Long-term effects of footwear on gait may be most critical

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BioMechanics March 2004

Function Analysis

Long-term effects of footwear on gait may be most critical

Biomechanical aspects of shoe alterations should be differentiated and

studied as a function of time.

By: Anne Mundermann, PhD

In addition to protecting feet from potentially harmful environmental

factors, footwear also acts as the interface between the body and the

ground during gait, and can thus be modified to alter mechanical loads

on the lower extremity during the stance phase of gait.

Too often, footwear initially perceived as comfortable (whether in a

store or a clinician's office) may later not be comfortable at all. In

clinical practice, patients are usually instructed to increase foot

orthosis wear-time per day slowly over a two- to three-week period to

break in their orthoses gradually.1,2 This clinical rule is based on the

assumption that the body adapts slowly to new or different footwear. Yet

most scientific studies quantify the effects of footwear in a laboratory

setting where different conditions are tested for only a few minutes,

and thus may not describe adaptation to footwear appropriately.

Footwear used for physical activities is typically composed of an

outsole, midsole, sock liner, and upper. The upper and sock liner are

primarily responsible for the fit between the foot and the shoe, while

the outsole is responsible for traction between the shoe and the ground.

The geometrical and material configurations of the midsole determine the

degree to which it cushions the impact when the foot meets the ground,

supports the subtalar joint during the initial contact of the foot with

the ground, and guides the movement of the lower extremity during the

remainder of stance phase.3 Several footwear modifications are designed

to optimize the efficacy of footwear in accomplishing these functions.

Footwear modifications

Footwear changes can be classified into three categories: shoe

modifications, shoe inserts, and foot orthoses. Shoe modifications

comprise variations in the mechanical properties of sole materials

including resilience, viscosity, and bending stiffness and flex point

location (in medial, lateral, rearfoot, or forefoot regions). Shoe

modifications also include geometric modifications such as heel lifts

(thicker material in the rearfoot relative to forefoot regions), heel

flares (extension of the outer sole beyond the boundaries of the shoe

upper), and posts (greater thickness on one side). The main disadvantage

of shoe modifications is that in many cases a shoe's preexisting

specifications preclude adjustments or customization.

Shoe inserts are placed within the shoe and commonly replace sock

liners. Like shoe modifications, shoe inserts may be composed of a

variety of materials and may have structural features such as heel

lifts, posting, or arch supports. Foot orthoses are also positioned

within the shoe; however, they are typically molded specifically to a

patient's feet to provide optimal fit between foot and orthosis. The

advantage of shoe inserts and foot orthoses is that the same device can

be used with different pairs of shoes; a disadvantage is that these

devices take up space within the shoe.

Lower extremity load during gait

Footwear is often modified with the goal of altering mechanical loads

placed on the lower extremity during the stance phase of gait. Many

variables contribute to the movement, muscle activity, and forces acting

upon the lower extremity during gait; the primary variable of interest

relative to lower extremity load may change depending on the type of

physical activity and mode of gait. This article concentrates on only a

few variables that appear to be functionally important in terms of the

development of lower extremity injury or disease.

The variables that have attracted most scientific scrutiny for both

walking and running are foot eversion and ground reaction force. Foot

eversion describes the amount of pronation that occurs at the subtalar

joint (Figure 1).4 Although only limited scientific evidence5 is

available, it is assumed that excessive pronation in turn causes

excessive internal tibial rotation. When the forces rotating the tibia

internally (via the coupling mechanism at the subtalar joint) are

combined with forces rotating the femur externally (via the gluteus

maximus and adductors), the oppositional forces may lead to excessive

stresses at the knee. Although ground reaction force provides an

indirect measure of both magnitude and rate of external forces acting on

the lower extremity (Figure 2),6 the role of impact force and rate of

impact loading in the development of overuse running injures and other

injuries to the biological tissues within the lower extremity is still

controversial.7-9

If the motion and inertial properties of the lower extremity segments

and the ground reaction force are known, resultant moments and forces at

the joints of the lower extremity can be calculated. For instance, the

knee adduction moment is that produced by the ground reaction force

about the anterior-posterior axis of the knee (Figure 3) and is an

estimate of the dynamic load distribution at the knee during gait.

Degenerative joint disease at the knee will progress faster10 and is

more severe11 in patients exposed to greater knee adduction moments.

The muscles of the lower extremity mainly generate forces that

facilitate movement,12 but they also ensure joint stability while the

foot is in contact with the ground13 and dampen soft tissue

vibrations.14 Muscle activity during gait is typically measured using

surface electrodes that are placed on the skin overlying the muscle of

interest. Increases in muscle activity will result in greater metabolic

expenditure12 that may not only lead to poorer performance and earlier

onset of fatigue in the elite athlete but may also make walking more

difficult for elderly patients. In addition, greater cocontraction of

antagonist muscles that span the same joint will produce greater contact

forces that may lead to degeneration of biological tissues if sustained

over long periods of time or a large number of loading cycles.15

Although muscle activity is crucial for gait, the effects of footwear

modifications on lower extremity muscle activity have received very

little attention.

If the body's reaction to footwear modifications-be they shoe

modifications, shoe inserts, or foot orthoses-depends on how long the

body is exposed to a particular kind of footwear, then why do we assume

that these effects can be generalized from a few minutes in a laboratory

test? In fact, the biomechanical effects of footwear modifications

should be differentiated as a function of time to match the body's

adaptation to such modifications, which seems to occur in phases.

Phases of adaptation to footwear modifications

Gait adaptation to footwear modifications can be divided into three

phases: short-term, medium-term, and long-term adaptation. Short-term

adaptation may be defined as the immediate adjustment of the body's gait

mechanics to a modification in footwear. Most studies3,16-19 in the area

of footwear biomechanics are concerned with these immediate adjustments.

Footwear modifications are typically presented to a subject for the

first time in the experimental session, the order of conditions is

randomized, and data for repeated walking or running trials are

collected. Randomization is important for data collection as the effects

measured for a particular footwear modification depend on the previously

worn condition.20

Using a randomized protocol with a neutralizing control condition, a

recent study21 showed that the immediate effects of foot orthoses on

variables describing lower extremity kinematics, kinetics, and muscle

activity are consistent across several days (Figure 4). Based on this

very important finding, it can be assumed that changes, or lack of

changes, measured following longer wear periods will be primarily due to

longer term accommodation rather than extended data-collection periods.

Medium-term adaptation describes the adaptation to footwear

modifications that occurs within a few days of using a new footwear

modification. Some studies22-24 acknowledge the distinction between

short-term and longer-term adaptation and allow for accommodation to

footwear modifications before collecting biomechanical data. Fisher et

al25 showed that the effects of footwear modification on the knee

adduction moment slightly increased over a one-week wear period in the

experimental condition. A limitation of this commonly used approach is

the possibility that a control condition is no longer a true control

condition, as the experimental condition may have become the condition

the body is most used to. This dilemma may be overcome by simultaneously

increasing wear-time for the control and all experimental conditions.26

During the medium-term phase of accommodation, the body may or may not

be able to adjust to the modified footwear. In clinical practice,

adjustments are made to the foot orthoses during this period; however,

there appears to be some question about the extent to which footwear

should be modified during this phase.

During the long-term adaptation phase, the body " fine-tunes " its gait

mechanics, possibly to minimize energy and improve gait efficiency.

Although elevated muscle activity observed during the short-term and

medium-term adaptation phases24,27 may initially be due to the

disturbing effects of footwear modifications on a finely tuned system,

it is possible that in the long term these modifications may create an

even better tuned system resulting in similar or even lower muscle

activation levels than those for the original control condition. Changes

in biomechanical variables in response to foot orthoses are related to

differences in comfort perception,27 and clinical experience shows that

comfort may change throughout the different phases of adaptation. Thus,

it is likely that long-term gait adaptation to footwear modification

indeed results in altered gait mechanics compared to those observed

after the short-term phase of adaptation. However, there is no

scientific evidence that gait mechanics in fact change throughout the

three phases of adaptation to footwear modification, partly due to the

difficulties of maintaining a true control condition across these

phases.

Future footwear research

Despite the large body of literature on footwear biomechanics, there are

still many unanswered questions as to the effects of footwear

modifications on gait mechanics. Many researchers23,24,28,29 studied

only a small selection of variables for a wide variety of subjects and

reported unsystematic results. However, there is certainly no question

that footwear modifications may be beneficial in terms of comfort and

pain relief, and a recent series of comprehensive studies21,27,30 by a

group of biomechanists and a podiatrist showed that footwear

modifications can, in fact, have systematic effects.

Posting of foot orthoses, for instance, reduced maximum foot eversion,

while molding of foot orthoses had little effect on foot eversion during

the stance phase of running. However, molding of foot orthoses lowered

the impact force and maximum vertical loading rate by more than 20% (in

these studies posting and molding were differentiated resulting in four

conditions: control [flat], isolated posting, isolated molding, and

molding and posting) (Figure 4).30 In general, the effects of posting

were different from the effects of custom-molding, and the effects of

custom-molding were dominant when both variables were combined. These

studies used very rigorous inclusion criteria, suggesting that

individuals with similar characteristics and limb configurations adjust

in similar ways to footwear modifications.

More research is needed to identify the characteristics and

configurations of individuals-including joint geometry, limb geometry,

muscle strength, and muscle properties-that determine how their bodies

will adapt to footwear. Other relevant factors may include specific

characteristics such as foot sensitivity or muscle coordination that

determine the sensory input into the neuromusculoskeletal system and the

use of this information, respectively-factors that were beyond the scope

of this article.

The human body is a very well designed yet complex system. Thus, it

appears appropriate to study a variety of aspects of the

neuromusculoskeletal system and its interaction with the environment

simultaneously. Collaborations between biomechanists and clinicians are

needed to advance the understanding of gait adaptation to footwear

modifications and to promote the use of noninvasive yet potentially

effective interventions for lower extremity injury and disease

prevention and treatment.

Anne Mundermann, PhD, is a postdoctoral research fellow of biomechanical

engineering at Stanford University in Stanford, CA.

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