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The validity and reliability of a simple semantic classification of foot posture.


The aim of the study was to establish the validity and reliability of the Simple Semantic Classification (SSC).

The SSC is a method of describing how an individual's foot responds when standing weight is applied to it. It is simple because it is restricted to four possible positions judged by observation only. It is semantic because it is based on descriptive terms and not on measurement. The primary reason that it was felt necessary to establish the reliability of the SSC was that evidence of an association between common forms of pathomechanical foot function and ulceration in a sample of neurologically impaired feet has been found. It would be beneficial therefore if a simple method to detect people at particular risk of plantar ulceration could be implemented.

In another study it was recorded that people who presented with pronated or hyper pronated subtalar joints (STJ) demonstrated significantly higher forefoot peak pressures than people who presented with either a neutral or supinated subtalar joint [1]. In the same study it was also found that people with neurologically impaired feet and ulceration were more likely to present with hyper pronated or pronated STJ; the relative risks were calculated as 3.9 and 2.2 respectively. (1) Recognising and identifying people with pronated subtalar posture therefore, may be a useful adjunct to current foot assessment protocols commonly in use.

Redmond et al. conducted an investigation which culminated with the presentation of the Foot Posture Index (FPI). (2) Stringent efforts were made to establish the validity of the measure which meant that even though other groups had established the reliability of an early eight indicator version of the instrument, (3,4) the final instrument relies on an observation of only five indicators (three were dropped because component validity could not be established).

The FPI was developed in response to the challenge that the complexity of foot function should be reflected in a reliable method of measurement that does not require sophisticated equipment, has a quantitative output and is simple to use. (5) Simple, however, is a relative term. Although these demands would be simple for podiatrists and others who routinely examine feet, calculating the FPI demands a visual assessment of key landmarks; it also requires the assessment of the position of the head of the talus based on palpation. The latter will be particularly challenging to many who are not more than ordinarily committed to foot care.

The essentials of the premise that supports the SSC are as follows:

At midstance, the optimal subtalar posture for the weight bearing foot is neutral (i.e. neither pronated nor supinated). Principal effects are that the foot is adequately prepared to adapt to the requirements for propulsion.

At midstance, if the subtalar joint is pronated the foot will be unstable and ill prepared for propulsion. Principal consequences include overloading of the central metatarsal heads and the hallux and/or excessive shearing stress at the first metatarsal head.

At midstance, if the subtalar joint is supinated the foot will be laterally unstable. Principal consequences include challenge to lateral ligaments and the lateral column. The fifth and fourth metatarsal heads will be exposed to excessive and prolonged force. The first metatarsal may be plantarflexed (cavus foot). (6-8)

The above concepts are generally accepted; they form the basis of established foot orthotic therapies and as such they suggest the face validity of a three-part classification (neutral, pronation or supination). The authors suggest that a four part classification is preferable due to the wide range of motion that is featured among people who present with pronated STJ. Justification for a four-part classification is that, in an earlier study, it was recorded that there were differences in peak pressure on the forefoot between groups classified as pronated and hyperpronated. Furthermore, feet with plantar ulceration were more likely to present with hyperpronated STJ than pronated STJ. (1)

It is widely agreed that the most appropriate conditions for assessing foot function from static observations is to examine the foot whilst weight bearing. McPoil and Cornwall established the validity of static stance observations of rear foot posture. Their research showed that the posture of the rear foot in static stance does reflect the posture of the rear foot as it interacts with the supporting surface during ambulation. (9)

Criteria for Classification of Foot Posture (Cross and Lehman)

Supination of the Subtalar Joint

* Inverted calcaneus

* Shallow concavity superior to the lateral malleolus

* No concavity inferior to the lateral malleolus

* High arch

Neutral Subtalar Joint

* Vertical or slightly inverted calcaneus

* Even concavities above and below the malleoli

* No curvature of the Tendo Achilles (TA)

* No bulging of the medial aspect of the foot

* Discernable medial arch

Pronation of the Subtalar Joint

* Eversion of the calcaneus (relative to the lower leg), but only to vertical (relative to the weight bearing surface)

* Bulging of the medial aspect of the foot

* Long sloping concavity superior to the lateral malleolus

* Small deep concavity inferior to the lateral malleolus

* Flattened medial arch (differentiated from a pathological pes planus on external rotation of the weight bearing leg: if the arch configuration changes with external rotation of the lower leg the indication is that the arch can recover).

Hyperpronation of the Subtalar Joint

* Eversion of the calcaneus (relative to the lower leg), beyond vertical (relative to the weight bearing surface)

* Bulging of the medial aspect of the foot

* Long sloping concavity superior to the lateral malleolus

* Small deep concavity inferior to the lateral malleolus

* Flattened medial arch

Helbing's sign may also be apparent

Construct Validity

Scharfbillig et al. reported low criterion validity for indicators used in the FPI, but gave a number of methodological reasons why they may have failed to establish a stronger association. (10) In another study Menz et al. sought to establish the relationship between the FPI and three arch-related measurements from radiographs: navicular height (NHr), calcaneal inclination angle (CIA), and calcaneal first metatarsal angle (C1MA). They reported that the FPI demonstrated significant associations with each of the radiographic parameters (P < .01) but that correlations were moderate to weak (NHr, r = 0.59; CIA, r = 0.36; C1MA, r = 0.42). (11)

Redmond et al. however pursued the validity of the criteria they chose through two independent measures: they compared FPI scores to an earlier validated method published as the Valgus Index and they conducted a laboratory study of static foot posture using the Fastrak TM [2]. Through those initiatives, they established the concurrent validity of the five components that they finally chose for inclusion into the FPI:

* Talar Head Palpation

* Curves above and below the lateral malleolus

* Inversion and eversion of the calcaneus

* Bulge in the region of the talonavicular joint

* Congruence of the medial longitudinal arch

* Abduction/Adduction of the forefoot on the rear foot

Four of the criteria that were validated by Redmond et al. are the salient features of the SSC:

* Curves above and below the lateral malleolus

* Inversion and eversion of the calcaneus

* Bulge in the region of the talonavicular joint

* Congruence of the medial longitudinal arch


The reliability of subtalar posture as an assessment procedure was established on the strength of inter-observer agreement on classification between therapists trained to identify and describe the foot under the criteria used for the SSC. Such training had been integral to courses on foot examination and foot appliance manufacture that the principal author had delivered to a wide range of Hansen's Disease programmes and projects in Asia and Brazil over a 5-year period. A cadre of trained therapists therefore was available for recruitment into the study.


It was not possible for such a widely dispersed group of therapists to have physical access to the same subjects of study. To standardise the examination protocol, a catalogue of photographs was created. Photographs of the posterior aspect of the lower third of 24 legs were taken (24 different people). The photographs were taken with the camera placed 10 cms from the supporting surface, positioning the camera lens parallel to the frontal plane of the leg; thus minimising the distortion that angulations of the lens would create. Markers were positioned adjacent to the lateral side of the feet. These appeared in the photographs to minimise the potential for observers to mistake lateral for medial aspects of the foot.

The photographs were arranged in a catalogue, hereafter referred to as the Semantic Classification Test (SCT). With each photograph, a text box was included which contained the subject ID and a choice of coded options to describe the posture of the subtalar joint. The coded options were: (Figure 1)

HP = Hyperpronated

* Eversion of the calcaneus obviously beyond vertical

* Pronounced Helbings' sign ('C' shaped, lateral oriented curvature of the Tendo Achilles, associated with valgus position of the calcaneus)

* Bulging of the medial aspect of the foot is very prominent

P = Pronated

* Eversion of the calcaneus, but only to vertical

* Helbings' sign, but not prominent

* Bulging of the medial aspect of the foot

N = Neutral

* Calcaneus vertical or slightly inverted

* No curvature of the TA

* Even concavities above and below the medial malleolus

S = Supinated

* Inverted calcaneus

* Medial malleolus not prominent


To gauge the credibility of the principal investigator's classification as the standard against which others would be tested, the SCT was first circulated electronically to three other British podiatrists, all of whom have a particular interest and expertise in podiatric biomechanics. They were requested to study the photographs and classify the feet according to the criteria given. Their responses and those of the principal author were tested to ascertain the level of agreement between them.

In January 2006, The Classification Test was taken to India. Thirteen physiotherapists, employed by LEPRA India, had gathered in Hyderabad for the purpose of a project evaluation. They had all undergone training in basic podiatric biomechanics at a course presented to them in November 2003 (a LEPRA India initiative for the development of foot care services in Andhra Pradesh, Orissa and Bihar). Whilst they were gathered in Hyderabad they were requested to complete The Classification Test.

In May 2006 The Classification Test was dispatched to an Associate in Brazil who circulated it electronically to 12 therapists and technicians (physiotherapists, occupational therapists and prosthetics technicians) and one physiatrist. All the candidates who could be recruited had attended a National Course on Foot Evaluation, Footwear Modifications and Simple Orthoses which had been delivered by the principal author at the Instituto Lauro de Souza Lima, Bauru (the course was similar in content to the course given to the LEPRA India Physiotherapists). The course had been repeated annually since 2003 to ensure that select therapists and technicians, associated with the National Hansen's Disease Control Program, from every state in Brazil would have the same training and orientation.

The issue of reliability was decided on the strength of observer agreement. The Kappa Statistic is one of the most common approaches for resolving such an issue and was the method of choice for this investigation. It is described as an index which compares the agreement recorded against that which might be expected by chance. Kappa can be thought of as the chance-corrected proportional agreement, and possible values range from +1 (perfect agreement) through 0 (no agreement above that expected by chance) to -1 (complete disagreement). (12)


Interpretation of Kappa Statistics: (Table 1)

British Podiatrists

Agreement on classification coding

Kappa = 0.76

C.I. = 0.6673-0.8613

Indian Physiotherapists

Agreement between Indian Physiotherapists and the principal investigator:

Kappa = 0.53

C.I. = 0.4431-0.6214

It was noted that there were two distinct groups amongst the Indian physiotherapists. A group of eight physiotherapists presented responses that were similar and consistent whilst the remaining five physiotherapists presented responses that were idiosyncratic. (In this context, 'consistent' applies to candidates who identified the correct direction in which the subtalar joint was oriented but may have disagreed on the extent of pronation: i.e. whether it was 'pronated' or 'hyperpronated'. 'Idiosyncratic' applies to candidates who frequently recorded the opposite orientation to that illustrated: i.e. they recorded 'supinated' for 'pronated' and vice versa. For further discourse on this topic please refer to the Discussion.) The group was split into two subgroups and the responses from each group were again tested for agreement.

Group A - n = 8 (consistent group)

Kappa = 0.65

C.I. = 0.5401-0.7701

Group B - n = 5 (idiosyncratic group)

Kappa = 0.35

C.I. = 0.2162-0.4924

Brazilian Physical Therapists

Agreement between Brazilian physical therapists and the principle investigator:

Kappa = 0.70

C.I. = 0.6372-0.7636


Sim and Wright (13) explain that Kappa weighting can be applied to a nominal scale with three or more categories, if certain disagreements are considered more serious than others. This applied to the study because hyperpronation has been found to be more serious than either pronation or supination. (1) They also reflected the opinion of others that if an assessment procedure is to be recommended for clinical practice, inter-observer reliability, demonstrated by a Kappa coefficient of at least four, should be established. The inter-observer coefficient describing the agreement between the Indian physiotherapists and the principal investigator was 0.53 (moderate) however, on examining the results it was clear that the outcome was skewed by the submissions of a minority who returned papers that scored notably less than the lower limit of the confidence interval for the entire group. When this sub-group was excluded from a second analysis the Indian physiotherapists and the principal investigator demonstrated substantial agreement with a kappa coefficient of 0.65. The 13 Brazilian therapists (with one physiatrist) also demonstrated substantial agreement with the principal investigator; reflected by a kappa coefficient of 0.70.

When Redmond et al. (2) pursued the challenge to devise a system to define the posture of the weight bearing foot they were acting in response to a number of august institutions and organisations that had repeatedly called for a method to measure the extent of foot abnormality. The demand for an acceptable method of quantitative measurement indicated the frustration that clinicians and researchers experienced with measurement methods that had become part of routine practice, even though such had shown questionable validity and poor reliability. (14-16) By developing the FPI Redmond et al. did successfully devise an objective method for assessing foot function, but the application of the method is limited to well trained specialists.

In advantaged societies, where multidisciplinary teams share responsibilities for health provision, a podiatrist's perspective and expertise would be an expected component of care for people with neurologically impaired feet. In the developing countries, however, such requirements often become the responsibility of occupational therapists, physical therapists or others who are compelled to expand their remit to assume responsibilities that would fall under the domain of podiatry in developed countries. It was considered that the development of the SSC would be a pragmatic compromise. The lack of specificity will obviously limit its use as an instrument for more precise diagnostic or prescription purposes, but since it relies on gross visible indicators of foot posture it is less likely to be vulnerable to application error or misinterpretation than the FPI.


This study presented evidence to demonstrate the reliability and validity of the Simple Semantic Classification. It may prove to be useful as a field method for identifying the risk of secondary problems for people with neurologically impaired feet and could also be used as a guide for appropriate foot appliance intervention.


The authors thank the following people for their cooperation and support:

Dr Rosa Soares (Director, 2005) and staff of the National Program for the Elimination of Hansen's Disease (PNEH), Brazil.

Dr Marcos Virmond (Director) and staff of Instituto Lauro de Souza Lima, Brazil

Dr Ranganadh Rao (Director) and staff of LEPRA, India

The following people kindly participated in the exercise and to them we also express our thanks:

Mr George Rendall

Mrs Katherine Rendall

Mr Steven MacMillan


(1) Cross H, Rendall G. A investigation of common forms of pathomechanical foot function and their association with force related variables and ulceration amongst people with loss of sensation due to leprosy. B J Pod, 2007; 10: 154-160.

(2) Redmond AC, Crosbie J, Ouvrier RA. Development and validation of a novel rating system for scoring standing foot posture: the Foot Posture Index. Clin Biomech (Bristol, Avon), 2006; 21: 89-98.

(3) Evans AM, Copper AW, Scharfbillig RW et al. Reliability of the foot posture index and traditional measures of foot position. J Am Podiatr Med Assoc, 2003; 93: 203-213.

(4) Payne C, Oates M, Noakes H. Static stance response to different types of foot orthoses. J Am Podiatr Med Assoc, 2003; 93: 492-498.

(5) Saltzman CL, Domsic RT, Baumhauer JF et al. Foot and ankle research priority: report from the Research Council of the American Orthopaedic Foot and Ankle Society. Foot Ankle Int, 1997; 18: 447-448.

(6) Root ML, Orien WP, Weed JH. Abnormal function of the foot. Clinical Biomechanics, Vol. 2. Clinical Biomechanics Corporation, Los Angeles, 1977.

(7) Kirby KA. Biomechanics of the normal and abnormal foot. J Am Podiatr Med Assoc, 2000; 90: 30-34.

(8) Rodgers MM. Dynamic foot biomechanics. J Orthop Sports Phys Ther, 1995; 21: 306-316.

(9) McPoil TG, Cornwall MW. Relationship between three static angles of the rearfoot and the pattern of rearfoot motion during walking. J Orthop Sports Phys Ther, 1996; 23: 370-375.

(10) Scharfbillig R, Evans AM, Copper AW et al. Criterion validation of four criteria of the foot posture index. J Am Podiatr Med Assoc, 2004; 94: 31-38.

(11) Menz HB, Munteanu SE. Validity of 3 clinical techniques for the measurement of static foot posture in older people. J Orthop Sports Phys Ther, 2005; 35: 479-486.

(12) Landis JR, Koch GG. The measurement of observer agreement for categorical data. Biometrics, 1977; 33: 159-174.

(13) Sim J, Wright CC. The kappa statistic in reliability studies: use, interpretation, and sample size requirements. Phys Ther, 2005; 85: 257-268.

(14) Elveru RA, Rothstein JM, Lamb RL, Riddle DL. Methods for taking subtalar joint measurements. A clinical report. Phys Ther, 1988; 68: 678-682.

(15) Menz HB. Alternative techniques for the clinical assessment of foot pronation. J Am Podiatr Med Assoc, 1998; 88: 119-129.

(16) Weiner-Ogilvie S, Rome K. The reliability of three techniques for measuring foot position. J Am Podiatr Med Assoc, 1998; 88: 381-386.

[1] The authors caution that whilst high peak pressure is a specific indicator of ulceration it is not sensitive: i.e. many neurologically impaired people with high peak pressures do not present with ulceration.

[2] The Valgus index indicates the extent to which a foot is affected by pronation or supination of the STJ. It is based on an analysis of the relationship between two axes that can be inscribed over a foot print impression. Redmond et al. used Fastrak TM to construct three dimensional models of the lower limb so that postural variations in all three cardinal planes could be calculated at five salient anatomical positions. Each component of the FPI was analysed as a dependent variable; the contribution of the electromagnetic motion tracking (EMT) measures to the explanation of variance in each FPI component score was established.

HUGH A. CROSS, American Leprosy Missions, Box 002 Mail and More, 2nd Level, Paseo Marina, Ayala Center, Cebu Business Park, Cebu City, 6000 Cebu, The Philippines

LINDA LEHMAN, American Leprosy Missions, Regional (Americas & Africa) Prevention of Disability Consultant

Accepted for publication 28 August 2008

Correspondence to: Hugh A. Cross B.Sc. (pod) Ph.D., Box 002, Mail and More, Paseo Marina, Second Level, Ayala Center, Cebu Business Park, Cebu 6000, The Philippines (e-mail:
Table 1.

Kappa      Agreement

0          None
0-0.2      Slight
0.2-0.4    Fair
0.4-0.6    Moderate
0.6-0.8    Substantial
0.8-1.0    Almost Perfect
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Author:Cross, Hugh A.; Lehman, Linda
Publication:Leprosy Review
Date:Dec 1, 2008
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