Background: It is important to highlight the advantages of ultrasound in assessing muscular and tendinous behavior due to its non-invasive nature and capacity for dynamic studies. However, evaluating tendons via ultrasound can be challenging given the complexity of anisotropic phenomena related to collagen fiber arrangement. This study aims to validate the reliability of fixed ultrasound compared to manual acquisition in measuring Achilles tendon thickness. Method: Twenty participants, six men and fourteen women, were recruited. Ultrasound was used to measure the Achilles tendon’s thickness at two specific points (4 and 6 cm from the calcaneal insertion of the Achilles tendon). The measurements were conducted by two examiners, one with previous experience and another without. Results: The measurements at 6 cm from the calcaneal insertion showed α = 0.996, α = 0.998 for measurements at 4 cm using manual acquisition, and α = 0.997 for measurements with fixed ultrasound at rest. For the weight-bearing and ankle dorsiflexion measurements, the reliability was excellent (α = 0.999 and α = 1.000). Conclusions: The findings demonstrated excellent reliability in the ultrasound measurements of the Achilles tendon’s thickness, even when performed by different evaluators and under load-bearing conditions. This study suggests the clinical utility of assessing anatomical structures under load, enhancing ultrasound’s applicability beyond the examination table. It is concluded that fixed ultrasound acquisition exhibits excellent reliability in measuring the Achilles tendon’s thickness, offering potential benefits for precise diagnosis of pathologies, planning surgical interventions, and reducing possible errors related to operator variability.
