The sample size was calculated by using the total score of the original FFI questionnaire (Budiman-Mak et al. 1991), with 95 % of confidence and 90 % of power and was obtained a number of 46 subjects (Sousa and Rojjanasrirat 2011). To prevent loss of volunteers, 50 patients were enrolled in this study. They were clinically diagnosed with plantar fasciitis (56 %), metatarsalgia (14 %) and lateral sprain of the ankle (30 %) in which the injuries occurred more than 6 months earlier. All these patients were seen at our institution’s Physiotherapy Unit and Orthopedics and Traumatology Outpatient Clinic.
The inclusion criteria for the plantar fasciitis were: (1) plantar medial heel pain, most noticeable with initial steps after a period of inactivity; (2) heel pain precipitated by a recent increase in weight-bearing activity; (3) pain with palpation of the proximal insertion of the plantar fascia; (4) positive windlass test (Martin et al. 2014). The criteria for metatarsalgia were: (1) pain on metatarsals heads during palpation; (2) Mulder positive sign (Espinosa et al. 2008). The lateral ankle sprain criteria were: (1) history of at least 1 significant ankle sprain; (2) inflammatory symptoms including pain and swelling; (3) interruption of the physical activity for at least 1 day after the sprain (Gribble et al. 2014).
The volunteers included in the study were of both genders (76 % female), with an average age of 32.9 ± 14.13 years (range 18–60 years) and all of them attended school until at least fully grading high school. Patients with cognitive alterations, neurological conditions and other lower-limb diseases were excluded.
Testing the psychometric properties
The validity of the Brazilian version of the FFI questionnaire was tested in order to establish the correlation among the proposed instrument and instruments that had already been translated and validated (Andresen 2000). The Brazilian-Portuguese version of the Medical Outcomes Study 36-item Short Form Health Survey (SF-36) (Ciconelli et al. 1999) and the Foot and Ankle Outcome Score (FAOS) (Imoto et al. 2009) questionnaires were used for the validity process. The questionnaires were applied by the same evaluator, through interviews, as the other questionnaires validity process conducted in Brazil (Ciconelli et al. 1999; Imoto et al. 2009; Yi et al. 2015). The completion time of the FFI questionnaire lasted from 5 to 10 min.
The calculus to obtain the score for each subscale is done by using the following formula: the sum of the score from all items answered by the patient/divided by the total score possible for the subscale multiplied by 100. If the patient does not perform an activity indicated by one of the subscale items (for example, not using auxiliary devices), this item is considered to be not applicable. Thus, the scores for such items will not be considered in the total sum for the subscale (Budiman-Mak et al. 1991).
The final score of the questionnaire is calculated using the formula: sum of the final percentages of all subscales divided by three (total number of subscales). The results may vary from 0 to 100 % and are directly proportional to the limb’s functional impairment, such that the higher the percentage is, the greater the functional alteration presented by the patient is (Budiman-Mak et al. 1991).
Short Form Health Survey (SF-36)
SF-36 is a multidimensional instrument used to evaluate quality of life. It is composed by 36 items distributed into eight scales: physical functioning, physical role functioning, bodily pain, general health perceptions, vitality, social role functioning, emotional role functioning and mental health. The total score from each scale varies from 0 to 100 points, where zero represents the worst state of health and 100 the best state (Ciconelli et al. 1999).
Foot and Ankle Outcome Score (FAOS)
FAOS is a questionnaire that is used to evaluate symptoms and functional limitations caused by ankle and foot alterations. The questionnaire consists of 42 items distributed into five subscales: pain (9 items), symptoms (7 items), function in daily living (17 items), function in sports and recreation (5 items) and quality of life (4 items) (Imoto et al. 2009).
The score is calculated for each subscale using the following formula: 100 − (subject score multiplied by 100 divided by the maximum score possible in the subscale). Scores vary from zero to 100, where zero indicates extreme symptoms and 100 indicates absence of symptoms (Imoto et al. 2009).
Reliability was tested by evaluating the reproducibility (test/retest) and internal consistency (IC). Reproducibility is the extent to which the same results from the questionnaire are obtained through different administrations. Internal consistency indicates the extent to which the items in a subscale are correlated, in order to evaluate homogeneity (Andresen 2000; Eechaute et al. 2007; Kirkley and Griffin 2003).
Inter-observer reliability is measured through applying the FFI questionnaire on two occasions: initially by evaluator A and after a period of at least an hour, by evaluator B, as the other validity questionnaires process conducted in Brazil (Ciconelli et al. 1999; Imoto et al. 2009; Yi et al. 2015). A third administration of the questionnaire was made in order to evaluate intra-observer reliability, performed by evaluator A after a time interval of a maximum of seven days after the first application (Terwee et al. 2007). The questions and alternatives of the FFI were read out loudly to all volunteers, without providing explanations, with the aim of not influencing the responses. During the period between administrations of the questionnaires, the volunteers were instructed to maintain their daily activities.
Ceiling and floor effects
Ceiling and floor effects of FFI were calculated. These effects are calculated in order to evaluate the entire spectrum of a condition’s severity with the items it contains (Eechaute et al. 2007). The ceiling and floor effects occur when the maximum or minimum possible score is achieved by a substantial portion of the sample of participants (Eechaute et al. 2007).
This study was approved by the Research Ethics Committee, under protocol number 226.521, and all the participants signed the free and informed consent statement.
Descriptive statistical analysis on the sample was performed using means and standard deviations (SD). The total FFI score was correlated with the subscales of the SF-36 and FAOS questionnaires, by means of Pearson’s linear correlation coefficient. The alpha error was set at p < 0.05.
To ascertain the intra and inter-observer reliability, the interclass correlation coefficient (ICC) was calculated. The IC of the FFI subscales was evaluated by means of Cronbach’s alpha coefficient. The reliability results were combined with the total FFI scores and their subscales in order to define the standard error of measurement (SEM) (Beaton et al. 2000). The SEM was calculated using the following formula: SEM = SD1.√(1 − ICC), where SD1 = is the standard deviation of the initial evaluation and ICC = the ICC found in the reproducibility analysis (Beaton et al. 2000; Leggin et al. 2006; Lopes et al. 2008; Martin and Irrgang 2007).
The error associated with applying the FFI over this time interval also defined the minimal detectable change (MDC), i.e. the minimum amount of change necessary for the FFI score (with 90 % confidence) to be considered to be a real change over the time frame of a maximum of one week. The MDC was calculated using the following formula: MDC = 1.65.√2.SEM (Leggin et al. 2006; Lopes et al. 2008; Martin and Irrgang 2007).
A ceiling and floor effects were considered if more than 15 % of the respondents achieved the lowest or highest (0–100) possible score of FFI (Eechaute et al. 2007).