wall, frog, bars and
collateral sulci are visible.
19
.
Fig 3 Each dermal papillae fits a corresponding hole within the coronary groove and
produces individual horn tubules.
20
P3
LC
DL
EL
ZONE 4
ZONE 3
ZONE 2
ZONE 1
Fig 4 A transverse section through the hoof and internal structures. The section is
taken from the region of mid toe. Illustrated are P3, the laminar corium (LC)
attached to the parietal surface of P3 the primary dermal (DL) and epidermal (EL)
lamellar interdigitation, (secondary lamellar are not visible at this magnification) the
stratum internum and medium demonstrating the 4 zones of density (1, 2, 3 & 4).
21
TENSILE FORCES
W
LI
P
BEARING BORDER
Fig 5 Weight of P3 (W) is transferred to the wall as tensile forces via the dermal
epidermal laminar interface (LI). The LI acts as a suspensory apparatus for the distal
phalanx (P3). The tensile forces are transferred distally along the hoof wall (Green
Arrow) and converted into pressure (P) at the ground bearing border or the bearing
border shoe interface
(Budras et al. 1998)
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Coronary Plexus
Lateral
Digital
Vein
Laminar Plexus
Coronary
Plexus
Solar Plexus
Fig 7 Lateral Venogram of a normal foot illustrates the complexity and magnitude of
the venous return system of the equine foot. Clearly visible are the coronary, solar and
laminar venous plexus. (www.thehorse.com)
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z
YAW
y
x
x
y
PITCH
ROLL
z
Fig 8 Diagrammatic representation of 3 dimensional static foot balance.
X axis represents mediolateral coordinates viewed dorsopalmar / plantar and
Y axis dorsopalmar viewed mediolateral whilst Z represents the proximodistal
(longitudinal) axis of the foot. Movements around the dorsopalmar axis Y are
referred to as “Roll” (indicated by the blue arrow). Movements around the
mediolateral axis X are referred to as “Pitch” (indicated by the red arrows).
Movements around the longitudinal axis Z are referred to as “Yaw” (indicated
by the green arrow).
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