## 1 Introduction
Only two articles included in the review assessed tendon stiffness after running, one study found no difference in AT stiffness post running [23]. The other study found no significant difference in stiffness, however, it only looked at changes after a 6-min warm up jog and stretching [19]. Also noted in the [PERSON] et al. (2013) paper was the lack of studies evaluating the mechanical properties of the 'free' AT [10], with 'free' AT referring to the part of the tendon without any other attachment to either bony or muscular structures. This current study therefore aims to focus on the 'free' portion of the AT to address this gap in the literature. Although not directly assessing stiffness, research using ultrasound tissue characterisation (UTC) has assessed the structure of the human in vivo AT before and after exercise. The results of studies using UTC suggest there were changes in tendon structure and integrity following a bout of exercise, including a decrease in aligned tendon fibrils and an increase in the separation and waviness of fibrils, both of which returned to baseline over the following 72 h [25]. A decrease in the alignment of the tendon fibres may result in decreased tendon stiffness. Habitual running of long distances (>80 km/week for >3 years) results in a marked increase in the cross-sectional area of the AT of approximately 22% in comparison with a non-running control group [26]. This increase in cross-sectional area may be a compensatory mechanism as pathological tendons can compensate for significant areas of disorganised fibres by increasing thickness to reduce stress and maintain structural homeostasis to ensure adequate load bearing [27; 28]. Despite a large body of work surrounding the effects of habitual running, studies exploring the impact of a single acute bout of running on stiffness measures of normal, healthy human ATs in vivo remain scarce [10; 19]. Most research into tendon stiffness utilise direct methods of calculating stiffness using ultrasound to measure change in tendon length and a dynamometer to measure force. This can produce accurate results, but is time consuming, requires complex procedures and a lot of equipment and space to run. Hence, a quick, easy and non-invasive measure of tendon stiffness is required. This study proposes to measure shear wave velocity (SWV), a surrogate measure of stiffness, in the AT in vivo using shear wave elastography (SWE). The relatively recent introduction of SWE offers a novel way to quantitatively assess tendon stiffness by measuring SWV through a tissue, providing information on tissue stiffness [29]. In recent years, there have been several studies published using SWE to assess tendon stiffness [30; 31; 32; 33; 34] and many reporting the reliability and validity of SWE [33; 35; 36]. Despite this, the reported information on the many variables that potentially influence SWE measurements remain unclear [32]. It is necessary to understand normal variation in AT stiffness with relation to leg dominance [37], as ATs have been shown to be significantly different between dominant and non-dominant legs [38] having different mechanical properties attributed to different loading profiles of both legs during daily activity due to foot dominance [39]. No studies have examined differences in AT stiffness with leg dominance using SWE or the alterations in SWE measurements experienced within a healthy human AT in vivo, in response to the time of day or an acute bout of running. It is important to understand the normal variation in SWV measures within the AT in vivo and the influence of external loads on these values. Without such information, abnormal or clinically relevant values cannot be decided upon and accurate interpretation is not possible. The aim of this study is three-fold. Firstly, it will measure the stiffness of the AT using SWE in the morning (08:00 h), afternoon (12:30 h) and evening (17:00 h) to see whether any measurable differences are apparent dependent on time of day.