## Discussion
This study however, demonstrated that a 30-min acute bout of running exercise resulted in a decrease in maximum anterior-posterior tendon thickness (mm) following an acute bout of exercise. This decrease in tendon thickness has also been shown in other research and hypothesised to be attributed to a loss of fluid from the tendon to the peritendinous space caused by the mechanical load inducing increased hydrostatic pressure [10]. This temporary 'dehydration' within the tendon could be a potential cause of differences found in SWV, as it may affect tissue density. Alterations in tendon mechanical properties including increases in stiffness may be the bodies response to a new level of loading and potentially aid in reducing tendon damage caused by mechanical fatigue [50]. An increased level of stiffness detected immediately after an acute mechanical load, may allow for less extension of the tendon [50] which potentially may help reduce macro-trauma risk. Previous research demonstrated that in young healthy ATs, an acute bout of eccentric exercise resulted in a significant increase in AT stiffness as measured with SWE [51]. Other studies however show that a single 30-min bout of running did not impact the stiffness of the AT and concluded that the mechanical properties of tendons remain constant throughout locomotion [23]. The research of [PERSON] et al. utilised the same number of participants as in this study, but required participants to complete a 30-min run at a set pace of 12 km/ph. This speed was selected as being representative of a recreational run, as all participants said they were recreational runners, therefore the speed of the run between these studies could have varied. In contrast to [PERSON] et al., this study utilised a self-paced run, asking participants to maintain an RPE level between 13-15 to ensure the run was the same subjective intensity for all participants. The mechanical properties of the ATs of each subject in the [PERSON] study were recorded both before and after the run during a series of hops, with tendon stiffness estimated using the traditional format using AT length data obtained using an ultrasound probe secured to the participants leg using bandaging tape and AT force as measured using force plates [23]. In contrast, this study utilised SWE. Reductions in tendon stiffness following unaccustomed acute bouts of exercise may increase injury risk [10], however the participants in this study all self-reported as being moderately active on a regular basis in activities that involved running, and who therefore should have been accustomed to this dose and manner of exercise used. This study is the first to use SWE to trace alterations in AT stiffness in vivo over the course of a day, the first to compare AT stiffness between dominant and non-dominant standing leg ATs and the first to use SWE to assess alterations in AT stiffness in vivo following an acute bout of running. As with all studies, it does carry some limitations including a saturation limit to the measures the Siemens ACUSON S3000(tm) HELX EVOLUTION Ultrasound System (Siemens Medical Solutions, USA) can obtain. SWVs above 10 m/s simply return a value of 'High'. Although there were very few 'High' values noted throughout data collection, it is not possible to say how fast the SWV was for these measures, therefore any measures returned as 'High' were discounted from the study. This study examined variation in the AT of 15 healthy subjects to assess leg dominance and time of day, based on the numbers used in previous literature (range 8-12 subjects) as a guideline [2; 3; 52]. The impact of these variables on SWE data has not been previously examined and therefore replicating the number of subjects from previous research was the best starting place. The impact of exercise on SWE measures has also not been previously studied and therefore it is not possible to complete an accurate power analysis. Therefore 12 participants were analysed, to match the number used in very similar research [23].