# Difference in Stiffness between Biceps Brachii Muscle Bellies Using Shear Wave Elastography
## 4 Discussion  
The SWV of the short and long heads of the biceps brachii muscle was measured in both arms with the elbow at full extension and flexed at 30\({}^{\circ}\) and 60\({}^{\circ}\), with the aim of determining the difference in stiffness between the muscles and the effect of elbow angles. The difference in stiffness between the short and long head of the biceps brachii is due to their different attachment sites and functions. The SHB consistently displayed velocities \begin{table}
\begin{tabular}{c c} \hline \hline
**Muscle and Elbow Angle** & **ICC (95\% CI)** \\ \hline SHB at full extension & 0.64 (0.51–0.77) \\ SHB at 30\({}^{\circ}\) flexion & 0.69 (0.57–0.80) \\ SHB at 60\({}^{\circ}\) flexion & 0.67 (0.54–0.79) \\ LHB at full extension & 0.90 (0.85–0.94) \\ LHB at 30\({}^{\circ}\) flexion & 0.92 (0.87–0.95) \\ LHB at 60\({}^{\circ}\) flexion & 0.90 (0.85–0.94) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The ICC values with 95% confidence intervals for each combination of muscle belly and elbow angle. Figure 4: A boxplot distribution of the posterior predictive SWV (m/s) for each category of elbow position and muscle. lower than the LHB at all three elbow angles. Anatomically, the arm positioned directly by the participants side resulted in the stiffness of the LHB to be greater than the SHB. This is an important distinction to make as previous research has not reached a consensus on the stiffness of the bicep muscles of stroke survivors [3]. Such previous research approaches, however, have not addressed the separate muscle bellies nor assessed the biceps without distinction, and it can be theorised that this is a leading reason for the discrepancies. Future comparisons of clinical assessments, such as the Modified Ashworth or Tardieu Scale, to the SWV measurements should distinguish which muscle belly has been selected for measurement by elastography. In considering biomechanics, extending the elbow increases the length of the biceps brachii; however, one or both proximal origins might require shoulder extension in combination with elbow extension to reach maximum length. A recent paper by [PERSON] et al. [19] found that the stiffness of the biceps increases with humeral abduction. There is potential that altering the shoulder position may lead to less of a difference in stiffness between the two muscle bellies, though this requires specific investigation. The selected supine position with arms adjacent to the body is comfortable, relatively reproducible, and has good clinical feasibility. In particular, this position can be easily created in a hospital setting, where participants may be restricted to a bed, especially in the initial time period after a stroke. Consistent with previous research, the stiffness of the biceps muscle, both short and long heads, was greatest at full extension of the elbow joint. The estimated mean SWV decreased with flexion to 30\({}^{\circ}\); however, this trend was not consistently seen at the 60\({}^{\circ}\) elbow angle. The SWV of both the SHB and LHB at 60\({}^{\circ}\) elbow flexion demonstrated the largest standard deviations despite being lower in velocity. Concerning the analysis, the stiffness continued to decrease with increasing elbow flexion in 18 (47%) participants, as was hypothesized; however, in the remaining participants, the stiffness of the bicep muscle at 60\({}^{\circ}\) flexion was higher than the 30\({}^{\circ}\) SWV measurement. On further analysis, this finding was not specific to either gender, as seen in Figure 4.  
We theories that the muscle of the arm could have been activated inadvertently, increasing the stiffness. When the participant is in a supine position and the elbow is flexed to 60\({}^{\circ}\), the forearm and wrist could have felt less stable, requiring minor activation of the biceps brachii during the evaluation with the SWE.