The gastrocnemius-Achilles tendon (AT) complex is the largest and strongest muscle-tendon in the human body (O'Brien, 2005). The complex is subject to changes especially the mid-portion of AT depending to the tensile loads during its elongation or contraction, make it susceptible to overuse injuries. According to statistics, the incidence rate of Achilles tendinitis is 0.2% in the general population (Romero-Morales et al., 2019). Pre-exercise static stretching (SS) is commonly performed to reduce muscle stiffness to enhance functional range of motion (ROM) and avoid AT injuries (Rosario and Foletto, 2015). SS has also typically been used in clinical practice, especially in rehabilitation training (Santos et al., 2019). SS is a common method for decreasing the muscle-tendon unit (MTU) stiffness and passive torque to prevent or improve limited mobility (Nakamuraet al., 2011). Thus, stretching is a recommended intervention, whether in healthy people or patients, whether in daily exercise or in clinical rehabilitation training.
Understanding of the effect of SS on the architecture and mechanical properties of the gastrocnemius-AT complex has been a topic of interest among clinicians and researchers. This musculoskeletal system is composed of MTUs, whereas each MTU is divided into a passive component and an active component (muscle). Onefunction of the passive component of MTU is to store and release elastic energy (tendons--proximal and distal to the muscle), while the function of the active component is force generation (muscle) (Cenni et al., 2018). Thus, the capacity to contract muscle and the proper muscle force transmission and movement generation are influenced by the operating ratios between muscle and tendon (Cenni et al., 2018). Recently, the behaviours of the muscle-tendon complex were evaluated using ultrasound shear wave elastography (SWE). Our previous studies demonstrated that SWE is a valid and reliable tool to estimate the elastic properties of tendon (Zhang and Fu, 2013; Zhang et al., 2014) and muscle (Zhang et al., 2017). In addition, a positive significant correlation was found between the elastic modulus of muscle from SWE and the elastic modulus from a material testing system (Eby et al., 2013). Similarly, grey-scale ultrasound has the ability to estimate architectural parameters of muscles, such asfascicle lengths, and muscle thickness (Hodges et al., 2003). As described, accurately quantifying the morphology and stiffness of AT and gastrocnemius by SWE and grey-scale ultrasound before and after a SS may provide a more comprehensive understanding about the physiological characteristics of MTUs.
Previous studies estimated the passive behaviour of the gastrocnemius by measuring the stiffness of the proximal 30% or the middle region of the lower leg length of the MG and LG during passive dorsiflexion or stretching using SWE (Akagi and Takahashi, 2013; Ce et al., 2015; Hirata et al., 2016; Payne et al., 2018). as well as the passive behaviour of AT by measuring in the proximal or distal region of tendons (Kato and Fukunaga,2010; Nakamura et al., 2013; Ce et al., 2015; Chiu et al., 2016; Wren et al., 2003). Interestingly, Porta et al (2014) evaluated the proximal, middle, and distal portions of patellar tendons in healthy subjects by ultrasound. They found that the proximal portion of the patellar tendon was significantly stiffer than the distal portion. However, the acute effect of SS on passive stiffness among the different regions of gastrocnemius and AT remains unclear. Contrasting scenarios about the outcome of SS on the gastrocnemius muscle-AT complex stiffness have been reported, but the values of shear modulus of the complex were measured in only one muscle region or one tendon region in those reports. However, the local stiffness does not represent whole tissue stiffness. In terms of anatomy, the muscle fibre of the gastrocnemius has a complex multipennate arrangement (O'Brien, 2005), whereas the twisted structure of the AT rotates as they descend, but does not run parallel to each other (Edama et al., 2016). Also, tendon stiffness is expected to vary along the tendon length, whereas muscle stiffness varies along the muscle length. Thus, it is important to evaluate different regions within the same muscle and tendon to provide a better understanding of stiffness variations within the gastrocnemius-AT complex. Beyond a more in-depth description of the gastrocnemius-AT complex biomechanics, a better understanding about the intramuscle distribution and intra-tendon distribution of passive stiffness in vivo would allow the medical researcher and clinical worker to optimize treatment strategies by focusing attention specifically on the stiffer regions.
The objectives of this study were as follows: (1) to investigate the acute effects of SS on the shear modulus of the MG, LG and AT for different regionsby determining three proximal-distal regions for MG and LG (distal, mid, and proximal regions on the muscle) and three regions for the AT (0, 3, and 6cm above the calcaneal tuberosity); (2) to examine the changes on ROM before and after a SS;and (3) to investigate the change of thickness of the AT and fascicle length of the MG and LG before and after a SS.
This study was approved by the Human Subjects Ethics committee of the Clinical Medical College of Acupuncture, Moxibustion and Rehabilitation (GZUCM2017-003-01). The study abided by the principles of the Declaration of Helsinki. Before the commencement of the study, all recruited subjects were fully informed of the study purposes, experimental procedures, rights of volunteers, and safety of SWE by an experimental statement, and all signed the informed consent.
Thirty healthy subjects [15 males and 15 females; males: age: 21.33[+ or -]2.72 y; height: 1.73[+ or -]0.07m; weight: 69.07[+ or -]14.73 kg; body mass index (BMI): 22.99[+ or -]4.96 kg/[m.sup.2]; females: age: 21.13[+ or -]2.17 y; height: 1.62[+ or -]0.05m; weight: 52.93[+ or -]12.63 kg; body mass index (BMI): 20.19[+ or -]4.13 kg/[m.sup.2]] were recruited for this study. Participants were not trained, recreationally active or sedentary. Each participant was asked to avoid longer than usual walking, standing, or running for a week prior to imaging.The location of the study was the Department of Ultrasound Imaging of Luoyang Orthopaedic Hospital of Henan Province.The inclusion criteria were that all subjects were healthy and couldfollowthe instructions of the operator.The exclusion criteria were as follows: neuromuscular disease, tendon rupture, musculoskeletal injury of lower-limb, current use of corticosteroids.
Experimental setup and protocol
This experiment was an observational study. Before (PRE) and immediately after (POST) 5-minute SS for the AT and the gastrocnemius in the dominant leg, the shear elastic modules of AT and the gastrocnemius, fascicle length of the MG and LG, thickness of the AT, and ROM of the ankle joint were measured. The dominant leg of subjects was determined by kicking a ball.
Equipment and parameter settings
All ultrasound examinations were performed by the ultrasound SWE system (Aixplorer Supersonic Imagine, France) with a 50-mm linear-array transducer (SL15-4 Supersonic Imagine, France). The instrument used the default standard musculoskeletal (MSK) presets.The upper limit (800 kPa) of the system was adopted for measurement of the muscle-tendon elastic modulus. Other settings of the SWE systems were as follows:opacity was 85%, and depth of the B-scan ultrasound was 3.0 cm. The B-mode ultrasound was performed to assess the fascicle length in the middle regions of the MG and LG, and the thickness of the AT, whereas SWE was used to assess the stiffness of the MG, LG and the AT. In the SWE examination, the Q-box diameter of the AT was defined by the thickness of the AT (Zhang and Fu,2013). The Q-box diameter of the MG and LG were set as 5 x 5 mm, and the size of the regions of interest (ROIs) was 10 x 10 mm (Saeki et al., 2017).
Before stretching, the subject was asked to wear loose-fitting short pants. In addition, the subject was explicitly asked to refrain from strenuous exercise for 48 hours before the experiment (Payne et al.,2018). To reduce experimental errors, the room temperature was...