Pressure-pain thresholds (PPTs) occur at the minimum transition point when applied pressure (i.e., force) is sensed as pain (2). Pressure-pain threshold measures are used in clinical settings for determination of “hot spot” tenderness (3) and diagnosis of myofascial pain dysfunction syndrome and myofascial pain syndrome characterized by tender myofascial trigger points (8). Furthermore, PPTs have been used to assist in the diagnosis of hyperalgesia (6). Pressure-pain thresholds provide a quantified force reading of one's “tenderness” and, thus, are very useful in a variety of clinical situations. For example, body locations where unusually low force application elicits pain (possibly in relation to the contralateral body part) may be attributable to an underlying cause that may be hard to quantify by methods other than tenderness. By tracking tenderness levels by PPT, it may be possible to quantify recovery (and, thus, speed of recovery) of underlying problems or soreness levels.
The terms algometer and dolorimeter are used to describe instruments for pain sensitivity measures. The term algometer may imply pressure tolerance testing, the maximum amount of pressure one may endure and, by nomenclature, not indicate the purpose for PPT testing, the first point at which a pressure sensation is sensed as pain (1).
Many times, these devices are handheld and have a “maximum hold” function that displays the maximum pressure obtained in any one application. Most commonly, these devices have a 1-cm2 pressure application surface and display force readings in newtons or kilograms of force. It has been noted that the force application should be perpendicular to the body surface, and the rate should be constant at an approximate rate of 1 kg·cm−2·s−1 (2) or 10 N·s−1 (9) to increase reliability (5). Application of force at a faster rate may provoke a low false threshold reading (5).
Previous investigations have examined the characteristics of PPT algometry and found it to be reliable (4,9). However, many algometers only display the maximum force function, and application of force has not been thoroughly investigated. It was the purpose of this study to identify whether an algometer's maximum force reading is reliable and valid. This investigation aims to assess the algometer through construct validity. Most notably, this investigation aims to note whether it is possible for a practiced investigator to have a consistent rate of force application.
Experimental Approach to the Problem
The algometer tested (Wagner Force One Model FDIX 50™, Wagner Instruments, Greenwich, Conn) was handheld and had a 1-cm2 round rubber application surface (Figure 1). The device's resolution was to the 0.2 N, with 250-N capacity, and was set at a 1000-Hz sampling rate with a maximum hold reading function. The investigator was familiar with the device and had practiced using it for several weeks.
To calibrate the handheld use of the algometer, the investigator applied force with the algometer to the center of a force plate (Pasco 2141, Roseville, Calif; 500-Hz sampling rate). The output of both the algometer and the force platform were analyzed to determine their linearity, SEE, and correlation.
Set-Up and Procedure
The force plate sat on a rigid laboratory bench and was leveled. The investigator assumed a 126° elbow angle (as measured by a handheld goniometer) in the start position, holding the algometer. The investigator performed all trials with the right hand steadying the algometer perpendicular to the force plate while the left hand was placed on top of the device as the principal force producer (Figure 1).
Ten sets of 5 manual force application trials were performed to approximately 80 N. Then, 10 sets of 5 force application trials were manually applied to specified levels of force, 5 trials to each level: 20, 30, 40, 50, 60, 70, 80, 90, 100, and 110 N. The force plate was calibrated before trials and tared to zero after every set of 5 measures. There were approximately 10 seconds of rest between trials and 2 minutes of rest between sets.
The software programs SPSS 13.0 (SPSS, Inc., Chicago, Ill) and Microsoft Excel 2003 (Microsoft, Redmond, Wash) were used for data analyses. Pearson correlations were used to compare force plate readings with the maximum force algometer readings. The SEE of these correlations was 0.32 N.
To analyze the rate of force application, the slopes of the force-time curves were analyzed between 10 N and the maximum force of each trial. Ten newtons, a force well above the SEM, was set as an acceptable starting measuring point that force application had been initiated.
Average Pearson (r) correlations between the maximum force reading of the algometer and force plate were excellent in both the trials to 80 N (r = 0.990) and the incremental trials (r = 0.999). The application of force was reasonably constant with slopes averaging 6.8 ± 0.932 N·s−1. The SEE was 0.323 N.
Although the rate of force application was slower than the optimal rate described by others (2,9), the application rate was relatively consistent, thus leading the investigators to believe that practice with the device may lead to accurate and replicable application of force. It has been stated that painful regions have a decreased PPT (5); therefore, using an algometer may be a useful way to quantify pains and possibly track recovery/healing. It may be noteworthy for subsequent studies using algometers to take skinfold measures of sites where force is applied as a measure of the covariant, considering that force application to a solid surface or absorptive surface (e.g., skin) may be different.
The algometer tested had high reliability and validity values when compared with force plate readings. This supports previous research by Fischer (1), Ylinen et al. (9), and Nussbaum and Downes (7), demonstrating acceptable intraexaminer reliability of pressure rate application. Considering these results, it may be safe to claim that, with practice, the use of this algometer is reliable and valid.
When measuring any variable, it is vitally important that both the examiner and the instrument are consistent and valid. The results of this study suggest that, with some practice, an individual may be reliable in applying force with an algometer. The results further suggest that the device itself is valid when correlated to force plate measures.
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