Biomechanical Role of the Hip in Maintaining Balance during Standing Forward Reach

Abstract

During reactions to external anterior-posterior (AP) postural perturbations, it has been demonstrated that the hip strategy (HS) is employed only when balance risk is high and the ankle strategy (AS) is inadequate. In quiet standing the ankle plays a major role in maintenance of AP balance while the role of the hip is limited to medial-lateral (ML) control. This has led to the hypothesis that use of HS during self-initiated AP movements that are relatively safe or involve small center of mass (COM) excursions indicates poor balance, possibly due to deficits in ankle musculature. Consequently, comprehensive balance tests like BESTest focus on hip strength in the frontal plane and ankle strength in the sagittal plane. PURPOSE: To elucidate the biomechanical role of the hip during self-initiated standing forward reach. This movement pattern is used in several clinical balance screens including BESTest, Berg Balance Scale and Functional Reach Test. METHODS: 5 healthy young participants stood with their right shoulder flexed to 90°. A yardstick was placed parallel to the floor at the level of the acromion. They were instructed to reach forward as far as possible without taking a step and without raising their heels off the floor. 3 trials each were collected with 2 different instructions - do not bend the hip (AS) and bend the hip (HS). 2 force plates and an 11 camera motion capture system were used to obtain joint angles and moments, and whole-body COM and center of pressure (COP) excursions. RESULTS: Reach distance normalized by height was significantly higher during HS (Δ= 4.5±1.3 cm; p=0.008). This was accompanied by a significantly higher peak hip extensor moment (Δ= 0.41±0.13 Nm/kg; p=0.014). However, the maximum COM and COP excursions and peak ankle plantarflexor moment were not significantly different between strategies. CONCLUSIONS: Using HS allowed participants to reach further without increasing the balance risk as assessed by COM and COP excursions. Based on these findings, it is likely that in a functional situation where an individual is required to reach a fixed distance, using HS will achieve the goal with smaller COP and COM excursions compared to AS. Therefore, use of HS may not necessarily be a last resort indicating absence of ankle strength and the hip should not be ignored during assessment and retraining of AP balance.