Upper Trapezius Muscle Tonicity, Assessed by Palpation, Relates to Change in Tissue Oxygenation and Structure as Measured by Time-Domain Near Infrared Spectroscopy

Palpation is a diagnostic tool widely used by manual therapists despite its disputed reliability and validity. Previous studies have usually focused on the detection of myofascial trigger points (MTrPs), i.e., the points within muscles thought to have undergone molecular composition, oxygenation and structural changes, altering their tonicity. Time-domain near-infrared spectroscopy (TD-NIRS) could provide new insights into soft tissue oxygenation and structure, in order to objectively assess the validity and reliability of palpation. This pilot study aims at (1) assessing the ability of TD-NIRS to detect a difference between palpably normal and hypertonic upper trapezius (UT) muscles, and (2) to estimate the reproducibility of the TD-NIRS measurement on UT muscles. TD-NIRS measurements were performed on 4 points of the UT muscles in 18 healthy participants (10F, mean age: 27.6 years), after a physical examination by a student osteopath to locate these points and identify the most and least hypertonic. From TD-NIRS, the most hypertonic points had a higher concentration in deoxy- ([HHb]) (0.887 ± 0.253 μM, p < 0.001) and total haemoglobin ([HbT]) (1.447 ± 0.772 μM, p < 0.001), a lower tissue oxygen saturation (StO2) (−0.575 ± 0.286%, p < 0.001), and a greater scattering amplitude factor (AF) (0.2238 ± 0.1343 cm−1, p = 0.001) than the least hypertonic points. Moreover, the intraclass correlation coefficient one-way random-effects model (ICC (1,1)) calculated for each TD-NIRS parameter and for each point revealed an excellent reliability of the measurement (Mean ± SD, 0.9253 ± 0.0678). These initial results, showing that changes in TD-NIRS parameters correlate with changes in muscle tonicity as assessed by palpation, are encouraging and show that TD-NIRS could help to further assess the validity of palpation as a diagnostic tool in manual therapy.