We demonstrated that the daily number of steps was associated with walking independence after adjusting for confounding factors such as age, sex, stroke type, NIHSS, lower extremity FMA, and 6MWT. The cutoff value of the daily number of steps for walking independence was 4286 steps in patients with sub-acute stroke hospitalized in the rehabilitation unit. Previous studies found that increasing day-time physical activity was related to walking independence in patients with sub-acute stroke who could not walk alone [5], and gait independence increased the median percentage of time spent standing/walking [8] by stroke patients in the rehabilitation unit, which partially support our results. However, to our knowledge, the cutoff value of the number of steps for walking independence has not been investigated. In a previous study, the cutoff value of 6MWT for determining walking independence was 304 m [10] and the cutoff value of lower extremity FMA for predicting walking independence was 22.5 points [22]. As improving the walking ability is one of the goals of rehabilitation [7], combining the cutoff value of the number of steps in our results with that of other motor performance tests may be useful to facilitate early walking independence. Further studies with longitudinal designs are required to clarify the causal relationship between the daily number of steps and walking independence in stroke patients.
We presented the reference values of the number of steps according to FAC in patients with sub-acute stroke in the rehabilitation unit, were as follows: FAC 2 was 1970 steps/day (IQR, 1334.8–3105.3 steps/day); FAC 3 was 2646 steps/day (IQR, 1669–3521 steps/day); FAC 4 was 4518.5 steps/day (IQR, 2951.5–6511.0 steps/day); and FAC 5 was 6763.5 steps/day (IQR, 5382.8–10465.0 steps/day). The daily number of steps was 4935 to 5291 in patients with mild stroke (NIHSS score 1 to 2) [23] and 1514.3 to 3010.7 in patients with moderate stroke (FAC 3; range 2–5) [4]. Day-time physical activity differed according to severity measured by NIHSS in patients with acute stroke [24]. These results are partly consistent with our study findings that the number of steps decreases with increasing disability after stroke.
The number of steps in the walking independence group was 4518.5 steps/day (IQR, 2951.5–6511.0 steps/day) for FAC 4 and 6763.5 steps/day (IQR, 5382.8–10465.0 steps/day) for FAC 5. It was previously reported that 6025 steps per day is an initial target for reducing new vascular events after mild ischemic stroke [25]. The number of steps per day after discharge from the rehabilitation unit increased by 724 steps compared with during hospitalization in stroke patients [26]. In stroke patients with independent walking, preventing new vascular events is an important therapeutic target [25, 27]. However, FAC 4 stroke patients did not reach the target level for reducing new vascular events. For these patients, improved walking ability for outdoor walking independence and promotion of physical activity may be required during hospitalization. In addition to physical function, environmental factors are involved in physical activity in community–dwelling stroke patients [28]. Thus, the factors associated with physical activity during hospitalization and after discharge may differ. Even patients with mild stroke who are able to walk outdoors independently may need consider environmental factors after discharge to maintain physical activity.
On the other hand, the number of steps in the walking non-independence group was 1970 steps/day (IQR, 1334.8–3105.3 steps/day) for FAC 2 and 2646 steps/day (IQR, 1669–3521 steps/day) for FAC 3. Klassen et al. reported that the number of steps during a physical therapy session was 580 (SD, 440) for usual care physical therapy, 2169 (SD, 1106) for more than double the intensity of the control, and 4747 (SD, 2083) for more than quadruple the intensity of the control, and walking recovery was improved in the higher dose group [29]. Patients with FAC 2.3 in our study did not reach the values of the previous study; however there is still a possibility of improvement. In addition, daytime and non-therapy time light intensity physical activity were related to functional recovery in patients with sub-acute stroke who could not walk independently [5]. Thus improvement of the walking ability in patients with poor walking ability and increasing the number of steps during therapy time and light intensity physical activity during non-therapy time are important methods for recovering the walking ability.
Study limitations
There were several limitations in the current study. First, the average daily number of steps taken was used as the measure of physical activity; however, the Fitbit One mean error was higher for slower velocities (10.9% for speeds < 0.4 m/s) [12]. Thus, there may have been measurement errors of physical activity in stroke patients with low walking ability. Second, the daily number of steps was measured at 30 days after stroke onset in patients who were admitted to the rehabilitation unit. Thus, these results may only be applicable to stroke patients in the rehabilitation unit and not to acute stroke or community-dwelling chronic stroke patients. In addition, we could not measure long-term outcomes such as mRS or FAC at 3 or 6 months; thus the relationship between the number of steps at 30 days from stroke onset and long-term outcome was not clarified. Third, this was a cross-sectional study, and the causal relationship between the daily number of steps and walking independence was unknown. Thus, further studies with a longitudinal design are required to clarify the causal relationship between the daily number of steps and walking independence. Fourth, although pre-stroke physical activity was associated with stroke severity and post-stroke outcome [30, 31], pre-stroke physical activity or fitness was not measured. Rehabilitation content affects physical activity; however, the contents of rehabilitation are not standardized because they are determined by each physician. Thus, these factors may affect the result of our study. Finally, this study did not use randomization or assignment; thus, there was a possibility of selection bias and lack of internal validity. Further studies with comparative validity designs are required.