Abstract— We have developed an office based vector tissue Doppler imaging (vTDI) that can be used to quantitatively measure muscle kinematics using ultrasound. The goal of this preliminary study was to investigate if vTDI measures are repeatable and can be used robustly to measure and understand the kinematics of the rectus femoris muscle during a drop jump task. Data were collected from 8 healthy volunteers. Vector TDI along with a high speed camera video was used to better understand the dynamics of the drop jump. Our results indicate that the peak resultant vector velocity of the rectus femoris immediately following landing was repeatable across trials (intraclass correlation coefficient=0.9).The peak velocity had a relatively narrow range in 6 out of 8 subjects (48-62 cm/s), while in the remaining two subjects it exceeded 70 cm/s. The entire drop jump lasted for 1.45 0.27 seconds. The waveform of muscle velocity could be used to identify different phases of the jump. Also, the movement of the ultrasound transducer holder was minimal with peak deflection of 0.91 0.54 degrees over all trials. Vector TDI can be implemented in a clinical setting using an ultrasound system with a research interface to better understand the muscle kinematics in patients with ACL injuries. Keywords-Ultrasonography; vector Doppler; tissue motion; drop jump; anterior cruciate ligament injury; osteoarthritis; musculoskeletal imaging; signal processing; muscle motion I. INTRODUCTION NTERIOR cruciate ligament (ACL) tear in the knee is one of the most common musculoskeletal injuries [1]. Athletes who are involved in intense pivoting activities are at higher risk of ACL injury [2]. ACL injuries have significant public health implications as it takes a long time to rehabilitate, presents high costs, and often leads to long-term morbidity. There is a high prevalence of knee osteoarthritis (OA) post ACL injuries [3] with 45% to 70% of individuals presenting early onset of OA [4]. Early onset of OA in young individuals may result in adverse health conditions and could limit activities of A. Eranki and S. Sikdar are with the Department of Electrical and Computer Engineering, George Mason University, Fairfax, VA, USA 22030. N. Cortes is with the School of Recreation, Health and Tourism, George Mason University, Manassas, VA, USA 20110. Z. Gregurić Ferenček is with the School of Physics, Astronomy, and Computational Sciences, George Mason University, Fairfax, VA, USA 22030. J. J. Kim is with Northern Virginia Orthopedic Specialists, Manassas VA, USA 20110 Corresponding author: S. Sikdar (phone: 703-993-1539; e-mail: ssikdar@gmu.edu). daily life (ADL). Although, ACL injuries are highly prevalent, the underlying musculoskeletal mechanisms that lead to OA are poorly understood. Quantitative measurements of muscle kinematics and function can enhance the understanding of the contributing factors leading to the early onset of OA. Conventionally, joint kinematics and kinetics using 3D motion capture systems, ground reaction force and electromyography are used to understand the change in gait in these patients. Studies using 3D motion capture systems have shown increased frontal plane knee adduction moment during gait [5], and sagittal and transverse plane knee movements during drop jumps [6]. Others have also shown a possible relationship between quadriceps weakness and knee OA in all knee joint compartments [7]. Recently, muscle morphology during static trials using magnetic resonance (MR) imaging has also been studied [8]. However, the underlying muscle kinematics preventing adequate joint protection during dynamic tasks following ACL reconstruction remains poorly understood. Ultrasound imaging is an attractive alternative for measuring musculoskeletal dynamics and function during dynamic tasks, because it can enable direct measurement of muscle kinematics. Concurrently quantifying biomechanics and muscle function during dynamic activities can provide a unique opportunity to elucidate the mechanisms leading to early onset of OA post ACL injury. Previously, real-time B-mode ultrasound has been used for dynamic imaging of gastrocnemius muscle motion [9-10]. These methods are not suitable to measure fast contraction velocities during highly dynamic tasks (e.g., drop jumps) that maximally load the knee joint. A spectral Doppler ultrasound method could be suitable for fast moving scatterers; however, conventional duplex Doppler is angle-dependent and would introduce errors in measuring muscle velocities due to continuous changes in insonation angle throughout the contraction-relaxation phase of the muscle during dynamic tasks. This problem can be overcome by estimating the velocity of the object using two different ultrasound beams steered at different angles, a method known as vector Doppler. We have previously developed vector tissue Doppler imaging (vTDI) to quantitatively measure tendon and muscle kinematics in-vivo during dynamic tasks [11-12]. To investigate the repeatability and potential applicability to ACL deficient individuals of the vTDI system in measuring musculoskeletal velocities, we   Real-Time Measurement of Rectus Femoris Muscle Kinematics during Drop Jump using Ultrasound Imaging: A Preliminary Study Avinash Eranki, Nelson Cortes, Zrinka Gregurić Ferenček, John J. Kim, and Siddhartha Sikdar A