## 4 Discussion
### Reliability of the Muscle Hardness Measurement Using the Introduced Novel Probe Positioning  
The present study introduced a novel probe placement protocol on the GM, TA and RF muscles, and revealed a moderate to excellent intra- and inter-rater reliability. We followed the protocol of previous studies regarding the probe placement procedure [16; 23; 24]. Upon identified the targeted muscle, one step forward has been taken by marking the edges of the muscle and mid-point between the edges. In this way, we made sure the probe was in the central area and positioned along the muscle contraction direction. In a previous study, the intra-rater reliability of GM at MRL condition was reported to be moderate to good (ICC = 0.68) [24]. Another study reported moderate to good inter-rater reliability for the muscle measurement (ICC: TA = 0.616, RF = 0.679, GM = 0.728) [27]. Our probe placement procedure achieved better intra-rater (ICC: GM_RST = 0.696, GM_MRL = 0.862) and inter-rater reliability (ICC: TA_RST = 0.871, TA_MRL = 0.851, RF_RST = 0.732, RF_MRL = 0.711, GM_RST = 0.707, GM_MRL = 0.895) with the increase ranging from 4% to 26%. The reliability of GM was found to be the lowest among the three target muscles. This might be because the curved shape of the GM muscle makes the ultrasound probe slides easily on the GM muscle during the muscle contraction. This made it difficult to maintain the same probe location, resulting in the lowest ICC values of the GM muscle. According to our experience throughout the experimental procedure, we understand that the standardization of the probe placement procedure prior to any measurement of shear modulus is important, in  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & **TA\_RST** & **TA\_MRL** & **RF\_RST** & **RF\_MRL** & **GM\_RST** & **GM\_MRL** \\ \hline
**Intra-rater** & ICC (3,1) & 0.920 & 0.938 & 0.786 & 0.781 & 0.696 & 0.862 \\  & \(p\)-value & \textless{}0.001 & \textless{}0.001 & 0.011 & 0.009 & 0.030 & 0.001 \\
**Inter-rater** & ICC (3,1) & 0.871 & 0.851 & 0.732 & 0.711 & 0.707 & 0.895 \\  & \(p\)-value & 0.001 & 0.002 & 0.019 & 0.025 & 0.026 & \textless{}0.001 \\ \hline \hline \end{tabular}  
* TA\_RST: tibialis anterior at resting TA\_MRL: tibialis anterior at mid-range lengthened; \(\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\order to achieve accuracy and reliability. It is clear that shear modulus gets easily affected by changing intramuscular properties and dynamics, such as fiber orientation [8] and increasing intramuscular pressure during either reflex or voluntary muscle contraction [14]. In addition to this sensitivity caused by changing internal muscle dynamics, body position is another important factor which needs to be considered during the measurement. One of the body positions that can affect the measurement is the external and internal rotation of the lower extremity in supine position that can be observed anteriorly. This may cause a shift of the muscle beneath the ultrasound probe. This particular artifact needs to be considered especially when a probe is placed over TA muscle. Previous works took consideration of the ultrasound probe location along muscle contraction direction of the lower extremity; however, the probe location between the muscle boarders was usually undermined during experimental procedure [12; 16]. We believe that our defined probe placement procedure is an improvement on standardizing the measurement technique using SWE. The reliability of this method was investigated and showed a moderate to excellent intra-rater and inter-rater reliability. This novel probe placement procedure provides more detailed guidance to the SWE operator, which can help to eliminate bias during the SWE measurement.