Physical activity measured using a triaxial accelerometer in early after surgery rehabilitation in patients with hip fracture affects walking ability: a retrospective observational study

Abstract

[Purpose] To clarify the association between physical activity measured using a triaxial accelerometer in early after surgery rehabilitation sessions and short-term walking ability in older adult patients with hip fractures and investigate indicators for determining the appropriate intensity and volume of early mobilization. [Participants and Methods] We analyzed 109 patients aged ≥75 years (median, 88.0 years) with hip fractures who underwent surgical treatment. A triaxial accelerometer was used to quantify physical activity. We investigated the association between physical activity (≥1.6 METs) measured early after surgery and walking ability at 1 and 2 weeks after surgery. [Results] The rehabilitation intervention times were 119.0 [104.7–129.3] min/day. Factors associated with walking independence were as follows: 1 week after surgery, fracture type, cognitive function, and physical activity during rehabilitation sessions; 2 weeks after surgery, surgery type, cognitive function, and physical activity during rehabilitation sessions. The cutoff values for physical activity during rehabilitation sessions for determining walking independence were ≥21.6% and ≥17.8% at 1 and 2 weeks after surgery, respectively. [Conclusion] Physical activity during early after surgery rehabilitation sessions affects short-term walking ability. Our findings emphasize the importance of establishing indicators of physical activity and early mobilization during early after surgery rehabilitation sessions.

INTRODUCTION

Hip fractures are a common type of fragility fracture in older adults. With population aging, incidence is increasing, making hip fractures a major public health issue. In Japan, most patients (95.4%) undergo surgical treatment¹⁾, followed by rehabilitation to improve physical function and the ability to perform activities of daily living. Nonetheless, because approximately half of patients do not recover pre-injury walking ability²⁾, developing effective after surgery rehabilitation strategies is an urgent issue. Several guidelines recommend early mobilization as part of early after surgery rehabilitation for patients with hip fractures³⁾. Further, longer-standing and walking time early after surgery are reportedly associated with recovery of lower limb function⁴⁾ and walking ability⁵⁾. Nevertheless, the appropriate physical activity (intensity and amount) for after surgery early mobilization remain insufficiently discussed. Therefore, physical activity indicators are essential for implementing early after surgery rehabilitation in patients with hip fractures.

Currently, the most practical approach to assessing physical activity is measurement using triaxial accelerometers. We also used a triaxial accelerometer in this study. It demonstrates high correlation with the doubly labeled water method, which is the gold standard for measuring total energy expenditure during free movement⁶⁾, and it can accurately estimate low-intensity physical activity⁷⁾. Additionally, this device has been used to evaluate early after surgery physical activity in older patients with hip fractures⁸⁾. The clinical utility of a triaxial accelerometer is that it allows objective, quantifiable measurement of the intensity and duration of rehabilitation.

This study aimed to clarify the association between physical activity and short-term walking ability during early after surgery rehabilitation sessions in older patients with hip fractures and to identify indicators for determining the appropriate intensity and duration of early mobilization.

PARTICIPANTS AND METHODS

This study was a retrospective observational study conducted at a secondary emergency hospital (Aizawa Hospital). At our hospital, hip fracture surgery is scheduled on the day of admission or the following day whenever possible. Following surgery, patients remain in bed rest until the start of rehabilitation. Rehabilitation starts the next morning after surgery, after a doctor evaluates the wound and the overall condition. Rehabilitation is provided throughout the year, and activities of daily living are expanded as quickly as possible according to mobility. Pain management from the day after surgery consisted of acetaminophen four times daily (after each meal and before bedtime).

The study population comprised 299 patients aged ≥75 years who underwent hip fracture surgery and were treated at our hospital between August 2024 and September 2025. Exclusion criteria were inability to walk independently before injury, high-energy trauma fractures or concurrent upper limb fractures, basicervical femoral neck fracture, total hip replacement, weight restrictions, management outside an orthopedic surgery ward, isolation management for 3 days after surgery due to infection, and lack of physical activity measurement for 3 days after surgery (not worn, triaxial accelerometer detachment or difficulty in wearing it continuously, poor data recording or measurement errors).

Data collected from medical records included age, sex, fracture type (femoral neck, trochanteric), surgery type (bipolar hip arthroplasty, osteosynthesis), days to surgery, after surgery hospitalization period, cognitive function at admission, early mobilization (activity to sit or stand out of bed on the day after surgery), Global Leadership Initiative on Malnutrition (GLIM), walking ability, rehabilitation (physical and occupational therapy) intervention times, and physical activity.

Cognitive function was assessed using the Mini-Mental State Examination (MMSE) at admission. The MMSE is a simple screening test used to assess cognitive decline and is scored on a scale of 0–30. Based on the criteria of a previous report⁹⁾, the MMSE range was classified on a 5-point ordinal scale (no dementia, 30 points; questionable dementia, 26–29 points; mild dementia, 21–25 points; moderate dementia, 11–20 points; and severe dementia, 0–10 points).

Functional ambulation categories (FAC) were used to evaluate walking ability^{10, 11)}. The FAC is classified into six levels based on the degree of walking independence (independent anywhere: 5 points, independent on level ground: 4 points, requires supervision: 3 points, requires mild assistance: 2 points, requires moderate or greater assistance: 1 point, and cannot walk: 0 points). Patients were classified into two groups based on their walking ability at 1 and 2 weeks after surgery: an independent walking group (4 points or higher) and a non-independent walking group (3 points or lower). Here, to account for bias due to differences in assessment timing, walking ability at 1 and 2 weeks after surgery was used as the main outcome.

Physical activity was measured by using a triaxial accelerometer (Active Style Pro HJA-750C; Omron Healthcare, Kyoto, Japan). Because the evaluation of physical activity requires measurements over at least three days¹²⁾, the measurement period was 72 h from after surgery day 1 to day 4 (from the time of the first rehabilitation intervention on after surgery day 1 to the same time on after surgery day 4) (Fig. 1). The epoch length used for measurements was 10 s. The metabolic equivalent (MET) was used as the measurement index. The triaxial accelerometer was attached to the waistband of the pants on the healthy side and secured with a holder and a clip with a strap. Bathing time was excluded from the measurements. Here, physical activity was defined as light-intensity physical activity or higher (≥1.6 METs). In physical therapy sessions for hip fracture patients on the third day after surgery, the following physical activities were equivalent to 1.6 METs or more: standing and sitting, 1.8 METs; transferring, 1.8 METs; activity in a standing position, 2.1 METs; walking with parallel bars, 1.8 METs; walking with a walker, 1.8 METs; walking with a cane, 2.0 METs; and climbing stairs, 2.1 METs¹³⁾.

Fig. 1.

Timeline of the research.

Physical activity was classified into the following three periods: 1) total time, hours per day (24h); 2) rehabilitation sessions, total hours of rehabilitation per day; and 3) non-rehabilitation period, 15 h from the ward’s designated wake-up time to lights-out time minus rehabilitation sessions per day⁸⁾. Physical activity during 2) rehabilitation sessions and 3) non-rehabilitation periods was affected by the duration of the 2) rehabilitation sessions; therefore, physical activity was calculated as a percentage of the total time for each category⁸⁾. For physical activity relative to 1) total time, the mean per day was divided by hours per day (1,440 min=24h); for physical activity relative to 2) the rehabilitation sessions, the mean per day was divided by the total hours of rehabilitation per day; and for physical activity relative to 3) the non-rehabilitation period, the mean per day was divided by the total hours of the non-rehabilitation period.

A post hoc test of sample size was performed using G*Power^{14, 15)}. The mean ± standard deviation (%) of physical activity during rehabilitation sessions (independent walking group versus non-independent walking group) was 28.9 ± 7.7% versus 16.9 ± 7.4% and 23.9 ± 8.2% versus 15.6 ± 7.1% at 1 and 2 weeks after surgery, respectively. 15 patients were able to walk independently after one week, and 39 patients were able to walk independently after two weeks (independent walking group). 94 patients were unable to walk independently after one week, and 70 patients were unable to walk independently after two weeks (non-independent walking group). with the significance level was set at 5%, the statistical power was 0.99 at both 1 and 2 weeks after surgery.

The Shapiro–Wilk test was performed to assess normality. Sensitivity analyses accounted for differences in the timing of walking ability assessment. The Mann–Whitney U test or χ² test was used to compare walking ability between groups at each time point. Multiple logistic regression analysis (forward variance method: likelihood ratio) was performed to control for potential confounding in the association between walking ability and physical activity at each time point. The dependent variable was walking ability in each period, and the independent variables were age, sex, fracture type, surgery type, early mobilization, cognitive function, GLIM score, and physical activity (total time, rehabilitation sessions, and non-rehabilitation periods). Area under the curve analysis was conducted to determine cutoff values for physical activity during rehabilitation sessions and to assess walking independence at each time point. Analyses were performed using IBM SPSS Windows version 26 (IBM Corp., Armonk, NY, USA), and the significance level was set at 5%.

This study was approved by the Institutional Review Board of Aizawa Hospital (approval number: 2025-079). Personal information was anonymized and protected. The requirement for written informed consent was waived due to the retrospective observational nature of the study.

RESULTS

Among the 299 participants, 190 met the exclusion criteria leaving 109 cases for analysis. Fifteen and 39 patients were able to walk independently at 1 and 2 weeks after surgery, respectively (Fig. 2). The median age was 88.0 years; 82 were female, and 27 were male. The rehabilitation intervention times were 119.0 [104.7–129.3] min/day. Regarding physical activity, total time, rehabilitation sessions, and non-rehabilitation periods were 3.7 [2.5–6.5]%, 17.8 [12.4–23.2]%, and 3.0 [1.8–6.1]%, respectively (Table 1). Variables with medium or high effect sizes in the between-group comparisons were age (0.31), cognitive function (0.46), physical activity in total time (0.31) and physical activity in rehabilitation sessions (0.46) at 1 week after surgery, and cognitive function (0.57), GLIM (0.31), and physical activity in rehabilitation sessions (0.47) at 2 weeks after surgery (Table 1).

Fig. 2.

Flowchart of patient participation.

Table 1. Comparison of basic attributes and walking ability between groups at each time point.

Item		Total (n=109)	Walking ability one week after surgery					Walking ability two week after surgery
			Independent walking group (n=15)	Non-independent walking group (n=94)		|dij|	Effect size	Independent walking group (n=39)	Non-independent walking group (n=70)		|dij|	Effect size
Age (years)†		88.0 [83.0–91.0]	83.0 [77.5–86.0]	88.5 [84.0–92.0]	**	-	0.31a	86.0 [80.0–89.0]	89.0 [84.0–93.0]	**	-	0.29a
Sex (n)‡						-	0.04b				-	0.03b
	Female	82	12	70		0.5		30	52		0.3
	Male	27	3	24		0.5		9	18		0.3
Fracture type (n)‡					*	-	0.23b			*	-	0.19b
	Femoral neck	56	12	44		2.4		25	31		2.0
	Trochanteric	53	3	50		2.4		14	39		2.0
Surgery type (n) ‡					*	-	0.21b			*	-	0.22b
	Bipolar hip arthroplasty	51	11	40		2.2		24	27		2.3
	Osteosynthesis	58	4	54		2.2		15	43		2.3
Days to surgery (days)†		1.0 [1.0–2.0]	1.0 [1.0–2.0]	1.0 [1.0–2.0]		-	0.01a	1.0 [1.0–2.0]	1.0 [1.0–2.0]		-	<0.01a
After surgery hospitalization period (days)†		19.0 [17.0–25.0]	18.0 [15.5–19.5]	20.0 [17.3–28.0]	*	-	0.20a	19.0 [17.0–21.5]	20.0 [17.0–31.0]		-	0.16a
Cognitive function (n)‡					**	-	0.46c			**	-	0.57c
	Severe dementia	20	0	20		2.0		0	20		3.7
	Moderate dementia	39	2	37		2.0		7	32		2.9
	Mild dementia	27	4	23		0.2		15	12		2.5
	Questionable dementia	19	6	13		2.5		14	5		3.8
	No dementia	4	3	1		3.6		3	1		1.7
Early mobilization (n)‡					*	-	0.18b			**	-	0.29b
	Yes	90	15	75		1.9		38	52		3.1
	No	19	0	19		1.9		1	18		3.1
Global Leadership Initiative on Malnutrition (n)‡					*	-	0.28c			**	-	0.31c
	No malnutrition	74	15	59		2.9		34	40		3.2
	Moderate malnutrition	15	0	15		1.7		3	12		1.4
	Severe malnutrition	20	0	20		2.0		2	18		2.7
Rehabilitation intervention times (min/day)†		119.0 [104.7–129.3]	122.7 [115.3–134.5]	117.0 [103.8–127.8]		-	0.16a	120.7 [109.3–131.3]	115.0 [102.2–128.5]		-	0.13a
Physical activity in total time (%)†		3.7 [2.5–6.5]	7.0 [5.7–8.2]	3.4 [2.4–5.3]	**	-	0.31a	4.1 [3.2–7.2]	3.3 [2.3–5.5]		-	0.19a
Physical activity in rehabilitation sessions (%)†		17.8 [12.4–23.2]	27.0 [24.5–32.9]	16.1 [11.7–21.5]	**	-	0.46a	22.4 [18.5–28.2]	15.0 [10.1–20.1]	**	-	0.47a
Physical Activity in Non-rehabilitation periods (%)†		3.0 [1.8–6.1]	6.4 [4.3–8.9]	2.7 [1.7–5.1]	**	-	0.26a	3.4 [2.3–6.5]	2.8 [1.7–5.8]		-	0.11a

Data are provided as median [interquartile range] or number. dij, adjusted residual. ^†Mann–Whitney U test; ^‡ χ² test. a r; b φ; c Cramer’s V. *p<0.05, **p<0.01.

Factors associated with independent walking 1 week after surgery included fracture type, cognitive function, and physical activity during rehabilitation sessions. Factors correlated with independent walking two weeks after surgery were surgery type, cognitive function, and physical activity during the rehabilitation sessions (Table 2). Sensitivity analyses showed that the factors related to walking ability were similar across time points. Additionally, physical activity during rehabilitation sessions was associated with walking ability 1 and 2 weeks after surgery.

Table 2. Factors related to walking ability at each time.

	Item		Odds ratio	95% CI
Walking ability one week after surgery †1	Fracture type	*	0.09	0.01–0.61
	Cognitive function	**	3.02	1.35–6.74
	Physical Activity in Rehabilitation sessions	**	1.21	1.07–1.37
Walking ability two week after surgery †2	Surgery type	*	4.18	1.35–12.94
	Cognitive function	**	3.82	2.03–7.18
	Physical Activity in Rehabilitation sessions	*	1.10	1.02–1.18

^†1 Model χ² test: p<0.001, Hosmer–Lemeshow test: p=0.910, percentage of correct classifications: 89.9%, Nagelkerke R²: 0.58. ^†2 Model χ² test: p<0.001, Hosmer–Lemeshow test: p=0.572, percentage of correct classifications: 85.3%, Nagelkerke R²: 0.54. *p<0.05, **p<0.01.

The physical activity during rehabilitation sessions, which determined whether patients could walk independently, was ≥21.6% at 1 week after surgery. The physical activity in rehabilitation sessions, which determines whether patients can walk independently, was ≥17.8% at 2 weeks after surgery. The predictive performance was higher at 1 week after surgery (AUC, 0.89) than at 2 weeks after surgery (AUC, 0.79) (Fig. 3).

Fig. 3.

Cutoff values required for independent walking at each time.

(a): Walking ability 1 week after surgery, (b): Walking ability 2 weeks after surgery. AUC: area under the curve; CI: confidence interval.

DISCUSSION

We investigated the association between physical activity during early after surgery rehabilitation sessions and short-term walking ability in older patients with hip fractures. Physical activity during rehabilitation sessions was associated with walking ability at both 1 and 2 weeks after surgery, supporting the robustness of the results. These findings underscore the importance of developing physical activity indicators during early after surgery rehabilitation.

Physical activity during early after surgery rehabilitation sessions exhibited a large effect size in between-group comparisons stratified by walking ability at each time point and was also associated with walking ability at each time point in the multiple logistic regression analysis. Early mobilization³⁾ is recommended for early after surgery rehabilitation in patients with hip fractures and has been reported to affect short-term walking ability¹⁶⁾. Nevertheless, here, physical activity during early after surgery rehabilitation sessions had a greater effect on walking ability at each time point than early mobilization, and this effect was extracted as a factor related to walking ability. Therefore, in addition to implementing early mobilization in early after surgery rehabilitation sessions for patients with hip fractures, clinicians should emphasize the physical activity achieved and provide appropriate exercise intensity and volume.

The cutoff values for physical activity in rehabilitation sessions to determine walking independence were ≥21.6% at 1 week after surgery and ≥17.8% at 2 weeks after surgery. These cutoff values varied depending on the timing of the outcome. In early after surgery rehabilitation sessions, the findings suggest that, beyond early mobilization, achieving physical activity of ≥1.6 METs at ≥21.6% (walking independently 1 week after surgery) or ≥17.8% (walking independently 2 weeks after surgery) may serve as an indicator. Because physical activity measured using a triaxial accelerometer is considered to have small ceiling and floor effects, it may provide useful information for individualized rehabilitation strategies in this age of diversification.

This study had some limitations. First, generalizability is limited because this retrospective observational study was conducted at a single center and excluded many cases (190); further multi-center research is needed to develop a physical activity index. Second, outcomes were assessed at 1 and 2 weeks after surgery, which supports the implementation of acute rehabilitation; however, this design did not allow evaluation of mid- to long-term walking ability or other outcome indicators. Third, although the study demonstrated a correlation between physical activity during early after surgery rehabilitation sessions and short-term walking ability, it remains unclear whether increasing physical activity improves walking ability; interventional studies are needed to clarify factors associated with early after surgery physical activity. Fourth, although using physical activity as an indicator enables objective quantification of rehabilitation intensity and amount, measurement requires equipment such as a triaxial accelerometer. Fifth, after surgery pain affects rehabilitation progress; however, this study lacked an objective pain assessment. Sixth, it is highly possible that some patients experienced reduced after surgery mobility because of underlying medical conditions, such as heart failure or renal dysfunction. Since these comorbidities were not sufficiently considered in the analysis.

This study investigated the association between physical activity during early post-surgical rehabilitation sessions and short-term walking ability in older patients with hip fractures. Physical activity during the rehabilitation sessions affected the walking ability at 1 and 2 weeks after surgery. We propose that, in the early post-surgical rehabilitation of older patients with hip fractures, physical activity should be used as an indicator alongside early mobilization.

Funding and Conflict of interest

The authors have no conflicts of interest or financial support to disclose.