Association between the digit ratio (2D:4D) and muscle viscoelastic properties in women

The aim of this study was to examine the association between the second-to-fourth digit ratio (2D:4D), an indirect marker of prenatal sex hormone exposure, and the viscoelastic properties of skeletal muscle in women.

This cross-sectional study included 44 women. The 2D:4D ratio was measured using a digital caliper. Skeletal muscle viscoelastic properties (muscle tone, stiffness, and elasticity) were evaluated in the biceps brachii, rectus femoris, and erector spinae muscles using the MyotonPRO device. Relationships between the 2D:4D ratio and muscle viscoelastic properties were assessed using Pearson correlation analysis, while multiple linear regression identified independent associations after adjustment for age and BMI.

Both the right and left-hand 2D:4D ratios were negatively correlated with biceps brachii (p = 0.006 and p = 0.019, respectively) and rectus femoris stiffness (p = 0.003 and p < 0.001, respectively). No significant correlations were observed between the 2D:4D ratios and muscle tone or logarithmic decrement (elasticity) (p > 0.05). Multiple linear regression analyses further demonstrated that both right- and left-hand 2D:4D ratios were independent predictors of biceps brachii (p = 0.006; p = 0.024, respectively) and rectus femoris stiffness (p = 0.008; p = 0.001, respectively) after adjustment for age and BMI.

Our findings indicate that lower 2D:4D ratios are independently associated with greater stiffness of the biceps brachii and rectus femoris muscles. The 2D:4D ratio may therefore provide a potential biological indicator of skeletal muscle stiffness.