Excessive Weight Bearing Compromises Foot Structure and Function Across the Lifespan - The Mechanobiology of Obesity and Related Diseases - page 146

indicated by the presence of a lowered longitudinal arch height during static
weight bearing, but a normal arch structure during dynamic tasks such as standing
on the toes [ 14 ]. As this foot structure is only pathological when static, it is thought
that ligament laxity is the mechanism for relaxed flat foot. Congenital flat foot is a
rare condition whereby the foot is in a position of calcaneovalgus, folding laterally
on itself [ 8 ]. Rigid flat foot is where a reversal of the concavity of the normal
longitudinal arch is displayed [ 15 ] and is evident during both static weight bearing
and dynamic gait. These latter two conditions are difficult to treat as they involve
structural deformity to the longitudinal arch, whereas relaxed flat foot is only
functional and can therefore be modified [ 8 ].
Alignment of the foot and ankle differs depending upon whether one is non-
weight bearing or statically bearing weight [ 16 ]. As body mass increases, greater
stress is placed on the ligamentous and muscular structures of the foot during
weight bearing actions [ 9 ]. Excessive increases in these weight bearing forces may
cause microtrauma to the ligaments and muscular structures, damaging soft tissue
and increasing the risk of joint collapse and flat feet, particularly in obese indi-
viduals [ 1 , 9 , 17 ]. For this reason, it is important to understand the consequences of
bearing excessive body mass on foot structure and function, particularly during the
early years of life when a child's foot is developing. Early identification of any
structural abnormalities is required to minimise the risk of future functional
complications across a lifespan.
3 Normal Foot Structure and Function
Structurally, the foot is comprised of bones, ligaments and muscles that act in
synergy to provide two main functions: stability and mobility. Stability is achieved
through a fixed arch structure, which can convert the foot to a rigid lever and
distribute loading throughout the foot during weight bearing. Conversely, a foot
functioning as a non-rigid structure achieves mobility. The non-rigid structure
allows the shock of weight bearing to be reduced, the foot to adapt to various
terrain and the damping of superimposed rotations [ 1 ]. The ability of the foot to
achieve both stability and mobility is dependent upon its unique structure. The foot
has a plantar plate composed of a multi-segmental ligamentous and fascial
mechanism (windlass mechanism), which works both transversely and longitudi-
nally [ 18 ]. The plantar aponeurosis, long plantar ligament, short plantar ligament
and spring ligaments are part of this plantar plate and act as powerful energy
storing mechanisms [ 19 , 20 ]. The fibres in these plantar ligaments are intertwined
in a crosshatch arrangement that is thought to increase the ability of the ligaments
to withstand loading in different directions.
In mechanical terms the foot is a truss with a tie-bar mechanism (Fig. 2 ). When
a load is applied to the truss/tie-bar system from directly above, the two trusses
(formed by the bony architecture of the foot) experience compression forces,
whereas the tie-bar (formed by the plantar aponeurosis) experiences tensile forces