This study involved 11 patients who had strokes, excluding those with subarachnoid hemorrhage, who were admitted to the comprehensive inpatient rehabilitation ward of Fujita Medical University Nanakuri Memorial Hospital between March 2, 2018, and November 17, 2020. The inclusion criteria required participants to have entered the comprehensive inpatient rehabilitation ward within 60 days of stroke onset and consented to participate in this study. The patient demographic comprised 5 males and 6 females (Table 1). Among these, 9 and 2 patients had cerebral infarction and hemorrhage, respectively. The participants’ mean age was 64.5 (SD 12.0) years. Upon ward admission, the patients exhibited a mean Functional Independence Measure (FIM) total score of 52.2 (SD 16.8).

The measurements were performed using a wearable activity monitoring system, the hitoe system [21,22], which comprises a measuring garment (wear or strap) with integrated electrodes, a hitoe transmitter for data transfer, and a smartphone app (Figure 1). This system measures heart rate based on electrocardiogram waveforms obtained from the built-in electrodes. A chest strap was used as the measuring device in this study. The transmitter has a built-in accelerometer that quantifies activity based on acceleration and posture determination (supine or otherwise) at a sampling rate of 25 Hz. The MSDA of the measured acceleration over a 2-second duration was used to indicate the amount of physical body movement [17].

The activity was measured within 1 week of admission to the convalescent rehabilitation hospital and repeated 4 weeks post admission. Patients wore the hitoe system continuously for 48 hours, excluding bathing times when measurements were halted and resumed postbath. On the second day of measurement, the belt and transmitter were replaced.

The FIM was used to assess the patients’ daily activity levels. Specifically, it assesses the degree of independence of activities on a total of 18 items, including 13 motor and 5 cognitive, on a 7-point scale [23]. This study used the 13 motor items as an index of daily activity level. The training (the time during the rehabilitative training session) and nontraining (the time other than the training period) periods were recorded by the physical and occupational therapists who supervised the patients.

Our study used 48 hours of data measured with the abovementioned system. The mean heart rate was calculated from the measured data for supine, sitting, and standing positions. Additionally, the heart rate was used to quantify exercise intensity, which is a common practice in exercise prescription [24]. The American College of Sports Medicine guidelines recommend using a percent heart rate reserve (%HRR) for measuring exercise intensity using heart rate [25]. The %HRR was calculated as heart rate—resting heart rate or maximum heart rate—resting heart rate, where the maximum heart rate was measured or estimated using age. The resting heart rate was determined using the mean heart rate during the night while lying down (median heart rate when the patient was judged to be lying down between 12 and 5 AM) when measurements were stable. The mean of the 24-hour %HRR values calculated with these values was used to analyze exercise intensity.

The Wilcoxon signed-rank test was used to test for significant differences, and statistical significance was set at P<.05. All statistical analyses were performed using the JMP11 (SAS Institute Inc).

The Ethics Committee of Fujita Health University (HM17-220) approved this study protocol, ensuring adherence to ethical standards. All participants provided informed consent. Identifiable data were securely stored in a locked cabinet, separate from the analysis data sets that were anonymized, to safeguard privacy and confidentiality. Participants did not receive financial compensation for their involvement in this study.

This study used the hitoe system to continuously monitor the heart rate and activity status of 11 patients who had strokes and who were admitted to a comprehensive inpatient rehabilitation ward for over 48 hours at admission and 4 weeks post admission. A significant increase was observed in activity time and total physical activity at 4 weeks post admission compared to that upon admission. However, no significant change in the average physical activity intensity was observed. Additionally, the increase in total physical activity primarily resulted from increased activity during training. A significant correlation was found between activity during the training period and the FIM, but not for activity during the nontraining period. Instead, activity during the nontraining period negatively correlated with age.

The activity levels of patients who had strokes, along with their long-term changes, have been thoroughly examined using accelerometers. It is well established that activity levels of patients with stroke are generally lower than those of healthy individuals [1,2]. These levels typically decrease during the acute phase and rise slightly over the following 3 months, but do not significantly increase afterward, remaining lower than in healthy individuals [26]. Variations have been observed in the postdischarge activity levels following acute treatment. In some cases, activity levels increase over time, while in less active groups, there is a further decrease in activity after discharge [3,27].

Heart rate, as a reflection of exercise intensity, signifies changes in cardiac output, and thus, represents the effort required for exercise in comparison to the absolute activity measured by acceleration [28]. As such, heart rate can offer valuable insights into a stroke patient’s mobility function since it quantifies the effort exerted in movement. For example, patients who had strokes with mild motor impairment may move relatively efficiently, whereas those with severe impairment may struggle to move despite expending a lot of effort.

In this study, we noticed an upward trend in physical activity, as measured by an accelerometer, from admission to 4 weeks later. Nevertheless, a significant increase was only observable during training sessions. The strong correlation between body movement measured by an accelerometer during training and the FIM could indicate a rise in dynamic body movement associated with the ability to perform activities of daily living (ADL). Patients who had strokes usually experienced incremental improvements in basic motor skills [29-31], mirrored by an increased walking speed and frequency of standing up, contributing to improved accelerometer readings [32,33]. Additionally, enhancements in exercise tolerance could lead to better accelerometer measurements since individuals can move for longer durations with fewer breaks [34,35].

Interestingly, the amount of activity during the nontraining time did not correlate with FIM but displayed a strong negative correlation with age, and several factors may be attributable to this age-related decrease in activity. Older individuals are more likely to experience conditions, including sarcopenia [36-38] and osteoarthritis [39,40], that could limit their basic fitness and reduce voluntary exercise. Furthermore, they may also actively avoid activities that require significant effort and cause fatigue [41,42]. From these insights, 2 key implications emerge. First, for older patients who can manage, engaging in additional activities during nontraining periods could be beneficial to boost their exercise tolerance, given the importance of maintaining activity levels for improving motor function and exercise tolerance [43,44]. Second, the data also suggests that the daily energy output of older adults may be naturally restricted. Considering this, some individuals might find it beneficial to manage their daily activities based on an “Energy Conservation Strategy.” This approach, commonly used for patients grappling with fatigue issues, focuses on the strategic allocation of energy throughout the day [45].

Despite the increase in physical activity, the average exercise intensity remained relatively unchanged, which could be due to improved motor efficiency following stroke and rehabilitation, enabling patients to perform daily activities more efficiently. Thus, the measurement of both acceleration and heart rate may be used to monitor changes in efficiency, which is observed through the process of poststroke rehabilitation [46,47]. The lack of change in heart rate might also result from reduced effort in certain segments of the population, potentially due to factors such as depression, low motivation, and pain—issues frequently associated with poststroke conditions [48]. Further investigation, such as comparisons with age-matched healthy populations, may help explore the factors that kept the exercise intensity unchanged.

This study had some limitations. First, a small sample size was used in this study, which precluded subgroup analysis. Consequently, biases such as the severity of illness may have influenced the results. For instance, previous studies have reported that acceleration-measured exercise intensity did not correlate with heart rate at low exercise intensities rather than at high exercise intensities, complicating the generalization of these findings to all patients who had strokes [26]. Therefore, the current analysis should be considered preliminary, and a more comprehensive investigation into the factors influencing activity increase needs to be conducted with larger sample sizes in the future. Nevertheless, the findings of this study still have important implications, emphasizing the importance of simultaneous measurement of heart rate and acceleration, which may reflect the efficiency of daily activities, as well as the need to monitor patients’ nontraining activities during rehabilitation programs.

Rehabilitation inpatients experienced a noticeable increase in activity volume during the subacute poststroke phase, even while exercise intensity remained fairly steady. The rise in activity that corresponds with ADL was primarily driven by scheduled training. Activity volume during nontraining periods did not correlate with ADLs; however, a strong correlation was noted with age, highlighting the significance of providing opportunities for activity, particularly for older patients. These observations may also suggest the need for adjusting the activity amount to accommodate the potentially limited fitness levels of this demographic. Future studies with larger and more varied patient groups are warranted to validate and further elucidate these findings.