## 1 Introduction  
It is common to participate in a warmup routine prior to athletic performance. The goal of this warmup routine is to improve performance while also minimizing injury [1]. Physiological changes resulting from a warmup routine often stem from increased body temperature, which can allow for greater muscle metabolism and fiber conduction velocity [2]. Physiological changes also seen during warmups include increased muscle blood flow, increase neural activity, and improved oxygen uptake [1, 2]. A variety of modalities have traditionally included a passive warmup like static stretching or more of an active warmup that is dynamic in nature and slowly increases in intensity [2]. In the last 20 years, passives warmups have changed to various soft-tissue modalities because of short-term weakness following static stretching [3, 4] and little evidence of the belief in the injury reduction effects of static stretching [5, 6]. Soft-tissue modalities prior to performance have consisted of manual massage, foam rolling, and percussion massage [7]. The biggest difference in these techniques is manual massage, as it varies drastically by practitioner and feedback [8], and has not demonstrated to have an effect on performance [9], while stretching, foam rolling, and percussion massage give the participant direct feedback and control over the modality. Due to the ease (one person) and availability of foam rolling (commercially produced) [10] and percussion massage [11], these have become popular choices for soft-tissue modalities. Static stretching involves the lengthening of a muscle until the point of discomfort or desired sensation level and holding that position for a specified amount of time [4]. Acute static stretching has been shown to increase range of motion and flexibility [12, 13, 14]. Staticstretching prior to performance may actually have a negative effect (\(\sim\)5%) on muscular performance [3; 4; 12; 13; 15; 16; 17], with little benefit outside of the increase ROM. Static stretching when combined with a myofasical release technique, such as massage or foam rolling, has resulted in an increased flexibility without the loss of muscular performance [14; 17; 18]. The development of diagnostic ultrasound has made it feasible to measure changes in soft tissue as measurements of muscle size (both in cross-sectional area and thickness), fascicle length [19], pennation angle [20], and echo intensity. Echo intensity has been used as a measure of muscle quality [21] and to measure increases in blood flow [22] and swelling [23]. There are mixed results as to whether stretching changes any ultrasound measurements, as a meta-analysis by [PERSON] et al. [19] concluded that stretching protocols produce trivial changes to fascicle length and muscle thickness with no changes in pennation angle or cross-sectional area. This analysis was carried out by looking at long-term stretching protocols, but when you start to investigate acute bouts of stretching, the research is much less and shows mixed results, as [PERSON] et al. [20] reported an increase in pennation angle accompanied by a decrease in vertical jump power, thus concluding that the change in pennation angle alters the muscle ability to produce force. [PERSON] et al. [24], however, found no difference in pennation angle, fascicle length, or muscle thickness following an acute bout of static stretching. Regarding stretching and echo intensity, there is very little research on this relationship. One study by [PERSON] et al. [25] reported that an increase in echo intensity following stretching is correlated with muscle elongation and shear modulus (muscle stiffness) [26]. As muscle stiffness increases, so does echo intensity. Echo intensity has been shown to be a valid way to measure increased blood flow to a muscle [22]. Since static stretching has been reported to increase blood flow following two minutes of stretching [27], the effect of stretching on echo intensity needs to be further studied.