## 4 Discussion  
In an ex vivo model of legs of lambs, it was observed that the application of a continuous infusion resulted in less needle force generated on the epineurium compared to that of the bolus injection. The difference in the magnitude of the force applied was found to be at the maximum at a 60-degree angle. Upon analysis, it was found that continuous infusion reduced the frictional forces, thus leading to the smooth flow of the injected substance. On the other hand, the bolus injection had to overcome the resistance of the surrounding tissue in order to initiate flow, resulting in a comparatively greater force being applied on the epineurium. The infusion technique has a principal advantage over the bolus technique in that needle insertion and fluid injection are linked together. This technique is similar to the continuous hydrolocation technique used in clinical practice to detect circumneural spaces, where sustained pressure is applied to the syringe. However, in the bolus technique, the times between needle tip positioning, the initiation of injection, and the time to peak pressure are different. Our research has shown that the mean (SD) time between the initiation of injection and the attainment of peak pressure is 2.8 (1.0) seconds. It is important to note that the delay may be such that the needle may be removed before the peak pressure response has been attained (Figure 2a,e). This pressure-monitoring facet has not been emphasised previously, but has important implications for clinical practice. If the pressure response is delayed sufficiently during injection, then an anaestisologist is powerless to intervene and prevent potential damage from excessively high pressures. Therefore, it is crucial to ensure that pressure monitoring is performed accurately and promptly to avoid potential harm to the patient. During our injection experiments, we observed an interesting anomaly, which involved a sudden rise in pressure before the force of the injection at a 60-degree angle. This phenomenon occurred because the cutting edge of the needle was more perpendicular to the tissue surface at this angle, requiring a greater force to penetrate the tissue. As a general principle, we found that the greater the angle of the needle to the perpendicular fibres and the greater the stiffness of the tissues are, the greater the resistance during tissue penetration is. This increased resistance made the needles more likely to deflect and bend, and the injected liquid was more likely to flow retrograde along the path of least resistance into compliant adipose tissue. This research has identified a wide range of forces and pressures that are measured in muscles when subjected to a gentle touch and the indentation of the epineurium. This variation in force can be attributed to the needle tip position and is consistent with the previous studies conducted on pigs [6] and soft embalmed cadavers [10,21]. However, the standard ultrasound technology used in medical procedures does not provide a sufficient  
Figure 5: * shows a comparison of the time (s) difference from injection to peak pressure, focusing on the impact of tissue location and angle during injection. Image (**a**) precisely displays the effect of gentle touch on the epineurium, epineurium indentation, and muscle, while image (**b**) examines the difference in time involving needle insertion angle and injection techniques with notable significance at 60\({}^{\circ}\). resolution to accurately determine the needle tip's precise location. When gently touching the epineurium, the needle tip may partially reside within the epineurium and partly face the circumneural fascial fibres, making it difficult to determine the exact location. This scenario makes it highly likely that any injection will take the path of least resistance and flow around the nerve. Upon indentation of the epineurium, the needle tip may partially penetrate the epineurium, making the injection intraneural, but not visible via ultrasound.