Bossom, Juliana Grace Ludvig2026-04-302026-04-302026-04-302026-04-28https://hdl.handle.net/10012/23122Trunk neuromuscular control is commonly assessed using sudden external perturbations to elicit unexpected instability, enabling the evaluation of stabilization strategies through trunk kinematics and muscular responses. Previous analyses focus on sagittal plane loading to assess these responses, with many studies exploring how the trunk responds in a dysfunctional state, such as muscular fatigue. Despite relatively inconsistent findings across literature, a general idea has emerged that the trunk’s stabilizing system will compensate for the loss of the force production associated with muscular fatigue to maintain trunk kinematic response, limiting extreme ranges of motion. However, most investigations primarily assess neuromuscular control in the sagittal plane, leaving the effect of muscle fatigue on frontal plane trunk neuromuscular control less understood. Additionally, a limited focus on between-sex comparisons in trunk neuromuscular control research represents an important knowledge gap, due to previously reported sex-differences in trunk kinematics during dynamic tasks and potential differences in fatigue characteristics. Given the generation of moments to oppose frontal plane loading is inherently more complex than those in the sagittal plane, and frontal plane sudden loading occurs in many dynamic sports, this work aims to characterize the effect of lumbar muscle fatigue on the response to sudden medio-lateral (M-L) perturbations between sexes. It was hypothesized that the trunk neuromuscular control system will prioritize task performance and joint safety through maintaining spine angular displacement, while compensating through decreasing time to peak velocity and increasing muscular activation peaks, with males having higher peaks than females. It was further hypothesized that muscle onset latencies will not vary with fatigue, since altering muscular activation is likely more favourable in response to decrease force producing capabilities. Twenty-eight (14M, 14F) contact-collision athletes experienced bidirectional M-L perturbations before and after low back muscle fatigue, in a randomized order. Participants were positioned in a semi-seated apparatus restricting lower-extremity motion with cables connecting a chest harness to a pulley-load supporting system with no external pre-loading. Perturbations were elicited through an unexpected drop force applied to the trunk on one side, equivalent to 30% of lateral bend strength. Spine angular displacement and muscle activation dynamics, including activation peak and onset latency, were used to understand trunk response to sudden loading. Muscles assessed included bilateral lumbar erector spinae (LES), thoracic erector spinae (TES), rectus abdominus (RAB), internal (INO) and external obliques (EXO). The first right and left trials of each fatigue condition were used for statistical analysis, and responses were assessed within an approximate 2 second window following perturbation onset. Generalized linear mixed models (GLMM) of the gamma log family were used to evaluate the effects of fatigue and sex on trunk kinematic and muscle activation dynamics variables. No differences in spine angular displacement were present while fatigued, yet notable neuromuscular compensations were found through condition-sex interactions. When fatigued, males exhibited greater bilateral LES activation, demonstrating sex-specific compensatory trunk control strategies. Across conditions and sexes, the abdominal musculature, particularly INO, had the highest peak activation and shortest onset latencies, suggesting an important role in frontal plane trunk stabilization. These results advance the understanding of frontal plane trunk neuromuscular control under fatigue and support the need for future work exploring sex-specific trunk stabilization strategies and their implications for training and injury risk.enneuromuscular controlspine biomechanicstrunkmedio-lateral perturbationssudden loadingMaster Thesis