The construct and concurrent validity of brief standing sway assessments in children with and without cerebral palsy

Abstract

Background: Standing postural sway is often quantified from center of pressure trajectories. During assessments of longer durations, children may fidget, thus limiting the feasibility and validity of sway recordings.

Research question: Do postural sway sample durations less than 30 s maintain construct and concurrent validity?

Methods: In this case-control, observational study, we measured postural sway in 41 children (age 5-12 years, 23 typically developing (TD); 18 with spastic cerebral palsy (CP), 13 diplegic and 5 hemiplegic, 11 GMFCS level I and 7 level II) for 30-second eyes-opened and eyes-closed conditions. From a single recording, 5-second incremental durations of 5-30 s were considered in this analysis. We quantified anteroposterior, mediolateral, and transverse-plane sway using seven time-domain variables: root-mean-square error, total excursion, mean frequency, mean distance, sway area, and 95 % confidence circle and ellipse areas. Variables were calculated in eyes-opened and eyes-closed conditions, as well as the ratio of the two. Construct validity was evaluated by the persistence of large effect sizes (Glass's Δ ≥ 0.80) between CP and TD participants at shorter durations than 30 s. Concurrent validity was evaluated by the correlations of shorter duration measures to the 30 s measure.

Results: Seven sway measures had large between-group effects (Glass's Δ ≥ 1.02) for the 30 s measure that persisted (Glass's Δ ≥ 0.81) at shorter durations (5-25 s) and also maintained concurrent validity (r ≥ 0.83). Six of these seven measures were taken in the eyes-closed condition, and all seven measures were in the mediolateral direction or transverse plane.

Significance: Our analysis suggests that sway durations less than 30 s can uphold construct and concurrent validity. These measures were primarily in the eyes-closed conditions and mediolateral direction. These results are a promising indicator that shorter-duration sway measures may be of utility when fidgeting prevents longer recordings.

Keywords: Balance; Center of pressure; Fidgeting; Postural control; Stability; Validity.

Figure 1 –. Commonly observed fidgeting results.

Participants were instructed to “stand as still as a statue”. After review, ten participants (23% of total), including seven with cerebral palsy (39% of CP) and three with typical development (13% of TD), were removed from the analysis due to fidgeting during either the eyes-opened or eyes-closed trial or both. Fidgeting was defined as moving the head, shoulders, arms, hands, torso, legs, or feet in a manner atypical of someone attempting to maintain a static standing posture. Movements related to tremor, spasticity, or postural adjustments to maintain balance were not to be considered as fidgeting. No participants presented with involuntary tremor. See also supplementary video.

Figure 2 –. Representative analysis of root-mean-square error (RMSE) variable.

Analysis completed in five-second increments starting from the full 30 seconds to a five-second measure (i.e. six measurements from 30 seconds to five seconds in duration). All measurements started from the beginning of the trial. (A) Illustrates the evaluation of construct validity using Glass’s Δ effect size between cerebral palsy and typical development participant groups. Variables measuring large effects at the 30-second duration were further investigated for the shortest measurement duration with persisting large effects (i.e. Glass’s Δ ≥ 0.80 for that duration and all of the longer durations corresponding to that measure). (B) Illustrates the evaluation of concurrent validity using Pearson’s correlation for shorter durations to the 30-second measures. Concurrent validity was evaluated for each measure where construct validity was maintained. All variables formatted as condition-direction/plane. Conditions: eyes opened (EO), eyes closed (EC), and Romberg Ratio ($\mathrm{RR}=\frac{\mathrm{EC}}{\mathrm{EO}}$). Directions: anteroposterior (AP) and mediolateral (ML). Plane: transverse (TP). Variables in the anteroposterior and mediolateral directions are shown with solid lines and in the transverse plane with dashed lines.

Figure 3 –. Construct and concurrent validity during eyes closed condition.

Variables shown are those for the eyes closed condition with large between-group effects (Glass’s Δ ≥ 0.80) for the 30-second duration. Analysis completed in five-second increments starting from the full 30 seconds to a five-second measure (i.e. six measurements from 30 seconds to five seconds in duration). All measurements started from the beginning of the trial. (A) Illustrates the evaluation of construct validity using Glass’s Δ effect size between cerebral palsy and typically developing participant groups. Variables measuring large effects at the 30-second duration were further investigated for the shortest measurement duration with persisting large effects (i.e. Glass’s Δ ≥ 0.80 for that duration and all of the longer durations corresponding to that measure). (B) Illustrates the evaluation of concurrent validity using Pearson’s correlation for shorter durations to the 30-second measures. Concurrent validity was evaluated for each measure where construct validity was maintained. All variables formatted as variable-direction/plane. Variables: root-mean-square error (RMSE), total excursion (TOTEX), mean frequency (MFREQ), mean distance (MDIST), sway area (AREA_SW), 95% confidence circle area (AREA_CC), and 95% confidence ellipse area (AREA_CE). Directions: anteroposterior (AP) and mediolateral (ML). Plane: transverse (TP). Variables in the anteroposterior and mediolateral directions are shown with solid lines and in the transverse plane with dashed lines.