Excessive Weight Bearing Compromises Foot Structure and Function Across the Lifespan - The Mechanobiology of Obesity and Related Diseases - page 148

The second method by which the foot absorbs shock is via its mobility, which
allows the muscles and ligaments to help absorb the forces generated with each
step [ 20 ]. Muscles of the foot provide a secondary support role for the longitudinal
arch by maintaining the arch during dynamic tasks. The role of extrinsic and
intrinsic muscles in maintaining stability of the arch has not been clarified for
dynamic conditions. Although early researchers found that these muscles played
no major role in stabilising the foot during static tasks [ 35 , 36 ], recent studies
using intra-muscular electromyography have reported that activation of the
intrinsic foot muscles is correlated with postural task difficulty [ 37 ]. Ligaments do
not usually incur physiological fatigue and therefore offer greater resistance to
stress compared to muscles [ 15 ]. Although providing stability to the foot, if
ligaments are stretched beyond their elastic limit they are unable to reform to their
original state, thus reducing their ability to absorb forces. Repeated excessive
loading may cause ligaments to exceed their elastic limit, damaging soft tissues
and increasing the risk of foot discomfort and subsequent development of foot
pathologies such as flat feet. A structurally and functionally efficient foot is
therefore imperative for non-pathological stance and gait, with any deviation from
normal growth and development likely to cause problems when performing daily
tasks.
4 Effects of Excessive Body Mass on Foot Structure
and Function
Increased loading of the feet can be classified according to timeframe and
described as temporary, short-term, or long-term. A temporary loading effect
occurs, for example, when carrying a backpack or wearing a weighted belt that
temporarily increases body mass. The increased loading caused by this temporary
body mass increase can be terminated immediately by removing the external item.
Short-term loading increases result from more ''permanent'' body mass changes
such as the body mass increase associated with pregnancy. This increase in body
mass, although applied for up to 9 months, then ends with birth. In contrast, a
long-term loading effect occurs over an extended period, such as in obesity, where
the increase in mass is continuous. Although studies pertaining to temporary and
short-term loading effects on lower limb and foot mechanics are available [ 38 , 39 ],
this review will focus on the long-term loading effects of obesity on the muscu-
loskeletal system, particularly in reference to the feet. Although children's loco-
motor skills develop at different rates, most children are walking independently at
some stage between 9 and 18 months of age [ 2 ]. As this chapter is focusing on the
effects of bearing excessive body mass on foot structure and function, we will look
at the effects of overweight and obesity in children older than 2 years of age, and
then how these changes continue into adulthood.