## Discussion  
This study is the first to trace the stiffness of the human AT in vivo over the course of a normal working day using SWE and to compare SWE results between dominant and non-dominant legs. This study also examined whether an acute bout of running for a period of 30 min leads to any significant alterations in SWV values experienced in the AT in vivo as measured using SWE. The main findings of this study advance our understanding of SWE by showing that measures taken on healthy asymptomatic ATs experience no significant alterations in the SWV (and hence stiffness) measured in either the dominant or non-dominant AT of participants. The data also demonstrates that healthy ATs do not experience significant alterations in SWV (and hence stiffness) throughout the day indicating that time of day does not need to be considered when performing repeated scans of the AT in varying clinic appointment times. Our data also provides clear evidence that in comparison to a control group, a significant increase in SWV was apparent immediately after a 30 min run. The implications of these results are that a prior bout of physical activity may initiate changes within the physical properties of the AT that could result in a significantly different SWV measure. A clinician should be aware of the possibility of obtaining a significantly increased SWV measure from the AT using SWE if 30 min of weight bearing exercise has been performed within the previous 6 h. The responses to longer periods of tendon loading on SWV, and its time course have not been investigated and may merit further enquiry. The purpose of assessing the impact of time of day on SWE measures was to examine whether time of day should be considered when performing repeated scans of the AT for diagnosis or during treatment. Although this study did not encompass a whole 24 h period, it does relate directly to times where most clinical assessment is likely to occur, providing direct significance for the clinical usage of SWE. This study examined the SWV experienced within healthy, asymptomatic ATs over the course of a working day using times similar to those used in previous research [2; 3], with measures taken at 08:00 h, 12:30 h and 17:00 h. This study explicitly controlled for out of the ordinary, intense/heavy exercise both before and during the testing period and allowed insight into stiffness changes throughout the day. The results of this study will be important when considering the use of SWE for both early diagnosis and monitoring of recovery, which would likely have a clinical impact on not only long-term rehabilitation but also return to activities, specifically in high-level athletes [33]. Previous research has reported decreases in tendon stiffness throughout the course of the day to the magnitude of 20.2% and 21% [2; 3]. It was hypothesised these decreases in stiffness were attributed to either hormonal changes or the action of general mobilisation throughout the day [2]. The results of this study however, were markedly different, with the individual changes showing no systematic change in stiffness and the magnitude of change in tendon stiffness was only approximately 2%. These changes were not statistically significant and therefore this study can report no change in tendon stiffness with time of day. The decrease in tendon stiffness noted above, between morning and evening, was only shown to be true when tendon stiffness was calculated at low force levels (calculated from the gradient of the tangent over force levels corresponding to 1205N). However, when tendon stiffness was calculated a high force levels (100% MVC), there was no significant change noted between morning and evening (\(p\) = 0.10). This finding was potentially attributed to the fact that when measured during MVC, the tendon was stretched to a position on its curve where stiffness is highest [2].