The aim of this cross-sectional study was to assess the activation of the lateral abdominal muscles in hypermobile and matched non-hypermobile control (N=10) participants. The University Institutional Review Board approved this study. All participants were informed of the benefits and risks of the investigation prior to signing the institutionally approved informed consent document to participate in the study. In 2019 and 2020 we recruited ten hypermobile participants first and then matched them by sex and height (within 2cm). This was a sample size of convenience. None of our participants had a diagnosed conditions associated with hypermobility (e.g. Marfan syndrome, Ehlers-Danlos syndrome or osteogenesis imperfecta). After completing an intake questionnaire, participant range of motion was measured as previously described elsewhere.(8, 11) Briefly, using a goniometer and tape measure, the following joint ranges of motion and distances were obtained: passive extension of the fifth metacarpalphalangeal joint, passive apposition of the thumb to the flexor aspect of the forearm, passive hyperextension of the elbow, passive extension of the knee and active forward flexion of the trunk from a standing position with the knees fully extended, attempting to have the palms of the hands rest flat on the floor. A participant was considered hypermobile if five of the nine joints measured(9) met the criteria established by Beighton (i.e. elbow extension greater than 9 degrees, knee extension greater than 9 degrees, little finger extension greater than 89 degrees, able to touch the forearm with the thumb and able to touch the floor with palms with knees extended). The modified, quantitative version of the Beighton score(11) was calculated as the sum of nine continuous measurements, as opposed to the sum of nine categorical (positive test = 1; negative test = 0) measurements. The nine items are the same as above. Each continuous measurement was calculated (in degrees or cm) based on the ratio between the test outcome and a result that would correspond with a positive test using the Beighton score criteria (upper limit). The range of score per item was 0-1, whereas the maximum overall score ranged 0-9, where higher scores indicated greater laxity. The test-retest reliability of the BOM score is ICC = 0.99 (SEM = 0.4).(11)
Participants were asked to lie on their side on a treatment table with a pillow between their knees for ultrasonic data collection. Ultrasound images were gathered with the GE Logiq s8 system and the ML6-15 MHz or 9L sound heads. Images recorded panoramic views of the TrA, EO and IO muscles, at rest and during the abdominal hollowing maneuver (actively drawing in the navel to spine to engage of contract the abdominal muscles). Prior to imaging the participants practiced this maneuver. Images were recorded for the right side and left side of the body in an order randomized external to the researcher collecting data.
Marks were made with a soft tipped marker at the apex of the 1st, 3rd, and 5th lumbar spinous processes. To create a panoramic view, the sound head was glided over the skin from posterior to anterior starting from these marks transversely across the participant’s trunk. We used a custom-made probe guide made out of a flexible body-contouring material to ensure a straight line suitable for panoramic ultrasound picture (FIGURE 1). Participants were asked to lay still while two consecutive images at each of the three marked locations were imaged and recorded. Each panoramic scan took about five seconds. Participants were then asked to perform the abdominal drawing in (hollowing) maneuver while two more images were recorded at each of the marked levels. Participants were allowed to relax and rest 10 seconds between these active muscle contractions. The participants practiced contracting their abdominal muscles while observing the muscle movement on the ultrasound screen as real time feedback. The contralateral side was then imaged in the same manner.
The ultrasound images were measured at a later time following the data collection by the assigned researchers, who were blinded to the participant’s group allocation. The postprocessing measurements were conducted using the Osirix DICOM Viewer (Pixmeo SARL, 266 Rue de Bernex, CH-1233 Bernex, Switzerland). As part of the participant’s characteristics we measured the cross-sectional area of the TrA. It was always measured first, using the closed polygon function to trace the internal fascial borders of the muscles. The length measurements were performed using the open polygon function through the center of the muscle, staying equidistant from the superficial and deep fascial borders. The thickness measurements for the three muscles were obtained by measuring three separate locations using the straight-line function. The first measurement was taken at the half-way point of the measured length. The other two measurements were taken approximately equidistant of the halfway point on either side. An average of these three measurements was then taken and reported as the overall thickness (FIGURE 2).
Isometric trunk flexor and extensor strength were measured using peak force with a MicroFET 2 dynamometer (Hoggan Scientific LLC, Salt Lake City) that was attached to a pole. A cushioned bar that connected to the dynamometer was placed around the participants with them facing away from it for flexor strength and facing towards it for extensor strength measurements. The bar was placed on the sternum for the assessment of flexion strength, and on the level of mid-scapulae for the assessment of extension strength. A belt was used to fixate the pelvis during isometric trunk flexion testing. The testing order (flexion/extension) was randomized by a researcher external to that of who collected the data. The participants performed a set of three isometric contractions in each direction with 20 second pauses in between trials. For all testing a research assistant observed the hip flexion angle and pelvic rotation to ensure that it stayed relatively constant. Test-retest reliability of isometric strength testing was ICC=0.991 (95%CI: 0.984, 0.996) for extension and ICC=0.992 (95%CI: 0.986, 0.996) for flexion.
Trunk flexion and extension endurance testing were performed according to the methods described by Reiman et al.(13) During trunk flexion testing, the participant assumed a sit-up position with the back initially resting against a device that was wedged between the back rest of the treatment table, set at 60 degrees, and the back of the participant. The hip and knee flexion angles were placed at 90 degrees, the arms were folded across the chest with the hands placed on the opposite shoulder, and the feet were secured. The timer was started once the device was pulled out from behind the back of the participant and ended when the participant was not able to hold this position any longer (i.e. their back touched the back rest of the treatment table). This test was capped at 900 seconds (15 minutes). For the trunk extension endurance test participants laid prone on a back extension bench with their upper body hanging over the end of the bench. The pelvis, hips and knees were touching the bench and the feet were secured posteriorly. The timer was started when the participants lifted their upper body to be in line with their lower body with their hands resting on opposite shoulders and was ended when they were no longer able to hold the position. Verbal cues were given for both tests to assist in maintaining the correct position.
All analyses were conducted using Stata statistical software version 16 (StataCorp, College Station, TX, USA). Between-group differences in change between conditions (rest vs hollow) for each muscle parameter were examined by independent t-test. Within-group differences between conditions were examined by dependent t-tests. The strength and direction of association between muscle parameter change between conditions, joint mobility scores, trunk muscle strength and trunk muscle endurance were assessed with Pearson’s correlation coefficient. An alpha level of 0.05 was adopted for all statistical tests.