Among the 4289 participants, 2400 (56%) were females and the median age category was 75-79 years. The median age for the fast group was 70-74 years and increased in the moderate and slow groups. The slow gait speed group contained a greater relative representation of Blacks than the other gait speed groups. All nine chronic health conditions were more prevalent with slow gait. Gait speed, demographic characteristics, and chronic conditions of the cohort are summarized in Table 1 by tertiles of gait speed at year 2. Overall, the mean gait speed at year 1 was 0.72 m/s (SD, 0.25), and 0.77 m/s (0.26) at year 2. Gait speed increased slightly by a mean of 0.04 m/s (0.20) from year 1 to year 2. Over the three-year of follow-up, 482 (11.2%) participants died. Results and model fits for all models are reported in Table 3.
Table 1. Characteristics, and Chronic Conditions in Aged 65+ Older Adults of the NHAT Study at baseline (2011)
|
Overall
|
Fast
|
Moderate
|
Slow
|
Sample size (%)
|
4289 (100)
|
1420 (33.1)
|
1437 (33.5)
|
1432 (33.4)
|
Gait speed Y1, m/s (mean [SD])
|
0.72 (0.25)
|
0.91 (0.19)
|
0.74 (0.18)
|
0.53 (0.21)
|
Age (%)
|
65-69
|
946 (22.1)
|
509 (35.6)
|
306 (21.3)
|
167 (11.7)
|
70-74
|
985 (23.0)
|
413 (28.9)
|
349 (24.3)
|
236 (16.5)
|
75-79
|
893 (20.8)
|
249 (17.4)
|
307 (21.4)
|
308 (21.5)
|
80-84
|
823 (19.2)
|
198 (13.9)
|
297 (20.7)
|
336 (23.5)
|
85-89
|
422 (9.8)
|
48 (3.4)
|
121 (8.4)
|
242 (16.9)
|
90+
|
220 (5.1)
|
11 (0.8)
|
57 (4.0)
|
143 (10.0)
|
Women (%)
|
2400 (56.0)
|
683 (47.8)
|
807 (56.2)
|
924 (64.5)
|
Race (%)
|
White
|
3127 (72.9)
|
1182 (83.2)
|
1066 (74.2)
|
879 (61.4)
|
Black
|
809 (18.9)
|
152 (10.7)
|
256 (17.8)
|
401 (28.0)
|
Hispanic
|
121 (2.8)
|
39 (2.7)
|
48 (3.3)
|
34 (2.4)
|
Other
|
232 (5.4)
|
47 (3.3)
|
67 (4.7)
|
118 (8.2)
|
Chronic conditions (%)
|
|
|
|
|
Arthritis
|
2463 (57.4)
|
965 (67.4)
|
831 (57.8)
|
965 (67.4)
|
Cancer
|
1214 (28.3)
|
404 (28.2)
|
404 (28.1)
|
404 (28.2)
|
Diabetes
|
1051 (24.5)
|
449 (31.4)
|
350 (24.4)
|
449 (31.4)
|
Heart disease
|
844 (19.7)
|
338 (23.6)
|
284 (19.8)
|
338 (23.6)
|
Hypertension
|
2938 (68.5)
|
1081 (75.5)
|
1001 (69.7)
|
1081 (75.5)
|
Lung Disease
|
696 (16.2)
|
274 (19.1)
|
231 (16.1)
|
274 (19.1)
|
Osteoporosis
|
938 (21.9)
|
372 (26.0)
|
313 (21.8)
|
372 (26.0)
|
Stroke
|
438 (10.2)
|
226 (15.8)
|
139 (9.7)
|
226 (15.8)
|
Dementia
|
158 (3.7)
|
104 (7.3)
|
39 (2.7)
|
104 (7.3)
|
Gait speed at year 2 and change in gait speed: models 1 & 2
At year 2, each 0.1 m/s increase in gait speed was associated with decreased mortality (HR, 0.81 [0.78, 0.84]; Model 1). Net increase in gait speed from year 1 to year 2 decreased mortality, with a HR of 0.95 (0.92, 0.99) per 0.1 m/s increase (Model 2).
Gait speed and change in gait speed: models 3 to 5
When considering gait speed at year 2 and change from year 1 to year 2 together (Model 3), greater gait speed remained protective (HR, 0.80 [0.76, 0.83]), but greater improvement in gait speed from year 1 to year 2 was associated with increased mortality (HR, 1.05 [1.00, 1.11).
When including both gait speed at year 1 and change in gait speed from year 1 to year 2 (Model 4), greater gait speed was again shown to be protective (HR, 0.80 [0.76, 0.83]). Contrary to Model 3, improvement in gait speed from year 1 to year 2, was associated with decreased mortality association (HR, 0.83 [0.79, 0.88]), rather than the increased mortality reported. This discrepancy is discussed below.
Gait speed trajectories (trichotomized at year 1 and year 2; Model 5; Figure 1) show that remaining in the fast groups from year 1 to year 2 is most protective (reference). Conversely, remaining in the slow group at both years 1 and 2 was associated with the greatest HR of mortality (HR, 4.43 [3.03, 6.48]). Model 5 also shows that any decline in gait speed systematically worsens prognosis compared to staying in the same category. The greater the decline, the worse the prognosis, as seen in the drop from fast to moderate (HR, 1.50 [0.89, 2.54]) versus fast to slow (HR, 3.15 [1.59, 6.24]) gait speeds. As seen in Model 3, participants who improved from slow to fast (HR, 2.05 [0.91, 4.63]) compared to slow to moderate (HR, 3.26 [2.06, 5.17]) showed decreased mortality, suggesting that a greater improvement in gait speed is protective. Of the three improved gait speed trajectories, the moderate to fast gait speed group had the lowest hazard ratio of mortality (HR, 2.33 [1.44, 3.79]).
Table 2. Exposure Models for Gait Speed Examined
Model number
|
Predictors and Covariates in Cox PH Models
|
Stage 1. Gait speed at Y2
|
1
|
GSY2 continuous
|
Stage 2. Change in gait speed from Y1 to Y2
|
2
|
∆[GSY2 – GSY1] continuous
|
Stage 3. Gait speed and change in gait speed
|
3
|
GSY2 continuous + ∆[GSY2 – GSY1] continuous
|
4
|
GSY1 continuous + ∆[GSY2 – GSY1] continuous
|
5
|
Trajectory [GSY1 → GSY2] 9 categories (FF, FM, FS, MF, MM, MS, SF, SM, SS)
|
Stage 4. Gait speed, change in gait speed, covariates, and interactions
|
6
|
GSY1 continuous + ∆[GSY2 – GSY1] continuous + age + sex + race + comorbidities
|
Note. GS: gait speed; Y1: year 1; Y2: year 2; D: change; F: fast; M: moderate; S: slow.
Interaction with age
Statistical analysis studying the interaction of gait speed with age did not show a significant difference in any of the models (Model 1 p=0.23; Model 2 p=0.99; Model 3 p=0.85; Model 4 p=0.85; Model 5 p=0.44).
Gait speed, change in gait speed, covariates, and interactions: Model 6
Similar to Model 4, Model 6 shows that when adjusted for race and comorbidities (beyond age and sex) gait speed and the change in gait speed remained associated with decreased mortality (HR, 0.81 [0.77, 0.85] and 0.85 [0.80, 0.90] respectively). Both associations, however, were lessened by inclusion of adjustment covariates.
Comparison of model fits
The adjusted model had the best model fit as measured by AIC (Model 6). Among the unadjusted, continuous models, those including gait speed and change in gait speed (AIC 7337) showed better fits than models considering only one of the two. The gait speed-only model had a superior fit than the change-only model (AIC = 7341, 7563 respectively).
Table 3. Results of Mortality on Gait Speed Exposure Models in Older Adults of the NHAT Study
Model Number and Description
|
HR (95% CI)
|
AIC
|
1. Gait speed at Y2 (per 0.1 m/s increase)
|
0.81 (0.78, 0.84)
|
7341
|
2. Change in gait speed (per 0.1 m/s increase)
|
0.95 (0.92, 0.99)
|
7563
|
3. Gait speed at Y2 and change (per 0.1 m/s increase)
|
GS Y2: 0.80 (0.76, 0.83) GS change: 1.05 (1.00, 1.11)
|
7337
|
4. Gait speed at Y1 and change (per 0.1 m/s increase)
|
GS Y1: 0.80 (0.76, 0.83) GS change: 0.84 (0.79, 0.88)
|
7337
|
5. Gait trajectory 3x3
|
F-F: Reference
F-M: 1.50 (0.89, 2.54) F-S: 3.15 (1.59, 6.24) M-F: 2.33 (1.44, 3.79) M-M: 1.67 (1.08, 2.58)
M-S: 2.86 (1.80, 4.56) S-F: 2.05 (0.91, 4.63) S-M: 3.26 (2.06, 5.17) S-S: 4.43 (3.03, 6.48)
|
7412
|
6. Gait speed Y1, change, adjusteda (per 0.1 m/s increase)
|
GS Y1: 0.81 (0.77, 0.85) GS change: 0.85 (0.80, 0.90)
|
7180
|
Note. GS: gait speed; Y1: year 1; Y2: year 2; D: change; F: fast; M: moderate; S: slow. (a) Adjusted covariates include age, sex, race, and comorbidities (hypertension, diabetes mellitus, heart disease, stroke, lung disease, arthritis, osteoporosis, cancer, and dementia)