# Difference in Stiffness between Biceps Brachii Muscle Bellies Using Shear Wave Elastography
## 1 Introduction  
Shear Wave Elastography (SWE) is a potential diagnostic imaging modality for determining the stiffness of muscles after a stroke. Accurate and early musculoskeletal rehabilitation has a greater chance of reducing post-stroke muscle complications; however, identifying those patients most at need of intervention can be challenging. Current clinical assessments for post-stroke spasticity, such as the Modified Ashworth Scale and Tardieu Scale, are subjective and ordinal scales [1] that are unable to differentiate between the neural and non-neural components [2] of altered muscle stiffness. Quantitative, reliable measurements of the muscle stiffness can be achieved by utilising an advanced ultrasound technology: Shear Wave Elastography (SWE). SWE is able to evaluate the passive muscle stiffness and detect changes that occur over time with good correlation to clinical assessments [3, 4]; however, more investigations are required before SWE can be integrated into the healthcare system. With SWE as a reliable diagnostic tool, stroke survivors may receiveearlier interventions or treatments more specific to their individual pathologies, resulting in improved limb function and better stroke recovery. Prior to applying the technology to stroke survivors, the stiffness of healthy muscles measured by SWE must be investigated in a range of settings and conditions. There are no current protocols for the application of SWE into the musculoskeletal system, in part due to the methodological inconsistencies in current research including participant positions [5]; however, trends are emerging such as the effect of joint angles. Muscles in a passive state will increase in stiffness when held in their lengthened position (by joint extension or flexion depending on the muscle biomechanics), as well as when actively stretched and actively contracted. This has been demonstrated by an increased Shear Wave Velocity (SWV) [6], shear modulus [7; 8; 9; 10], or Young's modulus [9] in recent studies. People who have experienced a stroke may not be able to maintain a fully extended elbow joint, thus understanding muscle stiffness measurements at multiple joint angles will allow stroke survivors to receive a diagnostic investigation regardless of their limited arm positions. Despite the constructive progress of research in musculoskeletal elastography, there has been limited investigations that take into account muscle morphology. The biceps brachii consists of two proximal origins, the short and the long head, and a distal insertion onto the radial tuberosity with an aponeurosis known as Lacertus fibrosis, which inserts on the superficial aponeurosis of the flexor muscles [11; 12]. The long head of biceps originates on the supraglenoid tubercle in the glenohumeral joint, extends horizontally into the rotator cuff interval, enters the bicipital groove, and sits underneath the transverse humeral ligament; the short head of the biceps' origin is on the coracoid process of the scapula [11]. While both the muscle bellies flex the elbow and supinate the forearm, the muscle of the long head of the biceps brachii (LHB) assists slight shoulder abduction. The muscle of the short head of the bicep brachii (SHB) instead assists in shoulder internal rotation, weak flexion, and horizontal adduction, and resists dislocation of the shoulder [12]. A recent paper by [PERSON] et al. [13] showed that the SHB and LHB respond differently to shoulder movements; however, no comparison of the SHB to LHB was commented on. The difference in origin and biomechanics of each bicep brachii muscle belly may create different levels of stiffness between the two. Previous unpublished research by the authors showed a difference in the SWV between the SHB and LHB at full extension and 30\({}^{\circ}\) flexion. This difference in muscle stiffness measured between the two bellies requires further investigation to confirm that muscle stiffness assessments should include consideration of muscle morphology.