Validation of the Narrowing Beam Walking Test in Lower Limb Prosthesis Users

Abstract

Objective: To evaluate the content, construct, and discriminant validity of the Narrowing Beam Walking Test (NBWT), a performance-based balance test for lower limb prosthesis users.

Design: Cross-sectional study.

Setting: Research laboratory and prosthetics clinic.

Participants: Unilateral transtibial and transfemoral prosthesis users (N=40).

Interventions: Not applicable.

Main outcome measures: Content validity was examined by quantifying the percentage of participants receiving maximum or minimum scores (ie, ceiling and floor effects). Convergent construct validity was examined using correlations between participants' NBWT scores and scores or times on existing clinical balance tests regularly administered to lower limb prosthesis users. Known-groups construct validity was examined by comparing NBWT scores between groups of participants with different fall histories, amputation levels, amputation etiologies, and functional levels. Discriminant validity was evaluated by analyzing the area under each test's receiver operating characteristic (ROC) curve.

Results: No minimum or maximum scores were recorded on the NBWT. NBWT scores demonstrated strong correlations (ρ=.70‒.85) with scores/times on performance-based balance tests (timed Up and Go test, Four Square Step Test, and Berg Balance Scale) and a moderate correlation (ρ=.49) with the self-report Activities-specific Balance Confidence scale. NBWT performance was significantly lower among participants with a history of falls (P=.003), transfemoral amputation (P=.011), and a lower mobility level (P<.001). The NBWT also had the largest area under the ROC curve (.81) and was the only test to exhibit an area that was statistically significantly >.50 (ie, chance).

Conclusions: The results provide strong evidence of content, construct, and discriminant validity for the NBWT as a performance-based test of balance ability. The evidence supports its use to assess balance impairments and fall risk in unilateral transtibial and transfemoral prosthesis users.

Keywords: Accidental falls; Amputees; Patient outcome assessment; Postural balance; Rehabilitation.

Figure 1

Illustration of the Narrowing Beam Walking Test. Participants attempt to walk along four progressively narrower beam segments with their arms crossed over their chest. If participants moved their arms or stepped off the beam that was considered a “balance failure”, and the distance walked to that point is recorded. Height of each segment is 3.8 cm.

Figure 2

Distribution of clinical balance test scores. NBWT scores were normally distributed across its range, suggesting that the NBWT is suitable to measure a broad range of unilateral lower limb prosthesis users. Scores on existing clinical balance tests were skewed towards their upper limit and lacked normality, suggesting ceiling effects.

Figure 3

Correlations between participants’ Narrowing Beam Walking Test (NBWT) scores and those on existing clinical balance tests. Correlations (Spearman’s Rho) between participants’ (n = 40) NBWT scores and A) Berg Balance Scale (BBS) scores, B) Four Square Step Test (FSST) times, C) Timed Up and Go (TUG) times, and D) Activities-Specific Balance Confidence (ABC) Scale scores. The best fit line and corresponding 95% CI are presented for each relationship.

Figure 4

Known-groups construct validity of the Narrowing Beam Walking Test (NBWT). NBWT scores differed significantly between participants with expected differences in balance ability as measured by A) fall history (faller=24; non-faller=16), B) amputation level (transtibial=25; transfemoral=15), C) amputation etiology (dysvascular=7; non-dysvascular=33), and D) mobility level (K-level 1 and 2=17; K-level 3 and 4=23). Modified boxplots (mean, 95% CI, maximum, minimum) for each group.

Figure 5

Receiver Operating Characteristic curves for the A) Narrowing Beam Walking Test (NBWT), B) Timed Up and Go (TUG),C) Four Square Step Test (FSST), D) Berg Balance Scale (BBS), and E) Activities-Specific Balance Confidence (ABC) Scale. The area under the ROC curve, a measure of a test’s diagnostic accuracy, was largest for the NBWT, and significantly greater (p < 0.02) than that of the BBS and ABC.