Fig 2. Illustration of pennation angle.

The aponeurosis is represented by the grey manifold, and its tangent plane at point p ₀ is shown in blue, with its normal represented by the grey vector, n. For a fiber (red curve) initiated at p ₀ and tracked through point p ₁, the pennation angle is defined as the angle between the line segment defined by points p ₀ and p ₁ and the tangent plane.