Literature Review methodology

A record research was performed, extracting articles from PubMed and Google Scholar databases to find articles related to PPT and the devices mentioned earlier by using the keywords "pain, pressure pain threshold, pressure algometry, algometer, dolorimeter, electromechanical algometer, and computerized algometer."

The search made by using each word separately resulted in 185 selected articles. From these 185 articles, scientific papers published before 2013 were discarded, except for the article ^[5], which was included to gain more data about the electromechanical algometers.

Inclusion and exclusion criteria was adopted to select the resulting 75 articles from the original 185. The inclusion criteria considered were the studies with precise results of PPT mean values, studies that used either a manual or an electromechanical pressure algometer, and studies that involved healthy subjects and subjects who suffered some pain or illness. This latter information has also been considered to analyze the differences in PPTs between healthy and ill people. On the other hand, the exclusion criteria adopted were the use of other instruments different from pressure algometers to generate a painful stimulus, studies that do not report the mean PPTs, and studies that do not specify the area of applied stimuli. A total of 20 scientific papers met the inclusion and exclusion criteria and thus were deeply studied and classified according to the body part where the stimulus was applied- trapezius muscles, back muscles, upper limbs, leg muscles, foot area, and head muscles. Figure 1 shows the article selection process described above and the final cardinality of papers considered for the six body parts mentioned before. As a first outcome of analyzing these 20 articles, the algometers list reported in Table 1 shows the existing models of algometers used in previous works.

Figure 1 Flow diagram of the followed methodology in paper selection. 

Data extraction

The following data were collected and summarized in Tables 2- 7. They show the subjects involved in the experiments (age, number, sex, physical conditions), algometer adopted (type, tip area, and speed rate), rest interval between trials, number of trials performed to obtain the mean value of PPT, the muscle on which the stimulus was applied, and in the last column the mean PPT values.

It is worth mentioning that the PPTs results were recorded when the stimulus changes from being comfortable to being uncomfortable, or a painful sensation was mentioned. Therefore, the PPT results collected from previous works have been obtained in each study as follows: a different number of trials were performed on each subject (varying from 2 to 5 applications of the stimulus), the average value of PPTs per subject was calculated over these trials, and finally, the mean values over all subjects were evaluated and corresponded to the PPT reported in Tables 2-7, for the body part where the stimulus was applied.

All the PPT values were properly converted and reported in the SI unit, kPa. For those studies where any kind of treatment was performed, only the baseline values were taken into account. For example, in ^[10], a baseline test was performed and then the subjects received a massage treatment; later on, they underwent another PPT test so, only the values of the first test were included in the tables.

A normalization of measurement units was also necessary for the stimulus application's speed rate (kPa/s) to compare all values shown in this study. As it can be seen the normalization was done for all collected data, except for the data extracted from ^[7] where the values used are in mm/s (see Table 6). It also shows that the range of speed rates varied from 10 to 126 kPa/s, and that in 5 studies the rate was not specified ^{[8] [10] [11] [12] [13]}. It can also be noted that the most used rate is 30 kPa/s (43.75% of the studies), followed by rates of about 124 kPa/s (18.75%), and 50 kPa/s (9.37%).

The age of the subjects involved varied from 18 to 88. In most studies, subjects were asked to stop taking any pharmaceutical drug (or medicine that alleviated pain) at least 24 hours before the test. Only in ^[14] were the patients allowed to continue their usual medical treatment during the experiment.

Since some studies conducted tests on different parts of the body, only the body parts that had reported values were included. Therefore, this work only presents the results of the trapezius muscles, back muscles, upper extremities and lower legs and head area. For example, in ^[15], a stimulus was applied in the middle trapezius muscle, low back, hand, and quadriceps. The mean values of PPT for each body region are reported in the corresponding table (Tables 2-5).

Most of the algometers considered in the articles were manual. As seen, the Somedic algometer was the most used (10 papers), followed by the Warner algometer (6 papers), while only in one paper ^[12] were the tests carried out with the Baseline Dolorimeter. Finally, in 6 studies, tests were performed using an electromechanical algometer.

All articles used an algometer with a flat circular tip that varied in size from 0.5 to 2 cm² (area of stimulation). In all the studies carried out with an electromechanical algometer, the subjects had an "emergency stop button," Some studies performed with a manual algometer ^{[2] [13] [15] [16] [17] [18] [19] [20]}. The stop button provided the researcher the time to record the PPT. In other articles, the subjects gave a verbal report to the examiner to indicate the change from the sensation of pressure to pain ^{[11] [12] [14] [21] [22] [23]} in order to record the PPT value.

Finally, the range of intervals between trials varied from 10 s to 300 s, the most used is 30 s, followed by 20 s. The intervals between trials were not specified ^{[7] [13] [21] [24]}.

Characteristics of the electromechanical algometers used in the studies

It was possible to list the adopted algometers from the analysis of the 20 articles selected from the literature which were either manual or electromechanical. These algometers, together with the producer, the model, and the reference for further details are reported in Table 1.

The Algoforce PA3 adopted in ^[5] has a pressure sensor and an actuator for what concerns the electromechanical ones. This algometer is controlled by electromechanical feedback to increase the pressure when the stimulus is performed.

The electromechanical algometer presented in ^[6], called multimodal automated sensory testing (MAST) system, is controlled by signals over a Bluetooth link. The subjects indicated the pain sensation using a rating scale over a touch screen on a separate computer. This algometer is portable and composed of a DC servo motor, hand-held thumbnail pressure stimulators, the Bluetooth control system, and a load cell-type force sensor that works by resistive measurement (for measuring the pressure applied).

The so-called Automatic Tissue Tester (ATT) electromechanical algometer was used to apply the pressure stimulus in ^[7], the subjects had to stand on a platform, and then the pressure stimulus was applied from the bottom up; therefore, the body parts that were affected were at the bottom of the foot. The electromechanical algometer comprises an indentation speed control, maximum force application of 1500 kPa, a maximum indentation of 35 mm, tension-compression load cell, and a 200 step/revolution stepper motor. In ^[9], an electromechanical algometer similar to the one described in ^[7] was built.

Finally, in ^{[8] [24]}, a Biaxial Algometer was used, designed with a pressure application device, a visual analog scale, a user interface, a controller, and an in-line attached load cell force sensor. The algometer can move linearly and rotationally.

Analysis of PPT of the trapezius muscle area

The analysis of pain pressure applied to the trapezius muscle area in healthy participants is presented in Table 2^{[15] [19] [21]}. One study did not specify the condition of the subjects ^[17]; in 5 works, participants with some disease or pain were studied. The disease and pain conditions that affect the subjects are fibromyalgia ^[15], low back pain ^[15], neck pain ^[20], tension-type headache ^{[2] [10]}, and pain in the upper trapezius (UT) muscle ^[11].

The highest mean value of PPT (512 kPa) was obtained in a test performed on healthy participants with a commercial algometer (Wagner Force Ten) in the upper trapezius muscle ^[15]. However, the lowest value (110 kPa) ^[11] has been recorded using the same manual algometer and in the same body part (upper trapezius muscle) on participants with local pain.

In two reports ^{[2] [10]}, patients that suffered tension-type headache (TTH) were involved in the study. The PPT values obtained from these studies were registered using Wagner Instruments algometers ^[2] and Somedic AB ^[10]. Together with the previously cited one (110 kPa), these values were the lowest among those measured in the trapezius muscle area.

In ^[2], a more in-depth analysis had been performed to evaluate the PPTs on patients that suffer different types of TTH. According to the frequency of the headaches, the patients were separated into two groups: chronic tension-type headache (CTTH) and frequent episodic tension-type headache (FETTH). However, even if the tests were performed when the patients were without headaches or with a low-intensity pain (at least ≤ 3 points, rated on a numerical pain rate scale, 0 - 10) the PPT values registered (215 kPa for FETTH and 222 kPa for CTTH) were significantly low when compared with values obtained in other studies on healthy people (see Table 2).

Instead, in ^[10], the pressure stimuli were applied it comes out that the intensity of pain directly affects while the patients presented a headache, obtaining the pain threshold, the more intense the pain, the even lower values (187 kPa on UT right and 186kPa on lower the threshold. There are no articles that report UT left) for the ones reported in ^[2]. From these studies, the use of an electromechanical algometer on the UT.

Analysis of PPT of the back muscle area

In this section, all the studies that applied a pressure stimulus on the back muscle area are considered, and the extracted data are collected in Table 3. All these studies involved healthy participants. Two of them also performed tests on subjects who had fibromyalgia ^[15], back pain ^[15], and sacroiliac joint pain ^[13].

The lowest PPT value was obtained in patients with pain in the sacroiliac joint region (235 kPa), whereas healthy subjects reached the highest values (833 kPa) ^[13]. Both values were obtained with a manual algometer (Wagner Force Dial TM FDK 40) that was applied to the sacroiliac joint region.

Out of the studies conducted on the back muscle area, an electromechanical algometer is used only in ^[9] (called computerized deformation-controlled indentation system). Therefore, tests were carried out on the lower back of healthy participants, both with a commercial Wagner FDN 100 algometer (695 kPa) and the electromechanical one (484 kPa). Since both devices applied the painful stimulus on the same area of the body and used the same sample of subjects, the values obtained should have been very similar. However, the difference between them was 211 kPa. The authors of such research explained the possibility of a mechanical problem related to the indentation speed of the system.

A value of 716 kPa obtained during the investigation in ^[15] on another patient using a manual Wagner algometer, further confirmed that the mechanical system was probably wrong. This value is 232 kPa above the PPT value reported by the electromechanical algometer in ^[9] but shows a difference of only 21 kPa for the manual algometer result for the same research ^[9].

Analysis of PPT of the upper limbs

In this section, PPT values obtained in upper limbs are collected and reported in Table 4. Among all the studies considered, only five involved healthy subjects ^{[5] [6] [15] [16] [18]}.

In ^[6], only female subjects are considered, while both males and females were included in the others. It should be noted that in ^[15], the pressure stimulus was applied between the thumb and the index finger on the dorsal part of the hand, while in the other articles, the pressure was applied on the biceps brachii muscle ^[16], thumb ^{[5] [6]}, the second metacarpal ^[2], and the middle finger ^[18]. In ^[5], tests were performed on the thumb, using a manual algometer (Algometer Type II, Somedic Production AB) and an electromechanical one, obtaining very similar PPT values, 357 kPa, and 371 kPa, respectively.

The PPT values of the second metacarpal, obtained from ^[2], were among the lowest ones (268 kPa and 241 kPa); it is worth mentioning that the patients presented TTH. On the other hand, the highest value (897 kPa) was obtained on the middle finger in [18], using a manual algometer (Wagner Force One FDIX 50). It is observed that the PPT value of the middle finger is higher than PPTs extracted from the hand ^{[2] [15]} and even higher than those presented on the brachii muscle ^[16], giving the impression that a more significant muscle gives higher values. These values can be related to the fact that the nerves responsible for perceiving the fingers' painful stimulus are more distant from the brain than those present in the arm.

The study reported in ^[6] made use of an electromechanical algometer. The pressure on the thumb of healthy subjects and the thumb of subjects with fibromyalgia produced PPTs of 260 kPa and 98 kPa, respectively, with a difference of 162 kPa. Thus, the PPT value of the patients with fibromyalgia had the lowest value of the whole table.

Analysis of PPT of the foot area

In the articles presented in Table 6, related to the foot area's pain pressure, both males and females were included, except for ^[7], where the participants were only five males. This article is also the only one found in the studied records that applied a pressure stimulus on the foot area with an electromechanical algometer.

In ^[22], an analysis of different studies was carried out, collecting the following average ranges of PPTs: from 400 to 900 kPa for the hindfoot area (medial malleolus), 200 to 500 kPa for the midfoot area (instep), and 400 to 700 kPa for the forefoot area (second or third MTP joint).

These ranges are valid only for PPT measurements in healthy people and with a stimulation tip of 1 cm². The same study's authors performed tests for three years on healthy people and people with painless diabetic neuropathy. The Somedic algometer used had an interchangeable flat round tip of 0.5, 1, and 2 cm². The PPT values reported with the stimulation area of 1 cm² taken in healthy people are within the previously established ranges.

The study described in ^[7] recorded the highest PPT values with respect not only to the foot area but concerning all the papers here considered (see Tables 2-7). In addition, they recorded three different values on healthy people- 1014 kPa, 1039 kPa, and 1074 kPa, on the heel pad with the ATT algometer aforementioned.

Analysis of PPT of the head muscles

Table 7 reports the results obtained from 2 articles ^{[2] [10]} that considered the head muscle area. These two studies were already analyzed in section 3.2, related to the trapezius muscle area, where the involved subjects' conditions were deeply detailed.

For the part of the study performed on the head muscles, in ^[10], the number of males and females that participated was not specified for both of these studies; pressure stimulations were performed in two places- suboccipital muscles and temporalis muscle, obtaining PPT values in a range from 151 to 212 kPa.

Design characteristic for a functional electromechanical pressure algometer (EPA)

Thanks to the analysis carried out in this work, it is possible to guideline an electromechanical algometer's characteristics. It was concluded that an ideal algometer should be able to measure the PPT in different parts of the body.

An ideal algometer must be designed to be applied to the trapezius muscles, back muscles, upper and lower extremities, and the head area because those areas are the most studied. Table 8 presents an overview of the design features included in the available literature.

A functional algometer should be able to monitor the actual pressure of contact between the algometer and the patient’s skin; as well as, the speed rate with which the stimulus is applied, and the penetration distance of the tip into the skin of the subject in order to obtain a more significant amount of standardized data ^[24]. It would then make it possible to avoid the error produced by the operator of a manual algometer.

Furthermore, error reduction in conjunction with the signals extracted from other devices (such as electroencephalograms, electrocardiograms, skin conductivity sensors, pulse oximeters) could be significantly helpful in future works related to measure physical pain ^{[23] [25] [26]}.

Based on the results obtained and the findings of the review performed in this paper, the following characteristics are proposed for the design of an electromechanical pressure algometer: