Fig 5. Theoretical error in pennation angle measurements.

(a) Measured pennation angle, (b) measurement error and (c) relative measurement error as a function of departure of the image plane from perpendicular to the aponeurosis. The analytical expression used to calculate the curves is provided in S1 Text. Note that pennation angles are always overestimated on ultrasound images, unless the image was obtained perpendicular to the aponeurosis, in which case the pennation is measured correctly. The overestimation does not originate from misalignment between the image plane and fascicles, but solely from not having the image plane perpendicular to the aponeurosis. The curves for 6 values of the true pennation angle ranging from 0° to 50° are shown.