ALTERED HIP EXTENSOR AND CORE MUSCLE SYNERGY IN SYMPTOMATIC CAM FEMOROACETABULAR IMPINGEMENT DURING THE HURDLE STEP TASK

Cam-type femoroacetabular impingement (FAI) is characterized by an enlarged femoral head deformity [1] and is a common cause for early hip osteoarthritis. Symptomatic FAI patients tend to compensate and off-load their painful hip to minimize discomfort. Although the hurdle step challenges our hips to coordinate stride motions and stability, the muscle activity during this task has yet to be investigated to delineate intricate muscle synergies for balance and postural control. The aim of this study was to examine the core and lower-limb muscle activity in patients with unilateral FAI and compare them with their contralateral, unaffected hip and healthy controls during the hurdle step task. Seventeen symptomatic unilateral cam FAI patients, awaiting surgery at our orthopaedic clinic (n = 17, m:f = 8:9, age = 25 ± 8 years, BMI = 24 ± 5); and seventeen healthy controls (n = 17, m:f = 7:10, age = 25 ± 7 years, BMI = 24 ± 4) were recruited. Twelve surface electromyography (EMG) electrodes (Trigno, Delsys; USA) were positioned to each subject's left and right lower-limb and core muscles (gluteus maximus, biceps femoris, tensor fasciae latae, rectus femoris, erector spinae, rectus abdominus). Each muscle was normalized using peak EMG during maximum voluntary isometric contractions. Each subject then performed the hurdle steps, yielding EMG data for the FAI patient's affected hip, FAI patient's unaffected hip, and control subject's hip in both swing and stance. The EMG waveforms were compared using statistical non-parametric mapping (SnPM; MATLAB, Mathworks; USA) between each FAI patient's affected and unaffected hip (Wilcoxon signed-rank test) and between the control group and each FAI group's affected and unaffected hips (Mann-Whitney U test). The FAI patients’ affected side demonstrated reduced gluteus maximus and biceps femoris activity from the initial step to the return step (25–75%) compared to their unaffected side and compared to the control group during the swing and stance position (Figure 1B, 1C; p ≤ 0.005). On the other hand, the FAI patients’ affected side displayed increased erector spinae and rectus abdominus activity compared to their unaffected side and to the control group. The most important finding was that the FAI patients altered their hip extensor and core muscle activations to help stabilize their symptomatic, affected hip during the hurdle step task. FAI patients struggled to effectively activate their hip extensors (gluteus maximus) to control and stabilize the hip during the swing and stance phase. Instead, they relied on their core muscles (rectus abdominis, erector spinae) to compensate for suboptimal hip extensor muscle activity and to maintain their hip-pelvic stability. A better understanding of compensatory mechanisms can help with preventative strategies prior to exacerbating any hip injury as well as developing prehabilitation/rehabilitation protocols for more personalized non-surgical management. For any figures or tables, please contact authors directly.