Muscle Contraction is Impaired in Hypermobile People: A Study of The Lateral Abdominal Muscles

Background: Emerging evidence suggests that muscle function can be impaired in people with generalized hypermobility. The Beighton scale was developed to assess for the presence of hypermobility. This cross-sectional study assessed the activation of the lateral abdominal muscles in hypermobile (N=10; Beighton score > 5) and matched non-hypermobile control (N=10) participants. Methods: : Panoramic ultrasound scans of the transversus abdominis (TrA) and the external and internal abdominal obliques (EO and IO, respectively) were obtained at three lumbar levels (L1, L3, L5) at rest and during the hollowing maneuver. Results: : Between-group differences in the TrA length, as well as TrA, EO and IO thickness changes between conditions (rest vs hollowing) and trunk strength and endurance were examined. Hypermobile participants exhibited less TrA shortening than controls (L1-L5: 5.1% difference, L3: 8.9% difference) and greater thickness changes at level L1 (12.7% difference). EO and IO thickness differed between groups for both conditions, where both of the muscles were 0.07cm - 0.18cm thicker at rest and 0.07cm - 0.19cm thicker with hollowing in the hypermobile participants compared to control (all: P<0.001). The ability to contract the EO was less in hypermobile participants compared to controls (P=0.002; -5.3% vs no change in thickness), while the ability to contract the IO was greater in hypermobile participants compared to in controls (P=0.038; +21.3% vs +17.6% thickness change). Hypermobile participants demonstrated over 30% greater average trunk flexion strength. Moreover, greater joint mobility was associated with less TrA muscle shortening, reduced EO thickness changes, greater isometric trunk muscle strength for both extension and flexion (all: P<0.001) and with reduced trunk muscle endurance for extension and flexion (P=0.006 and P=0.002, respectively). Conclusion: The results of this study indicate joint hypermobility is associated with reduced ability to contract TrA and EO, however, this did not appear to be associated with impaired function (e.g. muscle strength and endurance).