This study was conducted to determine the immediate effects of isometric flexion and extension exercise with external compression through ATM®2 on lumbar extension endurance, back extensor muscle fatigue, lower limb muscle contraction onset timing, and trunk flexion ROM in healthy subjects. The results showed a significant increase in trunk flexion ROM after exercise and a significant decrease in the relative GM contraction onset timing. For lumbar extension endurance, the Biering-Sorensen test showed a trend of increase in the duration time on lumbar extension and muscle fatigue in the ES showed a trend of decrease after exercise. Muscle fatigue in the LM, however, significantly increased.
The ATM®2 device has four belts for the safe fixation of the user’s body onto a vertical treatment table. The belt strength can be controlled for different exercises at various angles. In a case report on a 55-year-old patient with frozen shoulders caused by kyphoscoliosis, the effects of the exercise therapy based on ATM®2 were examined. The subject showed recovery of an almost normal ROM on both shoulders, and the kyphoscoliosis posture improved after the intervention of a few weeks . Lewis et al.  investigated the effects of the exercise intervention using a program with ATM®2 and home exercise for 4 weeks in 43 patients diagnosed with scoliosis. The results showed an increase in the spinal ROM after the intervention, with improvements in pelvic alignment, pain, and disability. Patients with LBP have reduced lumbar ROM and proprioception with slower motion than healthy individuals . Thus, therapists have used various treatment methods to increase lumbar flexibility. Mazloum et al.  reported a significant increase in the trunk flexion ROM through training based on pilates and extension exercise that significantly reduced pain in patients with LBP. In clinical practice, the use of ATM®2 in patients with LBP requires the belt tension strength to be set based on the limited ROM that pain provocation when performing trunk flexion or extension. As this study targeted healthy adult individuals, the belt tension strength during exercise was set to a maximum level without causing discomfort. Ito and Gamada  evaluated the trunk ROM using the finger-to-floor distance of healthy adult men between the group who performed ATM®2-based exercise and the control group after intervention. The results showed that the ROM significantly increased in the exercise group than in the control group. Nejishima et al.  also examined the effects of ATM®2-based exercise in 14 patients with LBP and reported that the level of pain and LBP disability (Roland-Morris Disability Questionnaire) had significantly decreased after exercise during the tests at week 4 or 8. Likewise, in this study, the SR test after exercise showed a significant increase in the trunk flexion ROM. The increase in trunk ROM through exercise suggested a potential intervention effect of the exercise on patients with LBP.
Lumbopelvic stability is considered a critical factor in the prevention and treatment of injury based on the potential contribution to recovery from injury and subsequent improvement . For humans to maintain a straight posture, it is necessary to produce a force against gravity. The weights of the upper limbs, trunk, and head are conveyed to the pelvis via the spine. The pelvis connects the spine and muscles of the trunk to the lower extremities. The pelvis allows a person to tilt and rotate forward and backward as well as support the weight of the body. However, abnormal pelvic tilt may shift the body's center of gravity, leading to gradual weakening of the neuromuscular system . Trunk muscles participate in the activities of the trunk and limbs, acting as synergistic or agonistic muscles for voluntary movements. Additionally, trunk muscles automatically respond to unexpected movements of the trunk and extremities and are involved in proactive posture control [35–37]. Since the trunk muscles play a role in maintaining postures in daily life, the strength and endurance of the trunk muscles must be maintained. One exercise for pelvic training is a trunk stabilization exercise that simultaneously activates the abdominal muscles and the local muscles of the spine for coordinated movements . Many previous studies focused on lumbopelvic stabilization and suggested methods to strengthen core stability, such as trunk stabilization exercises, training using biofeedback mechanisms, and self-training from a cognitive perspective . However, it is challenging for an individual to become aware of and control core-stabilizing muscle groups. Several methods have been suggested in previous studies for trunk stabilization [40, 41], including an abdominal draw-in maneuver, which is an exercise for lumbar stabilization. According to a previous study, this exercise shows the best stabilization effects in transverse abdominal muscle and multifidus muscle . Activation of the abdominal muscles is essential in stabilizing the pelvis against the pulling forces of the hip muscles. When the pelvis is stable, forces acting on the trunk are efficiently transmitted to the hip and lower extremities . Also, to maintain stability in the pelvis, the force of compression should be increased . Most treatments for patients with LBP include trunk muscle strengthening training, and recently, core stabilization exercise has frequently been applied . In addition, an external compression device may be used to provide stability. Arumugam et al.  reported that the external compression on the pelvis enhanced the form closure of the sacroiliac joint as well as the forced closure and exercise control through reduced compensation on lumbar stability muscles. In another previous study, the use of a pelvic compression belt affected the thickness of the LM and ES have been proved through ultrasound. Its potential use as an effective assistive tool in muscle strengthening exercise . Previous studies have investigated the effects of pelvic compression belts on the body during various treatments. Shin et al.  reported that hip abduction muscle strength was significantly greater in the group that wore pelvic compression belts than in the control group after two weeks of manual treatment in total knee arthroplasty patients. The ATM®2 device in this study may provide a pelvic compression effect during the isometric exercise. The application of the device to patients with LBP is anticipated to produce more effective therapeutic effects than the common muscle strengthening exercise.
The lumbar region has an important dynamic function of supporting the upper trunk while conveying the compression and shearing forces that arise during daily activities to the lower limbs . This provides spinal stability that is maintained based on the passive support from the bone and ligament structure, active support from the muscles, and interactions among the control systems via the central nervous system . Previous studies reported that, in patients with LBP, the limb or trunk exercise led to a different type of exercise control from that in healthy subjects [50–52]. In patients with LBP, compared with healthy subjects, the trunk muscle activity and the order of contraction onset timing changed . Causes spinal movement involved multi-segment joints, the limitation of a single segment to increase the movement of another segment in compensation, and movement of the limbs is related to the movement of the spine . Janda and Jull  reported that on PHE, the normal order of muscle activity is the contraction of the GM preceding the contraction of the HM and that the change in this order would induce excess compensatory lumbar extension, thus resulting in weakening of the GM. In contrast, Pierce and Lee  reported that on hip joint extension, the order of muscle contraction was consistently initiated by the HM before extension. Kwon and Koh  also reported that on PHE or hip joint extension in a standing posture, the HM began contraction first before the GM. Similarly, in this study, GM contraction preceded HM contraction before and after exercise in two subjects, while all other subjects showed the HM contraction first. Kwon and Koh  reported that on PHE, GM contraction was significantly delayed compared to HM contraction in the LBP group; the delay was by 0.03 s in the control group and 0.18 s in the LBP group. The measurements for PHE in this study showed that the mean delay in GM compare to HM was 0.38 s before exercise and 0.28 s after exercise, with a significant difference in muscle contraction onset timing before and after exercise. For PHE, it is easy for the HM to be overused when it becomes the dominant extensor as the GM is unable to function adequately, which can ultimately lead to LBP . In a study by Hungerford et al. , patients with sacroiliac joint pain were guided to stand on one leg and perform the hip flexion on the other leg and as the biceps muscle contraction occurred first, a delay in the GM contraction was observed. Thus, it is presumed that the adjustment of the order of muscle activity has a critical role in the prevention and treatment of LBP.
The spine itself is an unstable structure with appropriate support from the surrounding muscles and tissues for compensation . Regarding muscle endurance in the lumbar region, muscle fatigue is a frequently detected problem in patients with LBP and a risk factor for LBP [60–62]. Thus, the state of the trunk can be accurately determined through the assessment of lumbar muscle function and fatigue to provide suitable treatments. The Biering-Sorensen test in this study was used to assess the muscle endurance around the lumbar region based on the duration time and EMG. In a study by Jung et al. , the group that received the 4-week manual therapy and the group that received exercise therapy showed reduced pain after the intervention, while trunk stability and duration time in the Biering-Sorensen test increased. This study hypothesized that the duration time in the Biering-Sorensen test would increase after exercise to decrease muscle fatigue due to test motions. The results of this study showed a trend of increase, despite the lack of significance, in the duration time after exercise. In addition, muscle fatigue in the ES decreased, although without statistical significance. However, contrary to the hypothesis, muscle fatigue in the LM increased after exercise. This was attributed to the difficulty in performing test motions repeatedly within a short period between two measurements, despite the adequate resting time. Furthermore, in the Biering-Sorensen test used in this study, the subjects were guided to maintain posture up to as high as 120 s. For some subjects who could go beyond 120 s, the experiment ended as they reached 120 s to prevent potential influence on the statistical results. In addition, this study aimed to analyze the immediate effects of exercise intervention, and performing a single exercise might have prevented the predicted effects from being reached. In the future, the effects of a long-term exercise intervention with a 2- or 4-week training program in terms of muscle endurance and muscle fatigue should be investigated. Moreover, previous studies show that surface electromyography cannot fully measure deep muscles such as multifidus muscle . However, since multifidus muscle consists of both deep and surface segments , the activity of multifidus muscle has been measured using surface electromyography in many previous studies [66–68]. In this study, to avoid invasive procedures, the activity of multifidus muscle was measured using surface electromyography as previously described. In follow-up studies, it will be important to use methods such as needle electromyography for a more accurate assessment of muscle activity.
This study has several limitations. First, only the immediate effects of ATM@2-based isometric flexion and extension exercise were evaluated. Second, the subjects were not patients with LBP, but healthy individuals. Third, the results may have been influenced by subjective influencing factors during the procedures as they could not be completely excluded. In future studies, these limitations should be compensated to procure more scientifically valid clinical data.