M-mode ultrasound is a reliable and valid instrument for assessing muscle activity. However, it has not been studied in any of the muscles belonging to the shoulder joint complex, particularly in the infraspinatus muscle. The aim of this study is the validation of the infraspinatus muscle activity measurement protocol with the M-mode ultrasound in asymptomatic subjects. Sixty asymptomatic volunteers were evaluated by two physiotherapists who were blinded; each of them carried out three measurements with the M-mode ultrasound in infraspinatus muscle and analysed the muscle thickness at rest and contraction, velocity of muscle activation and relaxation and Maximum Voluntary Isometric Contraction (MVIC). Intra-observer reliability was significant in both observers, showing good thickness at rest (ICC = 0.833–0.889), thickness in contraction (ICC = 0.861–0.933) and MVIC (ICC = 0.875–0.813); moderate in the activation velocity (ICC = 0.499–0.547) and relaxation velocity (ICC = 0.457–0.606). The inter-observer reliability also had good thickness at rest (ICC = 0.797), thickness in contraction (ICC = 0.89) and MVIC (ICC = 0.84); poor in relaxation time variable (ICC = 0.474) and not significant at the activation velocity (ICC = 0). The muscle activity measurement protocol of the infraspinatus muscle measured with M-mode ultrasound has been found to be reliable in asymptomatic subjects, for both the intra-examiner and inter-examiner.

Background/Objectives: Aging is associated with physiological changes that impair thermoregulation and may increase vulnerability to thermal stress. Body composition, particularly adiposity and skeletal muscle mass, may influence skin temperature distribution and thermographic asymmetry; however, evidence in older adults remains limited. This study aimed to characterize thermographic asymmetry patterns in older adults and examine their association with body composition parameters potentially influencing thermoregulatory function during aging. Methods: A cross-sectional study was conducted in 127 community-dwelling older adults from Burgos, Spain. Body composition was assessed using multi-frequency bioelectrical impedance analysis (InBody S10), including the body fat percentage (PBF), appendicular skeletal muscle mass index (ASMI), phase angle, and extracellular water-to-total body water ratio (ECW/TBW). Infrared thermography was performed using an FLIR E6390 thermal camera (FLIR Systems Inc., Wilsonville, OR, USA), and thermographic images were analyzed with ThermoHuman software version 3.0. Variables included global temperature, trunk thermal asymmetry, upper-limb thermal asymmetry, mean absolute thermal asymmetry, the Thermal Recovery Index (TRI), and the Symmetry Assessment Parameter (SAP). Sex differences, Spearman correlations, and multiple linear regression analyses were performed. Results: Women presented significantly higher body fat percentage and greater thermographic asymmetry than men (p < 0.05). Body fat percentage was positively associated with upper-limb thermal asymmetry (ρ = 0.229, p = 0.015), mean absolute thermal asymmetry (ρ = 0.280, p = 0.003), TRI (ρ = 0.250, p = 0.008), and SAP (ρ = 0.276, p = 0.003). In contrast, the ASMI, phase angle, and ECW/TBW ratio showed no significant associations with thermographic asymmetry indices. In the adjusted regression analysis, body fat percentage showed a statistically significant coefficient for mean absolute thermal asymmetry; however, the overall model was not statistically significant and explained only 5% of the variance (R² = 0.050, p = 0.144). Conclusions: Body fat percentage showed modest associations with several thermographic asymmetry parameters, whereas skeletal muscle mass showed limited relationships with thermographic outcomes. Given the weak magnitude of the correlations and the limited explanatory capacity of the regression model, these findings should be considered exploratory. Body composition may represent one of several factors associated with thermographic variability in older adults.