## 1 Introduction  
The mechanical properties of human tendons are likely to be influenced by factors known to affect elastic structures, including patterns of loading and unloading during the day. The scale and relevance of these variables to tendon structure and function remain relatively unclear [1] and there are few systematic reports on how the stiffness of a human tendon alters throughout the course of a day [2,3]. Periods of sleep, rest and activity throughout a day will cause altered loading and unloading at differing frequencies and intensities on tendons, which will impact their stiffness. The research available into the effect of time of day on tendon structures in vivo has been conducted using the patella tendon, with tendon stiffness estimated using an isokinetic dynamometer to measure force and B-mode ultrasound to measure length [2]. This research measured a decrease in tendon stiffness of 20.2 \(\pm\) 9.5% between the testing times of 08:00 h and 18:00 h [2]. No research has yet assessed the impact of time of day on tendon stiffness using shear wave elastography (SWE). The term mechanotransduction refers to the processes converting mechanical loading, such as exercise, into measurable cellular responses that may result in structural or functional change [4]. Exercise has been shown capable of altering both the structural and chemical makeup of human tendon by inducing increases in its cross-sectional area and increasing the concentration of metabolic enzymes to increase collagen turnover and prostaglandin production [5; 6]. Mechanical stimulus is also postulated to initiate changes within the extracellular matrix of tendon that results in a more damage resistant tissue with optimal force transmission properties [7]. The early adaptive responses to mechanotransduction can initiate longer term alterations in the mechanical properties of a tendon [8], however the majority of alterations brought about by mechanotransduction are noted as the result of repeated exercise (loading) programmes. This could be because the adaptation in the mechanical properties of tendons takes a long time, or the impact of acute bouts of exercise have not yet been extensively studied. Regardless, the exact influence of acute bouts of specific forms of exercise on the mechanical properties of healthy human tendon remains relatively unclear [1; 9]. Long term exercise training such as running may increase the mass, collagen content, ultimate tensile strength and load to failure of a tendon [5], but changes in relation to acute exercise bouts are less well understood. A systematic review of the immediate effect of exercise on AT properties was conducted in 2013 [10]. The conclusion of this paper was that acute bouts of exercise impact AT mechanical properties in a manner dependent on both mode and dose of exercise as the differences in tendon stress-strain characteristics, such as rate, duration and frequency, between different types of exercise will cause varying changes to the mechanical properties of tendons over time [10]. Of the papers included in the review by [PERSON] et al. (2013), 14 assessed AT stiffness with 12 of these using data obtained from ultrasonography and dynamometry [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. Whilst the remaining two studies also utilised ultrasonography, they used either a force platform or a force transducer to obtain measures of force [23; 24]. Six papers reported a decrease in AT stiffness following exercise, one reported a significant increase and the remaining seven reported no differences [10]. Of the articles that found a decrease in AT stiffness, four assessed the effect of stretching, whilst others assessed isometric or concentric muscle contractions and not specifically running. One study found a significant increase in tendon stiffness post 10 minutes of static stretching with stiffness calculated using ultrasonography and isokinetic dynamometry [18].