PURPOSE: The Star-Excursion Balance Test (SEBT) is commonly used to assess dynamic balance. While the maximum reach distance (MRD) of the posteromedial (PM) direction of the SEBT is lower in individuals with ankle and knee dysfunction, we do not know whether MRD relates to feedforward and feedback movement control strategies. The purpose of this study was to assess the relationships between MRD of PM-SEBT and muscle activation levels, reflex responses to unexpected perturbations, and kinematics during a single-leg squatting (SLS) task.
METHODS: 20 healthy participants performed the PM-SEBT and two, 9-condition SLS tasks on a custom-built device, once with a flexing and once with an upright trunk. SLS conditions varied by speed and resistance with one random perturbation induced per condition. Sagittal motion was captured via Vicon. EMG were recorded from the Quadriceps (Q), Hamstrings (H), Gluteus Medius (GMed), and Soleus (Sol). EMG and kinematics during feedforward (FF) (-50-0 ms) and feedback (short-latency reflex (SLR) 0-50ms, long-latency reflex (LLR) 50-200ms) motor control were compared to MRD. Pearson Correlations were calculated for SLS EMG and kinematics (hip, knee, and ankle) to MRD. Step-Wise Regression to predict MRD was performed using significantly correlated SLS variables.
RESULTS: Significant correlations between SLS EMG and MRD: FF H (Flexing: p=0.014, R= -0.540; Upright: p= 0.015, R= -0.537), SLR H (Flexing: p=0.044, R= 0.454; Upright p=0.017, R= -0.528), SLR QH Ratio (Flexing: p=0.020, R= -0.516), LLR H (Flexing: p=0.007, R= -0.581; Upright: p=0.011, R=-0.557), and LLR QH Ratio (Flexing: p=0.042, R= 0.458). Significant correlations between SLS kinematics and MRD: SLR Hip flexion (Flexing: p=0.018, R=-0.522) and LLR Hip flexion (Flexing: p=0.021, R=-0.512). Step-wise regression results: LLR H Flexing explained 33.8% of the variance in MRD (p=0.007, R= -0.581).
CONCLUSIONS: Greater Hamstrings activation during feedforward and feedback control (SLR, LLR) and greater hip flexion during feedback control of a perturbed single-leg squatting task were associated with poorer dynamic balance during the PM-SEBT.