Background: Achilles tendon thickness has been associated with various diseases. Among imaging modalities, ultrasonography stands out because of its advantages in assessing Achilles tendon thickness. Previous studies typically recommend performing Achilles tendon ultrasonography with the patient in a prone position on an examination table. However, because it is not always possible or safe for each patient to assume the prone position, this study aimed to investigate the relationship between measurements obtained in the supine position and those obtained using the standard method.

Methods: A total of 180 healthy volunteers aged ≥18 years (70 female and 110 male; 360 Achilles tendons), with no history of disease or medication use, were included in the study. Achilles tendon measurements (thickness, width, and cross-sectional area) were obtained via ultrasonography in four different positions, including the standard method. The alternative methods included the supine position with the leg extended (Method 2), the supine position with the ankle resting on the opposite knee (Method 3), and the supine position with the leg held in the air at 90 degrees of hip and knee flexion (Method 4). Measurements were taken 2 cm distal from the Achilles tendon insertion on the calcaneus.

Inclusion Criteria:

• Healthy volunteers aged >18 years who agreed to participate in the study

Exclusion Criteria:

• Individuals with tenosynovitis, tendinitis, bursitis, tuberculous arthritis, rheumatoid arthritis, a history of Achilles tendon surgery, or tendon injuries.

The sample size for this study was calculated to include at least 180 volunteers, based on a moderate effect size, an error margin of 0.05, a power of 80%, and a correlation analysis approach.