It is well established that training of the "core" muscles of the trunk can contribute to the stabilization of the trunk, alleviate the symptoms of chronic low back pain, prevent injuries and possibly maximize sport performance (Akuthota and Nadler, 2004; Leetun et al., 2004; Nesser
et al., 2008; Richardson and Jull, 1995). Hence, specific core stability exercises are often incorporated not only in training programs of people who seek rehabilitation, but also in elite athletes' daily training sessions (Wirth et al., 2017). The "core" of the human body has been described as a complex of muscles, osseous and ligamentous tissues. It can be visualized like a box with its different sides represented by human parts; the abdominal muscles on the front, paraspinals on the back, pelvic floor and hips being the bottom and diaphragm the top cover of this construction (Akuthota and Nadler, 2004). Several muscles are responsible for trunk stabilization, yet each muscle has a different role when spine stability is required, due to their anatomical characteristics. For instance, transversus abdominis (TrA) and multifidus are local muscles with direct attachments to the spine. Hence, their contribution in lumbo-pelvic control is more obvious, even though it is not intuitive. TrA arises from thoracolumbar fascia, last six ribs and iliac crest wraps around the waist like a "corset" and finishes medially in linea alba (Bakkum and Cramer, 2014). When this muscle is activated it produces little to no trunk motion, however its contraction tends to reduce the circumference of the waist and thereby increase intraabdominal pressure and the tension in thoracolumbar fascia (Bakkum and Cramer, 2014). It has been reported that TrA is the first muscle which is activated when there is a lack of stability or a sudden limb movement, in order to prevent excess motion in the lumbar area (Bliss and Teeple, 2005).
Superficial muscles like the rectus abdominis (RA) are main contributors to trunk movement exercises. RA originates from the xiphoid process and 5th, 6th, 7th ribs covering the whole anterior abdominal wall and inserts in the pubic bone. RA's fascicles are usually interrupted by three to five tendinous intersections which add to muscle durability preventing rupture and support the muscle's biome-chanics (Rai et al., 2018; Yang et al., 2012). The main function of the RA is to flex the trunk on a fixed pelvis or to flex the pelvis on a fixed trunk (Bakkum AND Cramer, 2014). It has the ability to generate high torques through the thorax and pelvis and when contracting isometrically, it contributes to trunk stability (Lee, 2019).
Many exercises have been described as suitable for strengthening the trunk stability muscles. These include bridge exercises from supine, prone and side positions, bird-dog exercise and many other stability exercises with the use of Swiss balls, BOSSU balls, slings etc. (Bjerkefors et al., 2010; Bliss and Teeple, 2005; Marshall and Murphy, 2005; Saliba et al., 2010). It has been suggested that during core stability exercises, the activation of the RA ideally should be minimal (Czaprowski et al., 2014; Richardsonand Jull, 1995; Urquhart et al., 2005).This assumption has its foundation in two theories. First, "global" trunk muscles, such as the RA, are active overwhelmingly in most trunk flexion dynamic movements and hence, relaxation of this muscle could allow better activation of deep trunk muscles, such as the TRA (Richardson and Jull, 1995). Second, in order to contract the TrA muscle properly, pelvis and spine movement should be minimal, and, thus, the RA activity is low (Richardson and Jull, 1995; Urquhart et al., 2005). This is best achieved when performing an exercise known as abdominal hollowing or abdominal drawing-in maneuver (ADIM), which selectively activates the TrA and the lumbar multifidus but not the superficial muscles, like the RA (Richardson and Jull, 1995). In order to perform this technique correctly, one must draw their lower abdomen in, feeling that the navel is getting closer to the spine, while maintaining relaxed abdominal muscles, pelvis and spine (Richardson and Jull, 1995). Research studies have shown that during ADIM performance, TrA muscle's thickness increases significantly (Bjerkefors et al., 2010; Himes et al., 2012; Manshadi et al.,, 2011; Mew, 2009; Nagai et al., 2016; Saliba et al., 2010). Moreover, it has been proposed that improvement of TrA contraction thickness is achieved when using submaximal exercise loads (less than 30% of maximum voluntary contraction) (Hodges et al., 2003; McMeeken et al., 2004). Since local muscles contribute to spinal stability in a different way than global muscles, some researchers advocate that they should be trained separately (Hodges, 1999; Richardson and Jull, 1995). Others believe that the design of a core stability training program should follow the principles of progression, starting with simple tasks like the familiarization with proper hollowing execution and gradually incorporating more difficult exercises that recruit not only local muscles but also the superficial ones (Bliss and Teeple, 2005).
Early studies have examined the activation of deep trunk muscles with the use of intramuscular (Bjerkefors et al., 2010; Urquhart and Hodges, 2005) and surface electromyography (EMG) (Czaprowski et al., 2014). The former method requires the insertion of fine wire electrodes in the human body in order to detect the activation of certain muscles, such as the deep trunk muscles. Ultrasound (US) imaging allows a non-invasive visualization of muscle morphology at rest and during exercise (Hodges et al., 2003). This is because muscle architectural parameters (thickness, fascicle length and orientation) change from rest to contraction (Hodges et al., 2003). For deep trunk muscles, where visualization of fascicle length is difficult, changes in thickness from rest to exercise have been considered as being a valid measure of the result of muscle activation (Djordjevic et al., 2015; Ferreira et al., 2004; Hodges et al., 2003; Koppenhaver et al., 2009; McMeeken et al., 2004). In particular, many studies have shown there is a high correlation (r > 0.74) between US thickness of the TRA and EMG, with the relationship found to be either curvilinear (Hodges et al., 2003) or linear (Ferreira et al., 2011; McMeeken et al., 2004). Further, there is evidence that changes in US thickness during exercise can be used to discriminate patients with low back pain from controls (Djordjevic et al., 2015; Ferreira et al., 2011). For this reason, changes in thickness have been used to monitor the effects of specific exercises (Baek et al., 2012; Himes et al., 2012; Kim et al., 2017; Mew, 2009) or exercise programs (Cho, 2015; Gong, 2018; Yang et al., 2015) on abdominal muscle function. Due to very low levels of recruitment, changes in RA muscle thickness during core muscle exercises have rarely been examined; there are reports, however, that examined the effects of various trunk exercises on the thickness of RA and the other abdominal muscles (Kim et al., 2015) or they compared children with spasticity and typically developing children (Adjenti et al., 2018). RA muscle thickness has also been used to test the effects of exercise interventions on abdominal muscle function (Romero-Morales et al., 2018). Further, the reliability of US in the measurement of muscle contraction thickness has been extensively investigated in many studies (Gnat et al., 2012; Hides et al., 2007; Koppenhaver et al., 2009).
The majority of studies have focused on the importance of core stability training in alleviating the pain in people with non-specific chronic low back pain and increasing the sensory efficiency of soft tissues (Koppenhaver et al., 2009; Richardson and Jull, 1995; Teyhen et al., 2005). Even though this type of training was designed for individuals with back pain symptoms, it has become quite popular among elite athletes who wish to increase their performance (Sharrock et al., 2011) or to protect their lumbar spine from future injuries. The spread of core exercises in well trained athletes' training routines, has created the need for more challenging exercises and, hence, the existing drills have been modified in different ways in order to raise the difficulty level. These include supine bridge exercises at unstable surfaces (Saliba et al., 2010), exercises using slings and vibration (Gong, 2015), modified trunk curl-up exercises (Crommert et al., 2018) and sit-ups on BOSSU balls with added resistance (Saeterbakken et al., 2014). Among these, the back bridge exercise is a closed kinetic chain exercise which is widely used not only for increasing the muscular strength of the hip extensors, but also for improving lumbar stabilization as it induces the contraction of the abdominal muscles (Baek et al., 2012; Cho, 2015; Gong, 2018; Stevens et al., 2006; Yang et al., 2015). It is not however clear whether core muscle exercises can be classified based on their level of intensity. Himes et al. (2012) reported that TrA thickness does not increase when performing side-bridge exercises of increasing difficulty. In contrast, a bridging exercise utilizing a suspension system has been shown to result in greater TrA thickness changes than a traditional bridge exercise (Saliba et al., 2010), due to increased instability in the lumbar region.
Modern athletes must have the ability to adapt into different conditions, meaning that their muscles, especially the spine stabilizers, should be well trained in order to, either allow more mobility or ensure maximum stiffness depending on circumstances. Despite the rich literature, a parameter that has not been thoroughly examined, that could possibly...