Our results demonstrated a reduction in tibial nerve excursion (3.00 mm) compared to previous cadaver based studies [8, 9]. Coppieters et al.  reported a mean longitudinal tibial nerve excursion of 9.5 mm as the ankle was moved from 40° plantarflexion to approximately 15° dorsiflexion during a straight leg raise. The authors reported that excursion was quantified via a digital Vernier calliper with reference points being a suture around the nerve and a fixed marker screwed into cortical bone. Using a similar methodology for measurement of nerve excursion Alshami et al.  reported 6.9 mm of longitudinal tibial nerve excursion during a dorsiflexion eversion test, in which the ankle was moved from 0° to 17.1° dorsiflexion, then everted 10°. The major limitation of previous cadevar studies related to the experimental preparation in particular that the Achilles tendon was transected to obtain a physiological range of motion. Therefore, tibial nerve excursion maybe altered by the total ankle range of motion and eversion of the rearfoot as reported in previous studies [8, 9].
The differences in nerve excursion may also be attributable to the embalmment process and its potential effects on neural tissue elasticity. Cho et al.  speculated that there may be less tissue elasticity in fresh frozen cadavers compared to living tissue. Coppieters and Alshami  noted that limited information is known about the effects of freezing and thawing or the impact of embalmment on the mechanical properties of nerves, in particular elasticity. If the embalmment process had a major effect on elasticity and ultimately excursion we would have expected the current results to demonstrate similar levels or more excursion when compared to the cadaver studies. This factor in combination with the limitations of the experimental setup of the cadaver based models may indicate that tibial nerve excursion is influenced in large part by the position of surrounding joints.
The positioning of the knee and hip may have influenced the degree of nerve excursion in the current study. The movements of hip flexion, knee extension and dorsiflexion of the foot increase tension or pre-tension the sciatic and tibial nerve [8, 29]. Alshami et al.  reported strain in the tibial nerve at the tarsal tunnel is lowest when the positions of the hip or knee do not pretension the sciatic or tibial nerve. Shacklock  defined the dynamic relationship between neural tension and neural excursion as the following: in the early part of joint movement the primary event in the nervous system is taking up the slack. In mid-range, the slack is absorbed and the rate of neural sliding increases. Then later, in joint movement, the slack and capacity of nerves to slide has been consumed, causing tension in the nerves to rise. As demonstrated in the current study, mean nerve excursion was lower when measured in the weight-bearing position with the knee extended; preloading the tibial nerve, reducing the capacity of the nerve to slide.
We found longitudinal excursion of the tibial nerve demonstrated excellent intra-rater reliability. Based on a SEM of 0.28 and 0.22 mm for session 1 and 2 respectively, the SRD percentage was calculated and revealed a change in length of greater than 27% (0.84 mm) and 22% (0.67 mm) respectively for session 1 and 2 would be required to be 95% confident that a real change had occurred. Therefore, tibial nerve excursion of greater than 0.84 mm can be considered real change. The ankle range of motion was standardised to a total range of 30° (20° plantarflexion to 10° dorsiflexion) by the measurement platform. Consequently, results displayed low SEM but relatively high SRD values. These findings indicate the benefits of using the measurement platform with the foot in a standardised position to obtain tibial nerve measurements at the ankle for future investigations.
The impact of foot posture was not investigated, which may affect the degree of tibial nerve excursion. Differing foot postures such as a pronated [flatfoot] or supinated [high-arched] foot type may have an influence on the mechanical functioning of the tibial nerve. We can speculate that foot pronation may have a pre-tensioning effect on the nervous system similar to that of the knee extension and hip flexion, potentially reducing the capacity of the tibial nerve to slide longitudinally. A pronated foot type has been associated with increases in pressure in the tarsal tunnel, creating the potential for a compression neuropathy . Specifically the valgus position of the rearfoot associated with a pes planus foot posture is postulated to increase stretch on the tibial nerve, placing increased compressive force on the contents of the tarsal tunnel . Daniels et al.  conducted in-vitro investigations, concluding that tibial nerve tension was increased in a pes planus foot posture, and postulating that there may be a link to the development of the compressive neuropathy tarsal tunnel syndrome.
As indicated by the SRD there was a relatively high degree of error involved in the measurement of longitudinal nerve excursion. This error maybe explained by the limitations of ultrasound imaging technique employed. The success of ultrasound imaging is operator dependent with the placement of the probe in the exact same position and anatomical plane a potential source of sonographic artefact . The tibial nerve a three-dimensional structure was imaged in a two-dimensional plane, with the nerve essentially representing a thin line at an arbitrary angle in the body. To avoid the effect of anisotropy, the transducer was however kept perpendicular to the nerve throughout the measurement process to avoid creation of this artefact .