On the importance of the hip abductors during a clinical one legged balance test: A theoretical study

Abstract

Background: The ability to balance on one foot for a certain time is a widely used clinical test to assess the effects of age and diseases like peripheral neuropathy on balance. While state-space methods have been used to explore the mechanical demands and achievable accelerations for balancing on two feet in the sagittal plane, less is known about the requirements for sustaining one legged balance (OLB) in the frontal plane.

Research question: While most studies have focused on ankle function in OLB, can age and/or disease-related decreases in maximum hip abduction strength also affect OLB ability?

Methods: A two-link frontal plane state space model was used to define and explore the 'feasible balance region' which helps reveal the requirements for maintaining and restoring OLB, given the adverse effects of age and peripheral neuropathy on maximum hip and ankle strengths.

Results: Maintaining quasistatic OLB required 50%-106% of the maximum hip abduction strength in young and older adults, and older patients with peripheral neuropathy. Effectiveness of a 'hip strategy' in recovering OLB was heavily dependent on the maximum hip abduction strength, and for healthy older women was as important as ankle strength. Natural reductions of strength due to healthy aging did not show a meaningful reduction in meeting the strength requirement of clinical OLB. However deficits in hip strength typical of patients with peripheral neuropathy did adversely affect both quasistatic OLB and recoverable OLB states.

Significance: The importance of hip muscle strength has been underappreciated in the clinical OLB test. This is partly because the passive tissues of the hip joint can mask moderate deficits in hip abduction strength until it is needed for recovering OLB. Adding a follow up OLB test with a slightly raised pelvis would be a simple way to check for adequate hip abductor muscle strength.

Fig 1. Details of the double inverted pendulum model used to represent the task of balancing on one leg in the frontal plane over a stationary stance foot.

(A) One link represents the stance leg (SL) and the other the rest of the body (RB). The trunk is assumed to be straight without any lateral bending, and the contralateral leg is kept at a neutral abduction angle. (B) Parameters of the model are calculated from published mass-link information of a mid-size male aviator (m₁ = 14 Kg, m₂ = 67 Kg, l = 57 cm, h = 88 cm, r = 19 cm, α = 56°) [29]. (C) Two independent states (θ₁, θ₂) are required to describe balancing quasistatically on one leg. (D) Free Body Diagram for the equilibrium of the quasistatic double inverted pendulum.

Fig 2. A graphical comparison of the effect of ankle and hip strength on the mean calculated quasistatic active feasible balance region (FBR) for OLB in women (left) and men (right) of different ages and health status.

A point on each state-space plot represents a quasistatic state for the OLB double inverted pendulum model with a fixed foot link, where θ₁ is the angle that the stance leg makes with the vertical line and θ₂ is the stance leg’s hip abduction angle (please see Fig 1C). Because of limited ankle and hip strengths and ranges of motion, quasistatic OLB can only be maintained for the states inside the solid lines defining the FBR for each sex and group. The dashed-dot line in the middle of each FBR is the locus of states for which the COM lies in the same vertical plane as the ankle so that no ankle moment is required (T_S = 0). The top boundary of the FBR for all groups is the locus of points for which the pelvic inclination angle is raised a nominal 20º. For healthy young men and women, and healthy older men, the lower boundary of the FBR is the locus of points for which the pelvic inclination angle is decreased a nominal 20º. However, for healthy older women and older patients with peripheral neuropathy it is the maximum active hip abduction strength that constrains the lower end of the FBR. The left and right boundaries of the FBR are always determined by the medial and lateral margins of the functional base of support, respectively.

Fig 3. Example of calculated passive tissue contribution to hip abduction moment during OLB in older women with peripheral neuropathy.

(A) Calculated effect of passive tissue contribution to the net hip abduction moment on the FBR. Each line in the plot tagged with a small letter in gray shows a locus of quasistatic OLB states where a condition is satisfied: a: zero ankle moment (T_S = 0); b: pelvic inclination angle increased 20º; c: pelvic inclination angle held level with horizon; d: pelvic inclination angle decreased to -20º; e: FBR derived with only active hip abduction strength (red region); f: FBR derived with both active and passive hip abduction moments (region with diagonal lines); g: required hip abduction moment for maintaining the quasistatic state is equal to the net available hip abduction moment; h: required hip abduction moment for maintaining the quasistatic state is equal to the maximum active hip abduction moment strength; i: each green dashed line shows the states where the hip abduction moment from the passive tissues is equal to a percentage of the hip abduction moment load of OLB ranging from 10% to 100% in 10% increments which increase in the direction of the arrow. (B) Available hip abduction moment as a function of hip abduction angle.

Fig 4. Example of passive tissue contribution to hip abduction moment during OLB in older men with PN.

Calculated effect of significantly decreased active hip abduction strength on the FBR for older men with peripheral neuropathy. The locus of points tagged by grey letters a-g and the choice of colors are the same as described in Fig 3A.

Fig 5. Graphical comparison of calculated active and net FBR with recoverable laterally perturbed initial quasistatic OLB states in healthy older women and older women with peripheral neuropathy.

Letters a-d show the loci of quasistatic initial states similar to those shown in Fig 3. Letter e (green region) shows the active FBR, derived with only active hip abduction strength. Letter f (region with diagonal lines) shows net FBR derived with both active and passive hip abduction moment. Letter j (blue region) shows laterally perturbed initial quasistatic states which can be recovered by a strategy involving maximum acceleration of the center of mass. We can see that while for both groups the net FBR extends between lines b and d, weaker active hip abduction strength significantly ‘hollows out’ the recoverable states for the older women with PN (A and B). Repeating the simulations with zero ankle inversion / eversion strength collapses both active and net FBR to line a, and shows the same hollowing effect on the recoverable quasistatic states as the simulations with full ankle strength (C and D). Comparing the width of the FBR (top row) and the recoverable quasistatic states when considering no ankle strength (bottom row) shows that the effectiveness of the hip strategy in recovering OLB heavily depends on the maximum hip abduction strength. In the healthy older women, the hip strategy seems as effective as the ankle strategy.