The recorded ultrasound video was examined by the Camtasia software (Tech-Smith Corp, Okemos, MI, USA) to select three LTRs for latter analysis. Each LTR was tracked by the Lucas-Kanade optical flow (LKOF) algorithm with affine optic flow extension [12]. All analyses were performed using the Matlab® 2014 software (Mathworks Inc., Natick, MA, USA). For tracking muscle thickness, two ROIs (size 250 x 70 pixels) positioned on the superficial and deep aponeurosis were manually selected.