Fdx 50

Initially, within‐session reliability of PPTs was determined during the familiarisation to demonstrate stability of the measurements on acute, repeated recordings for the primary investigator. Participants were seated on the isometric chair with their feet off the ground, torso upright and arms relaxed in the lap supinated. Two measurements were taken on the right (exercised) and left (non‐exercised) quadricep (marked at 20 cm proximal to the base of the patella) and the right biceps brachi (10 cm proximal to the cubital fossa) as well as the left upper trapezius muscle (10 cm from the acromion in direct line with the neck) using a handheld digital algometer (FDX 50, Wagner Instruments, Greenwich, CT, USA). These measurements provide a comprehensive assessment of local and systemic hypoalgesia, including ipsilateral, contralateral and non‐homologous muscle groups, which allow for greater comparisons between studies, particularly to Hughes and Patterson (2020 (link)) who used the same sites. Pressure was applied manually with a 1 cm²‐diameter stimulation site at an ascending rate of approximately 1 kg force per second. Participants were instructed to verbally indicate when they first perceived the pressure stimulus as painful. An interval of 30 s was given between measurement sites and if the two measurements were >10% apart, then a third measurement was taken and the mean of the closest two values was used. Within‐sessions reliability of the PPTs can be seen in Table 1 below.

The PPT is a form of quantitative sensory testing that enhances the understanding of pain sensitivity and musculoskeletal pain [22 (link)-24 (link)]. Moreover, it has strong interrater reliability across multiple raters [25 (link)]. The PPT effectively depicts pain intensity and is widely applied in patients with KOA. PPTs will be measured using a portable machine (FDX50, Digital Force Gauge, Wagner Instruments) [26 (link),27 (link)]. The patient will be placed in a supine position. The variable collector will select four points for measurement: (1) 3 cm medial to the midpoint of the medial patellar border, (2) the center of the patella, (3) 3 cm lateral to the midpoint of the lateral patellar border, and (4) the belly of the deltoid muscle of the affected upper limb (Figure 3) [28 (link)]. The pain test needle will be moved slowly at 0.1 kg/s until the participant feels pain. The measurement will be repeated 3 times at each site with an interval of 25 s, and the average will be calculated [29 (link)].

A standard FDX 50 digital algometer (Wagner Instruments, Greenwich, CT, USA) was used for the study. This model of algometer is recommended for pain threshold and pain tolerance. Pressure Pain Thresholds (PPT) was defined as the amount of force required to produce a pain sensation distinct from pressure or discomfort, or otherwise; the point at which pressure transitions to discomfort or pain^{50 (link)}. The algometer consisted of a pressure gauge and a 1 cm² rubber piston tip with a digital display of force in 0.01 kgf increments. It was calibrated before each test^{51 (link)}.
The following points were tested:

Trigeminal nerve outputs (the output of the ophthalmic nerve (supraorbital nerve output (V1)), the output of the maxillary nerve (infraorbital nerve output (V2)), the output of the mandibular nerve (chin nerve output (V3)))^{52 (link)};

Two points on the masseter muscle (according to the Systematic Mapping of Pressure Pain Thresholds, two points were selected for MM—point number 7 (MM2) and point number 8 (MM1))^{53 (link)};

Three points on the temporalis muscle (according to the Systematic Mapping of Pressure Pain Thresholds, three points were selected for TA—point number 13 (TA1), point number 10 (TA2) and point number 7 (TA3))^{53 (link)};

One point on sternocleidomastoid muscle (in the middle course of the muscle at the height of C4 (SCM))⁵⁴ ;

Three points on the upper part of the trapezius muscle (points were determined in each subject in a straight line between the height of the seventh cervical vertebra and the lateral part of the shoulder process of the scapula, starting from the edge of the neck (UT 1) toward the shoulder process of the scapula at a distance of 2 cm from each other (UT2 and UT3))^{55 (link)} (Fig. 1).

Figure 1

Tested points on Pressure Pain Thresholds. TA—the temporalis muscle; MM—the masseter muscle; SCM—the sternocleidomastoid muscle; DA—the digastric muscle; UT—the upper trapezius; V1— the output of the ophthalmic nerve; V2— the output of the maxillary nerve; V3— the output of the mandibular nerve;

The study was conducted by an experienced physiotherapist (first author).