Abstract
Background.
Slow gait speed increases morbidity and mortality in older adults. We examined how preferred gait speed is associated with energetic requirements of walking, fatigability, and fatigue.
Methods.
Older adults (n = 36, 70–89 years) were categorized as slow or fast walkers based on median 400-m gait speed. We measured VO2peak by graded treadmill exercise test and VO2 during 5-minute treadmill walking tests at standard (0.72 m/s) and preferred gait speeds. Fatigability was assessed with the Situational Fatigue Scale and the Borg rating of perceived exertion at the end of walking tests. Fatigue was assessed by questionnaire.
Results.
Preferred gait speed over 400 m (range: 0.75–1.58 m/s) averaged 1.34 m/s for fast walkers versus 1.05 m/s for slow walkers (p < .001). VO2peak was 26% lower (18.5 vs 25.1ml/kg/min, p = .001) in slow walkers than fast walkers. To walk at 0.72 m/s, slow walkers used a larger percentage of VO2peak (59% vs 42%, p < .001). To walk at preferred gait speed, slow walkers used more energy per unit distance (0.211 vs 0.186ml/kg/m, p = .047). Slow walkers reported higher rating of perceived exertion during walking and greater overall fatigability on the Situational Fatigue Scale, but no differences in fatigue.
Conclusions.
Slow walking was associated with reduced aerobic capacity, greater energetic cost of walking, and greater fatigability. Interventions to improve aerobic capacity or decrease energetic cost of walking may prevent slowing of gait speed and promote mobility in older adults.
Key Words: Gait speed, Mobility, Fatigue, Epidemiology, Energetics.
Mobility is a key component of health across the lifespan and is necessary for older adults to maintain independent functioning and autonomy. Mobility limitation, defined as difficulty walking one-quarter mile or climbing one flight of stairs, is reported by 30%–40% of adults aged 65 years and older (1,2), and there is likely to be a high degree of unrecognized mobility limitation (3) because many older adults do not engage in regular volitional walking activity (4).
Gait speed is the most common marker of mobility (5). Preferred gait speed slows with age (6,7), and slow preferred gait speed is a strong predictor of subsequent health outcomes in older adults, including disability in activities of daily living (8), mobility limitation and disability (8,9), cardiovascular disease (9), hospitalizations and health care service use (10), and mortality (9,11). However, the mechanisms underlying the development of age-related slow gait speed are not well studied or understood, which hampers disability prevention efforts for older adults.
One possibility is that slow gait speed develops as an adaptive response to conserve energy and reduce the fatigue associated with walking. As peak aerobic capacity (VO2peak) declines with increasing age (12,13), the energetic requirements of walking at a given speed increase relative to VO2peak, such that usual walking becomes more intense. In turn, this may lead to higher levels of fatigue associated with walking and compensatory slowing of preferred gait speed in order to reduce the energetic requirements of walking and levels of fatigue (14). Indeed, an emerging body of evidence suggests that energy requirements during walking play a central role in the development of mobility limitation in older adults (15–18).
Fatigue refers to global self-reported tiredness, exhaustion, lack of energy, and weariness (19). Fatigue is associated with mortality (20), but it is challenging to assess how fatigue influences physical function, including preferred gait speed, because individuals likely modify their activities to maintain feelings of fatigue within an acceptable range (14,19). Therefore, two individuals may report similar levels of fatigue when the work performed to induce the fatigue is vastly different. To overcome this measurement issue of self pacing, the construct of fatigability was introduced as the degree of fatigue experienced during performance of a defined activity, which normalizes fatigue to activity level (19). Studies are beginning to show that higher levels of fatigability are associated with worse physical function (21).
Accordingly, the objectives of this study were to test the hypotheses that (i) slower preferred gait speed is associated with higher energetic requirements during walking and higher levels of fatigability and (ii) preferred gait speed is not associated with fatigue.
Methods
Study Participants
Community-dwelling men (n = 21) and women (n = 17) aged 70–89 years were recruited from the Pittsburgh, Pennsylvania area for the Study of Energy and Aging Pilot between March and May 2010. Individuals met the following criteria at telephone screening: able to walk without assistance of a device or another person; able to get in/out of bed and chairs and walk across a small room without difficulty; body weight less than or equal to 285 lbs for men, less than or equal to 250 lbs for women; and body mass index 20–32kg/m2. They reported no history of medical conditions that might alter gait or ability to safely complete treadmill walking tests: hip fracture; stroke in past 12 months; cerebral hemorrhage in past 6 months; heart attack, angioplasty, or heart surgery in past 3 months; chest pain during walking in past 30 days; current treatment for shortness of breath or a lung condition; usual aching, stiffness, or pain in their lower limbs and joints; and bilateral difficulty bending or straightening the knees fully. They had to be willing and able to undergo a skeletal muscle biopsy and a magnetic resonance scan. The study was approved by the Institutional Review Boards at the University of Pittsburgh and the California Pacific Medical Center. All participants provided written informed consent. To be included in the analysis dataset, participants had to complete a standard speed (0.72 m/s) treadmill walking test (n = 36).
Preferred Gait Speed
Participants completed a 400-m overground walk by walking 10 times around a 40-m course following the instruction to “walk at your usual pace without overexerting yourself” (22). Time to completion and rating of perceived exertion (RPE) on the Borg scale (6–20) were recorded upon completion; speed (m/s) was calculated. Participants also completed two 6-m walk trials as part of a modified Short Physical Performance Battery (SPPB) (23).
We divided participants into fast and slow walkers based on median 400-m walk speed (1.19 m/s) to ensure adequate sample sizes in both groups. A gait speed of 1.19 m/s is similar to previously published reference values for preferred walking speed in older men and women between 70 and 89 years (24,25). Speed over 400 m may be a more accurate reflection of preferred gait speed than speed over 6 m because it incorporates the time required to make appropriate stride length and frequency adjustments to settle into a sustainable speed.
Peak O2 Consumption, VO2peak
VO2peak was determined by a modified Balke graded treadmill exercise test (26). Major exclusion criteria were ascertained during telephone screening. Participants also had to have blood pressure less than or equal to 180/110 mmHg, resting heart rate less than or equal to 110 bpm, and no evidence of cardiac arrhythmias during resting 12-lead electrocardiogram conducted prior to the VO2peak test. To ensure participant safety during the VO2peak test, the electrocardiogram was monitored continuously, and blood pressure was measured every 2 minutes.
During the VO2peak test, walking speed was held constant at the participant’s fastest measured speed from the two 6-m walk trials from the SPPB. Treadmill grade began at 0% and was increased thereafter by 2% every 2 minutes until attainment of VO2peak. Expired air was collected to determine oxygen consumption (VO2) and carbon dioxide production (VCO2) via indirect calorimetry (Moxus, AEI Technologies, Pittsburgh, PA); data were saved as 20-second averages. VO2peak was defined as the highest 20-second average VO2 during the test and expressed as ml/kg/min.
The VO2peak test was symptom limited and followed criteria outlined in the American College of Sports Medicine guidelines (26), including strong encouragement to achieve a respiratory exchange ratio greater than 1.05 and a Borg RPE greater than 16. The test was terminated upon participant report of volitional fatigue.
O2 Consumption During Walking
Participants walked on a treadmill for 5 minutes at a standard speed of 0.72 m/s, rested for 5 minutes, and then walked on the treadmill for another 5 minutes at their preferred gait speed (measured from the fastest 6-m walk trial from the SPPB). The standard speed (0.72 m/s) was chosen because it was slow enough to minimize participant exclusion but not uncomfortably slow. VO2 (ml/kg/min) was measured as described previously; values from the first 3 minutes of each test were discarded to allow time for participants to adjust to the workload and reach stable oxygen consumption, and then values from the final 2 minutes were averaged.
Oxygen consumption during treadmill walking at 0.72 m/s and at preferred gait speed were expressed in ml/kg/min and divided by walking speed to determine the energetic cost of walking per unit distance (C w, ml/kg/m). They were also expressed relative to VO2peak (%).
Fatigue and Fatigability
To measure global perceived fatigue, participants were asked four questions from the Physical Energy Scale from the Motivation and Energy Inventory (27) during the clinic interview. They were asked to consider the past 4 weeks and rate how much time they felt “physically tired during the day,” “exhausted,” and “energetic” (0 = “all of the time” to 6 = “none of the time”). They were asked how often they “ran out of energy before the end of the day” (0 = “every or nearly every day” to 5 = “never”). Part way through the Study of Energy and Aging Pilot, an additional set of fatigue questions were added in which participants (n = 24) were asked if they had been feeling “unusually tired” during the past month, as well as to rate how often they felt “weak,” “sleepy,” “lively,” and “tired” (0 = “not at all” to 10 = “very”). Participants also rated their overall energy level during the past month (0 = “no energy” to 10 = “most energy you have ever had”).
Fatigability, a phenotype that normalizes fatigue to activity level (19), was assessed during the clinic interview with the Situational Fatigue Scale (SFS), which measured mental and physical fatigue in relation to situational demands of 13 items; individual items were scored from 0 = “no fatigue at all” to 5 = “extreme fatigue,” and item scores were summed to yield a total SFS score ranging from 0 to 65 (28). We also separately examined the item score for the activity of “taking a walk for 1 hour.” RPE at the end of the 400 m, standard speed treadmill, and preferred speed treadmill walks were also considered fatigability measures (21).
Other Measures
Weight (kg) was measured with a standard balance beam scale and height (cm) with a Harpenden Stadiometer; body mass index was calculated (kg/ m2). Race, smoking status, mobility impairment, and self-rated health were reported through clinic interview. Depressive symptoms were measured with the short form Center for Epidemiological Studies Depression Scale (29), and cognitive function was measured with the Teng Modified Mini-Mental State (3MS) Examination. Physical activity was assessed using the Physical Activity Scale for the Elderly (30). Physical function was evaluated using the SPPB. One repetition maximum leg extensor strength (lbs air pressure) and overall leg extensor power (Watts) were measured on a Keiser leg extension machine. Participants were asked to report physician-diagnosed medical conditions.
Statistical Analyses
Data were summarized as mean (SD) or N (%) for normally distributed variables and median (interquartile range) for skewed variables. Participants were divided into slow and fast walkers based on median preferred gait speed from the 400-m walk (1.19 m/s). Differences between fast and slow walkers were compared using independent samples t-tests, nonparametric Mann–Whitney U-tests, and chi-square tests. Pearson correlation coefficients were used to examine linear associations between preferred gait speed and measures of oxygen consumption and fatigability. Stepwise linear regression was used to examine energetic determinants of preferred gait speed. All analyses were performed using an alpha of 0.05 for statistical significance; we did not perform Bonferroni or other corrections for multiple comparisons due to the pilot and exploratory nature of the study. All statistical analyses were performed in SPSS (version 19.0; Chicago, IL).
Results
Preferred gait speed on the 400-m walk ranged from 0.75 to 1.58 m/s; mean [SD] preferred gait speed was 1.34 [0.12] m/s for fast walkers and 1.05 [0.13] m/s for slow walkers. Compared with fast walkers, slow walkers were older and had lower SPPB and Physical Activity Scale for the Elderly scores (Table 1). There were no differences between slow and fast walkers for other descriptive characteristics.
Table 1.
Baseline Characteristics of SEA Pilot Participants
| Total Sample (N = 36) | Slow Walkers (N = 18) | Fast Walkers (N = 18) | p Value | |
|---|---|---|---|---|
| Age (y) | 78.4 (5.0) | 80.3 (4.6) | 76.4 (4.8) | .017 |
| Men, N (%) | 20 (55.6) | 11 (61.1) | 9 (50.0) | .502 |
| White, N (%) | 34 (94.4) | 17 (94.4) | 17 (94.4) | .000 |
| Weight (kg) | 71.5 (12.2) | 74.8 (13.8) | 68.1 (9.8) | .104 |
| Height (m) | 1.66 (0.10) | 1.67 (0.10) | 1.65 (0.11) | .559 |
| Body mass index (kg/m2) | 25.7 (2.7) | 26.6 (2.9) | 24.9 (2.1) | .060 |
| Leg extensor 1 repetition maximum (lbs) | 353 (86) | 346 (65) | 361 (104) | .621 |
| Leg extensor power (Watts) | 328 (133) | 321 (115) | 336 (153) | .734 |
| Medical conditions, N (%) | .105 | |||
| None | 8 (22.2) | 3 (16.7) | 5 (27.8) | |
| 1 | 16 (44.4) | 6 (33.3) | 10 (55.6) | |
| 2+ | 12 (33.3) | 9 (50.0) | 3 (16.7) | |
| Medical history, N (%) | ||||
| Myocardial infarction | 3 (8.3) | 2 (11.1) | 1 (5.6) | .546 |
| Congestive heart failure | 2 (5.6) | 1 (5.6) | 1 (5.6) | 1.000 |
| COPD, asthma, emphysema or bronchitis | 2 (5.6) | 1 (5.6) | 1 (5.6) | 1.000 |
| Osteoarthritis | 10 (27.8) | 6 (33.3) | 4 (22.2) | .457 |
| Depression | 5 (13.9) | 3 (16.7) | 2 (11.1) | .630 |
| Cancer | 19 (52.8) | 11 (61.1) | 8 (44.4) | .317 |
| Depressed, CES-D score > 10, N (%) | 7 (19.4) | 5 (27.8) | 2 (11.1) | .206 |
| Teng MMSE score (/100)* | 95.5 (92.0–97.0) | 95 (93.5–97.0) | 96 (90.8–98.3) | .455 |
| PASE score | 135 (55) | 111 (45) | 159 (55) | .006 |
| SPPB (/12)* | 11.5 (10–12) | 10 (9–12) | 12 (11–12) | .002 |
| SPPB 6-m preferred gait speed (m/s) | 1.20 (0.20) | 1.08 (0.17) | 1.32 (0.16) | <.001 |
| 400-m gait speed (m/s) | 1.20 (0.19) | 1.05 (0.13) | 1.34 (0.12) | <.001 |
| Mobility impairment, N (%) | 2 (5.6) | 1 (5.6) | 1 (5.6) | 1.000 |
| Excellent/good self-rated health, N (%) | 34 (94.4) | 16 (88.9) | 18 (100.0) | .146 |
| Smoking status, N (%) | .717 | |||
| Never | 25 (69.4) | 13 (72.2) | 12 (66.7) | |
| Past | 11 (30.6) | 5 (27.8) | 6 (33.3) |
Notes: Continuous variables reported as mean (SD), p values from independent samples t-tests, unless otherwise indicated. Categorical variables reported as N (%), p values from chi-square tests. Data missing for N = 1 fast walker for leg extensor power. CES-D = Center for Epidemiologic Studies Depression Scale; COPD = chronic obstructive pulmonary disease; MMSE = Mini-Mental State Examination; PASE = Physical Activity Scale for the Elderly; SEA = Study of Energy and Aging; SPPB = Short Physical Performance Battery.
*Reported as median (interquartile range), p values from Mann–Whitney U-tests. Mobility impairment: unable to walk 1/4 mile (3–4 blocks) outside on level ground or walk up 10 stairs.
VO2peak was 26.3% lower among slow walkers than fast walkers (18.5 [4.9] vs 25.1 [5.1] ml/kg/min, p = .001) (Table 2). At the standard gait speed of 0.72 m/s, VO2 was not significantly different between slow and fast walkers, but slow walkers used a larger percentage of VO2peak (58.9% [12.8] vs 41.8% [11.2], p < .001). At preferred gait speed, VO2 was not significantly different between slow and fast walkers, but there was a trend toward slow walkers using a larger percentage of VO2peak (68.8% [18.1] vs 58.5% [16.2], p = .095), despite their slower gait speed (1.08 [0.17] vs 1.32 [0.16] m/s, p < .001). The energetic cost of walking (C w), which normalizes VO2 for gait speed, was higher among slow walkers than fast walkers at preferred gait speed (0.211 [0.169–0.229] vs 0.186 [0.167–0.200] ml/kg/m, p = .047).
Table 2.
Walking Energetics and Fatigability for Slow and Fast Walkers
| Slow Walkers (N = 18) |
Fast Walkers (N = 18) |
p Value | |
|---|---|---|---|
| Measures of walking energetics | |||
| VO2peak (ml/kg/min) | 18.5 (4.9) | 25.1 (5.1) | .001 |
| RPE at VO2peak | 17.3 (1.4) | 17.8 (1.8) | .666 |
| RMR (ml/kg/min) | 2.6 (0.1) | 2.7 (0.1) | .015 |
| VO2 at 0.72 m/s (ml/kg/min) | 10.9 (1.3) | 10.1 (2.1) | .188 |
| VO2 at 0.72 m/s relative to VO2peak (%) | 58.9 (12.8) | 41.8 (11.2) | <.001 |
| VO2 at preferred gait speed (ml/kg/min)* | 13.4 (11.1–15.2) | 14.3 (12.1–17.0) | .159 |
| VO2 at preferred gait speed relative to VO2peak (%) | 68.8 (18.1) | 58.5 (16.2) | .095 |
| C w at preferred gait speed (ml/kg/m)* | 0.211 (0.169–0.229) | 0.186 (0.167–0.200) | .047 |
| Measures of fatigability | |||
| Situational Fatigue Scale (SFS) total (13 items) | 14.7 (7.6) | 9.4 (7.2) | .042 |
| Fatigue while walking 1 h (0–5)* | 1.5 (0–3) | 0 (0–1) | .019 |
| RPE at 0.72 m/s* | 10.0 (9.0–11.0) | 8.5 (7–9.3) | .049 |
| RPE at preferred gait speed | 12.6 (1.9) | 10.8 (1.9) | .014 |
| RPE at end of 400-m walk | 10.8 (2.2) | 9.6 (1.8) | .066 |
Notes: Continuous variables reported as mean (SD), p values from independent t-tests, unless otherwise indicated. Categorical variables reported as N (%), p values from chi-square tests. Among slow walkers, data missing from N = 1 for VO2peak and associated variables; N = 1 was found to be an outlier for VO2 at 0.72 m/s and was excluded from this and associated variables. Data missing from N = 3 for VO2 at preferred gait speed and associated variables. Data missing from N = 1 for RPE at 0.72 m/s, from N = 4 for RPE at end of preferred gait speed test. Among fast walkers, data missing from N = 2 for VO2 at preferred gait speed and associated variables. C w = energetic cost of walking; RPE = rating of perceived exertion; RMR = resting metabolic rate.
*Reported as median (interquartile range), p values from Mann–Whitney U-tests.
Fatigability was greater among slow walkers (Table 2). Slow walkers had higher summary scores on the SFS (14.7 [7.6] vs 9.4 [7.2], p = .042) and reported higher levels of perceived fatigue for walking for 1 hour (1.5 [0–3] vs 0 [0–1], p = .019). Slow walkers also reported higher RPE while walking at 0.72 m/s (10.0 [9.0–11.0] vs 8.5 [7.0–9.3], p = .049) and at preferred gait speed (12.6 [1.9] vs 10.8 [1.9], p = .014).
There were no differences between slow and fast walkers on any measures of global fatigue (p > .2 for all comparisons).
Preferred gait speed (from the 400-m walk) was moderately to strongly correlated with VO2peak (r = .666, p < .001), VO2 at preferred gait speed (r = .454, p < .001), the SFS summary score (r = −.397, p = .017), and perceived fatigue associated with walking for one hour (r = −.357, p = .033) (Figure 1). The stepwise linear regression analyses considered the following candidate determinants of preferred gait speed: age, leg extensor one repetition maximum, Physical Activity Scale for the Elderly score, VO2peak, VO2 at preferred gait speed, VO2 at 0.72 m/s relative to VO2peak, RPE during preferred speed and standard speed treadmill tests, the SFS summary score, and perceived fatigue for walking for 1 hour. The final model included terms for VO2peak (p < .001) and VO2 at preferred gait speed (p = .033), which together explained 53% of the variability in preferred gait speed (Table 3).
Figure 1.
Associations between preferred 400-m gait speed, walking energetics, and fatigability. Regression lines are shown as dashed lines.
Table 3.
Parameter Estimates From Stepwise Linear Regression Model to Predict Preferred Gait Speed
| Variable | Coefficient (β) | SE | p Value |
|---|---|---|---|
| Intercept | 0.596 | 0.115 | <.001 |
| VO2peak (ml/kg/min) | 0.018 | 0.004 | <.001 |
| VO2 at preferred gait speed (ml/kg/min) | 0.017 | 0.008 | .033 |
Notes: Model R 2 = .53. Other candidate determinant variables (age, leg extensor one repetition maximum, Physical Activity Scale for the Elderly score, VO2 at 0.72 m/s relative to VO2peak, rating of perceived exertion during preferred speed and standard speed treadmill tests, the Situational Fatigue Scale summary score, and perceived fatigue for walking for one hour) were excluded by the stepwise procedure at p ≥ .10.
Discussion
Among 70- to 89-year-old adults, we found that slow walkers had reduced aerobic capacity compared with fast walkers, reflected by lower VO2peak values. As a consequence, the physiologic effort of walking was higher in slow walkers than fast walkers. Specifically, slow walkers used 59% of their VO2peak to walk at the slow speed of 0.72 m/s, whereas fast walkers used only 42% of their VO2peak. To walk at their preferred gait speed, slow walkers used 69% of their VO2peak and fast walkers used 59% of their VO2peak, demonstrating that preferred speed walking required a significant physiologic effort, especially for slow walkers. For comparison, young adults, regardless of fitness, consistently use 55% of their VO2peak to walk at a rapid pace (31). Thus, preferred speed walking for older adults requires greater effort than rapid speed walking for young adults. Our results corroborate those reported by Fiser and colleagues, who studied walking intensity in 60- to 88-year-old adults (15), and Schrack and colleagues, who studied walking energetics in 33- to 94-year-old adults in the Baltimore Longitudinal Study of Aging (32).
A novel finding is that higher rates of energy expenditure during walking were accompanied by greater fatigability. Slow walkers reported higher RPE levels when walking at 0.72 m/s and at preferred gait speed, and greater overall fatigability on the SFS, indicating that they tire more easily during daily tasks than fast walkers, including walking for 1 hour. This suggests that fatigability increases as the energy requirements of walking approach one’s energetic capacity. Interestingly, there were no differences in measures of global fatigue between slow and fast walkers. The phenotype of fatigability was introduced recently (19) to normalize fatigue to activity level, and our results show, for the first time, that measures of fatigability are more strongly associated with gait speed in older adults than global measures of fatigue. RPE during walking, which combines a self-reported measure of exertion with an objective measure of physical work, appears to be a simple and useful measure of fatigability (21). Others have measured fatigability as the degree of reported fatigue following performance of standardized lab-based tasks that simulate common activities of daily living such as sweeping and grocery shopping (33), the degree of performance deterioration during standard tasks (21,34), and the degree of tiredness reported after completion of standardized walks (34). These studies have shown fatigability is correlated with physical activity, physical function deficits including gait speed, and self-reported fatigue. Although the health outcomes associated with high levels of fatigue in older adults are well established (35), future research is needed to evaluate whether high levels of fatigability are also predictive of health outcomes, including incident mobility limitation, mobility disability, and mortality.
The energetic cost of walking (C w, ml/kg/m), which normalizes VO2 to gait speed, has a well-established U-shaped relationship with gait speed, with maximum economy or minimum energetic cost falling anywhere between 1.1 and 1.3 m/s (36,37). Mean preferred gait speeds of slow and fast walkers in our study were consistent with this range; nevertheless, we found C w was elevated among slow walkers compared with fast walkers, indicating that the energy expenditure required to cover a given distance is greater for slow walkers. This has important implications for daily mobility, as higher energy costs may limit the amount and extent of daily movement. Future research should examine the relationship between C w and mobility limitation in older adults.
Slowing of gait speed is a hallmark of the aging process, but the mechanisms underlying age-related slowing of gait speed are still poorly understood. Our results provide support for the hypothesis that age-related slow gait speed develops as an adaptive response to conserve energy and thereby manage the effort of walking and reduce fatigue associated with walking. This hypothesis was first introduced by Schrack and colleagues (17), and they later provided empirical evidence to support it by demonstrating that energy expenditure per minute was constant among adults aged 32–96 years, whereas gait speed declined substantially with increasing age (32). In addition, Willis and coworkers showed that young adults select a preferred walking speed in which perceived exertion is close to minimal (38). Our results suggest that the adaptation may be incomplete in older adults, as slow walkers still experienced higher intensities of walking and fatigability than fast walkers. Although slow and fast walkers had similar absolute levels of VO2 during standard and preferred speed walking tests, slow walkers experienced higher levels of physiologic effort due to reduced VO2peak and increased energetic cost.
The results of our correlation and linear regression analyses showed that preferred gait speed was independently associated with VO2peak and VO2 at preferred gait speed. If slow walking speed and increased fatigability develop as a consequence of deteriorating aerobic capacity, then interventions could focus on increasing VO2peak and making a larger reserve of energy available. Alternatively, interventions could reduce the energetic cost of walking (18), which would also reduce the physiologic effort associated with walking.
This study has limitations. It was cross-sectional and involved a small sample size. A larger prospective study is needed to determine whether measures of walking energetics are associated with the onset of declines in preferred walking speed, mobility limitation, and disability and to extend the current findings to older adults with a wider range of physical function; our participants were quite high functioning based on SPPB scores and gait speed. For instance, mean preferred gait speed among slow walkers (1.05 m/s) was faster than commonly used clinical cutpoints for slow walking such as 1.0 m/s. We hypothesize results will be exacerbated in a more representative population with lower functioning older adults. With the small sample size, we observed some nonsignificant trends in baseline characteristics between slow and fast walkers (eg, body mass index, number of medical conditions) that would likely be significant in a larger study. Also, the standard speed used in this study, 0.72 m/s, was quite slow; we expect that a faster standard speed would have elicited differences in VO2, given that the energetic cost of walking at preferred gait speed was elevated among slow walkers. To minimize participant burden, we measured preferred overground gait speed during a 6-m walk and used this as preferred speed on the treadmill; however, preferred overground and preferred treadmill speeds sometimes differ (39), which some (39) but not all (40) have attributed to differences in gait kinematics and kinetics. When speed is held constant, oxygen consumption during treadmill walking may be higher than during overground walking (40). Although we did not directly measure energy requirements of overground walking, our approach of using preferred overground speed during treadmill walking tests likely provided a better simulation of the energetic requirements of overground walking than using preferred treadmill speed (39). Furthermore, we expect similar, and perhaps stronger, relationships would be observed with overground walking as with treadmill walking.
In summary, we observed that slow walkers had reduced VO2peak and a greater energetic cost of walking. As a result, the effort associated with walking was higher among slow walkers than fast walkers, and slow walkers experienced greater fatigability than fast walkers. Interventions that improve aerobic capacity or decrease the energetic cost of walking in older adults may prevent slowing of gait speed and promote mobility.
Funding
This work was supported by the National Institute on Aging at the National Institutes of Health (1RC2AG036594).
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