Can these contributions to standing postural control be quantified from CoP trajectories in neurotypical adults?
Methods
Instantaneous contributions can be negative or larger than one, and integrated contributions sum to equal one. Proof-of-concept demonstrations validated these calculated contributions by restricting CoP motion under one or both feet. We evaluated these contributions in 30 neurotypical young adults who completed two (eyes opened; eyes closed) 30-s trials of bipedal standing. We evaluated the relationships between limb contributions, self-reported limb dominance, and between-limb weight distributions.
Results
All participants self-reported as right-limb dominant; however, a range of mean limb contributions were observed with eyes opened (Left: mean [range] = 0.52 [0.37–0.63]; Right: 0.48 [0.31–0.63]) and with eyes closed (Left: 0.51 [0.39–0.63]; Right: 0.49 [0.37–0.61]). Weight-shift contributions were small with eyes opened (0.00 [−0.01 to 0.01]) and eyes closed (0.00 [−0.01 to 0.02]). We did not identify any between-limb differences in contributions when grouped by self-reported limb dominance (p > 0.10, d 0.25) or between-limb weight distributions (0 0.20).
Significance
Across neurotypical participants, we observed a notable range of limb contributions not related to self-reported limb dominance or between-limb weight distributions. With this tool, we can characterize differences in the amount of CoP motion and the underlying control strategies. Changes in limb contribution can be measured longitudinally (i.e., across rehabilitation programs, disease progression, aging) representative of limb function, which may be particularly useful in populations with asymmetric function.
Introduction
Postural sway is often quantified from center-of-pressure (CoP) trajectories during quiet standing. Anteroposteriorly, destabilizing gravitational forces are countered with ankle plantar- and dorsiflexor moments—the primary postural control mechanisms during quiet stance—resulting in CoP movement underneath each foot [1], [2], [3], [4], [5]. Understanding how individual limbs contribute to this standing postural control as measured using the CoP is of particular interest when evaluating populations with asymmetric function (e.g., stroke, amputations) where net CoP measures may overlook differences. For example, persons with Parkinson's disease, who demonstrate asymmetric limb function and standing stability [6], may not show significant differences in standing postural control early in disease progression when characterized by traditional net center of pressure analysis, but do in later stages [7]. Previous efforts comparing the CoP between limbs showed both similarities and differences in the spatial and frequency domains, correlations to net CoP, and between-limb synchrony [2], [3], [5], [8], [9], [10]. However, these approaches did not quantify how each limb, and weight shifting, contributed to anteroposterior CoP control. Therefore, these previous methods ignored relevant biomechanical CoP control strategies.
The purpose of this analysis was to (1) validate a novel measure of standing anteroposterior CoP control and (2) assess the relationship between this measure and common limb dominance characterizations. We hypothesized that (1) our novel measure would be valid evidenced by proof-of-concept trials targeting each variable, and (2) limb contributions would be distinct from limb dominance.
Proof-of-concept results
The unrestricted condition showed asymmetric and consistent contributions from each limb, with small weight-shift contributions (Fig. 2A). With one limb restricted, the unrestricted limb was the primary contributor (Fig. 2B and C). With both limbs restricted, weight shifting was the primary contributor (Fig. 2D).
Exploratory results
Eyes-opened results are shown in Fig. 3. Eyes-closed results and a correlation matrix are provided in the Supplementary material. Across neurotypical participants, we observed a notable
Conclusion
These results validate our novel calculation of limb contributions to standing anteroposterior postural control. Our conclusions and variable contributions were consistent between eyes-opened and eyes-closed conditions. However, these contributions did not appear to be related to self-reported limb dominance (i.e., kicking limb and Waterloo Footedness) or between-limb weight distributions. Self-reported limb dominance has also poorly identified the preferred landing limb for young adults.
