Reliability of the use of foot pressure pain threshold in adults: a test-retest analysis

Introduction

According to the International Association for the Study of Pain, pain is defined as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage” (Raja et al., 2020). Pain can be classified based on its duration as either acute or chronic. Acute pain has limited duration with minimal psychological influence, while chronic pain persists indefinitely and is accompanied by a psychological component (Puebla Díaz, 2005).

Foot pain is very common, affecting 24% of adults over the age of 45 (Menz et al., 2013). Studies have shown that people suffering from foot pain experience a reduced quality of life, as it may hinder daily activities, cause balance and walking problems, and increase the risk of falls, especially in older individuals (Laslett et al., 2018; Cotchett et al., 2022).

Pain is difficult to measure and can be evaluated subjectively (as reported by the individual) or objectively (quantified after being triggered). The Visual Analog Scale (VAS) is commonly used for subjective pain measurement, assessing the intensity of pain as perceived by the individual (DeLoach et al., 1998). Precise and objective quantification of the pain threshold (the minimum tolerated pain) is crucial for many aspects of pain management, from clinical research to evaluating treatment efficacy. Mechanical stimulation devices, such as pressure algometers, are used for this purpose (Fischer, 1987). An algometer displays the pressure pain threshold (PPT), which is the minimum pressure applied to the evaluated area that triggers pain (Fischer, 1987; Kinser, Sands & Stone, 2009; Saban & Masharawi, 2016). It is important to note that the reliability of pressure pain threshold data depends on the technique used by the observer and the subject’s ability to provide clear verbal feedback on their perception of pressure pain thresholds levels (Chesterton et al., 2007). Within the framework of Quantitative Sensory Testing (QST), a standard protocol is used to measure pressure pain thresholds. This involves using a digital algometer to collect multiple PPT measurements at target points to obtain an average value, as well as factoring in variables such as patient positioning and applied pressure force to enhance reproducibility. The procedure is standardized by providing precise instructions and training for data collection (Rolke et al., 2006). This standardization helps minimize variability and ensures consistent results across different evaluators and sessions.

Intra-rater reproducibility refers to the degree of agreement between pressure pain threshold measurements taken by the same evaluator at different times. This reproducibility can be divided into two test-retest parameters: agreement and reliability. Agreement (or measurement error) refers to the magnitude of difference between pressure pain threshold measurements taken during the same session and those taken in different sessions. Agreement is expressed as an absolute value such as the standard error of measurement (SEM) or the coefficient of variation (CV). Reliability refers to the evaluator’s ability to distinguish between individuals based on their pressure pain thresholds and is expressed as a relative value such as the intraclass correlation coefficient (ICC) (Chesterton et al., 2007; Fischer, 1987; Kinser, Sands & Stone, 2009; Nussbaum & Downes, 1998; Walton et al., 2011).

Previous studies have demonstrated the reliability and validity of algometer measurements in different parts of the body (Chesterton et al., 2007; Fischer, 1987; Kinser, Sands & Stone, 2009; Walton et al., 2011), but there is a lack of data from individuals with healthy feet without previous pathologies. The digital pressure algometer is a diagnostic tool used to assess pain thresholds in patients, especially for musculoskeletal conditions like fibromyalgia, myofascial pain, and joint discomfort. The digital algometer also has several advantages, such as being easy to use and providing accurate and reliable measurements (Jones, Kilgour & Comtois, 2007; Nussbaum & Downes, 1998). The digital algometer is frequently used to measure pain thresholds in the head (Castien et al., 2021), tongue (Díaz-Saez et al., 2023), and neck (Knapstad et al., 2018). Other studies have used the digital algometer to evaluate pain thresholds in patients with rheumatoid arthritis (Huskisson & Hart, 1972), specifically in the lower limbs at the knee level (Stausholm et al., 2023), where the algometer showed good intra- and inter-rater reliability for measuring knee pain thresholds in arthritis patients.

In previous studies on foot pain, algometers have been used to assess pain thresholds before and after treatment of chronic heel pain due to calcaneal spurs (Yürük, Aykurt Karlıbel & Kasapoğlu Aksoy, 2022). Pressure pain thresholds may be an indicator of the effects of foot posture throughout the workday. Greater changes in pressure pain thresholds and increased foot discomfort were found in workers who spent more hours standing (Messing & Kilbom, 2001). Pain sensitivity varies according to the area of the foot—higher in the arch and dorsum of the foot and lower in the heel—which has a significant impact on the design of footwear (Xiong, Goonetilleke & Jiang, 2011). Algometers have also been used to measure treatment outcomes for plantar fasciitis (Yelverton, Rama & Zipfel, 2019), providing results before and after treatments (Boob, Phansopkar & Somaiya, 2023). Madeleine et al. (2014) used pressure pain threshold measurements to demonstrate the effect of cushioning footwear inserts. Additionally, pain measurement can be useful for tracking its progression over time or as a predictive tool for future outcomes (Walton et al., 2011).

However, no study has evaluated pressure pain thresholds of the feet in asymptomatic individuals. This study aimed to determine the intra- and inter-session agreement and reliability of foot measurements using pressure pain threshold in adults aged 55 to 75 years.

Materials and Methods

Procedure

A total of 31 patients were included in the study. Body Mass Index (BMI) was calculated by dividing weight in kilograms by the square of height in meters (kg/m²).

The pressure algometer was used to objectively assess foot pain threshold, defined as the minimum tolerable pressure before pain is perceived (Fischer, 1987; Trueba-Perdomo, Gasparini & Cuautle, 2021). The specific algometer used for these measurements was the Algometer Commander from JTECH Medical, manufactured in 2004. This instrument featured a 1 cm² probe for measurement, using Newtons (N) as the unit of measurement (Saban & Masharawi, 2016).

During the procedure, the patient was positioned comfortably in a supine position on an examination table. Measurements were taken on both feet, the right (R) and the left (L), at four specific points on the plantar aspect: the first metatarsal head (1st MH), the fifth metatarsal head (5th MH), the first toe pulp (1st TP), and the heel. These correspond to the areas of the foot that typically experience the greatest contact with the ground throughout the different phases of gait. For each point, the probe was placed perpendicularly to the foot, and pressure was applied gently to the target area (Castien et al., 2021). The process was halted as soon as the patient reported any discomfort. Measurements at each point were taken three times, with the same procedure repeated during the initial trial, after 30 min, and again after 7 days. The podiatrist who performed the measurements had previous clinical experience using the algometer in routine practice for the assessment of pressure pain thresholds. In addition, prior to the study, the assessor underwent a standardized process of calibration and familiarization with the device to ensure consistency of measurements. Figure 1 presents a visual representation of the methodology used in this study, illustrating the pressure areas measured and the parameters calculated both within and between sessions.

Results

A total of 31 participants were included in this study. Of these participants, 16 were women and 15 were men, with a mean age of 64.1 and 67.6 years, respectively. The average BMI was 29.6 kg/m² for women and 28.9 kg/m² for men. Regarding BMI categories, 12.5% of women and 20% of men were classified as normal weight, 50% of women and 40% of men as overweight, and 37.5% of women and 40% of men as obese (Table 1). Pressure pain threshold values from session 1 are shown in Table 2. All intra-session and inter-session agreement and reliability values are presented in Tables 3 and 4 and are illustrated in Fig. 2.

Discussion

This study was designed to examine the intra-rater reliability of the Commander JTECH digital algometer (2004) for pain pressure threshold measurements taken from the foot. The results showed good to excellent reliability (ICC: 0.76 to 0.91), supporting the hypothesis that the digital algometer is a reliable tool for measuring pressure pain thresholds on the foot, similar to its use on other parts of the body as shown in previous studies. Furthermore, the device provides consistently reliable results, making it a useful tool for both clinical practice and research.

Several previous studies have examined the reliability of pressure pain threshold (PPT) measurements on the foot, although with differing methodologies and objectives. Xiong, Goonetilleke & Jiang (2011) reported an ICC of 0.8 when evaluating pressure thresholds in the right foot of 20 participants, with results influenced by stimulus characteristics such as probe size and indentation speed. However, unlike the current study, their focus was on mechanical variables relevant to product design rather than clinical assessment using standard tools. Similarly, Wu, Liu & Qu (2024) used algometry to map foot regions in 20 adults and reported regional differences in pain sensitivity, with lower thresholds in the dorsum and lateral areas and higher thresholds in the plantar region, particularly the heel. While informative, these studies did not assess intra-rater reliability under clinical conditions. In contrast, Saban & Masharawi (2016) reported ICCs ranging from 0.75 to 0.92 when testing five heel regions, which aligns with the reliability values obtained in our study. Furthermore, Jerez-Mayorga et al. (2020) and Reezigt et al. (2023) confirmed the algometer’s reliability and usability, including among inexperienced raters, though they focused on body areas beyond the foot. Therefore, our study adds to the literature by being, to our knowledge, the first to evaluate the intra-rater reliability of a digital pressure algometer applied systematically across specific foot regions in an older adult population, offering clinically relevant insights and supporting the instrument’s applicability in routine assessments.

The findings of the current study support the use of the digital algometer as a fundamental tool for measuring pressure pain thresholds. Its demonstrated accuracy, intra-rater reliability, and ease of use make it a valuable instrument for clinical assessment of pain sensitivity. These characteristics enhance its utility in monitoring patients’ responses to interventions over time and in ensuring consistency in pain evaluations, thereby contributing to more precise clinical decision-making.

Conclusions

The digital algometer can be used to measure pressure pain thresholds in the healthy foot. Specifically, this study evaluated pain in four points on the plantar surface of the foot (the head of the 1st and 5th metatarsals, the pulp of the 1st toe, and the center of the heel), resulting in good to excellent reliability.

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