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The Journal of Spinal Cord Medicine logoLink to The Journal of Spinal Cord Medicine
. 2019 Feb 27;44(2):212–220. doi: 10.1080/10790268.2019.1576444

Validity of the Apple Watch® for monitoring push counts in people using manual wheelchairs

Kati S Karinharju 1,2,✉, Alexandra M Boughey 3, Sean M Tweedy 1,4, Kelly M Clanchy 5, Stewart G Trost 6, Sjaan R Gomersall 3
PMCID: PMC7952070  PMID: 30811310

Abstract

Objective: A recent Apple Watch® activity-monitoring innovation permits manual wheelchair users to monitor daily push counts. This study evaluated the validity of the Apple Watch® push count estimate.

Design: Criterion validity.

Setting: Southern Finland and Southeast Queensland, Australia.

Participants: Twenty-six manual wheelchair users from Finland and Australia were filmed completing a standardized battery of activities while wearing the Apple Watch® (dominant wrist).

Outcome Measures: Wheelchair pushes as determined by the Apple Watch® were compared to directly observed pushes.

Results: Agreement between Apple Watch® push counts and directly observed pushes was evaluated using Intraclass correlation coefficients (ICC), Pearson correlations and Bland-Altman analyses. Apple Watch® pushes and directly observed push counts were strongly correlated (ICC = 0.77, P < 0.01) (r = 0.84, P < 0.01). Bland Altman plots indicated that the Apple Watch® underestimated push counts (M = −103; 95% ULoA = 217; LLoA = −423 pushes). Mean absolute percentage error was 13.5% which is comparable to studies evaluating agreement between pedometer-based step counts and directly observed steps.

Conclusion: Apple Watch® push-count estimates are acceptable for personal, self-monitoring purposes and for research entailing group-level analyses, but less acceptable where accurate push-count measures for an individual is required.

Keywords: Apple Watch®, Monitoring, Physical activity, Push counts, Wheelchair

Introduction

In 2015, over 163,000 people in Australia who reported having a disability, used a manual wheelchair for mobility.1 People with spinal cord injury (SCI)2 are amongst the highest users of manual wheelchairs in Australia,3 but manual wheelchair use is also relatively common among people with cerebral palsy, spina bifida, osteogenisis imperfecta, multiple sclerosis and a range of other health conditions. People using manual wheelchairs have significantly high prevalence of cardiovascular diseases and higher mortality from cardiovascular diseases than general population.4,5 Furthermore, physical inactivity has been identified as a major contributor to cardiovascular disease among this population.6–8 Regular participation in physical activity has been demonstrated to reduce risk factors for cardiovascular disease and improve physical fitness in people using manual wheelchairs,9–11 however evidence indicates that participation rates are particularly low among manual wheelchair users11–14 and effective methods to promote physical activity participation in this population are required.

Wearable devices, which record step counts in people who do not use manual wheelchairs, have been shown to be an effective method to increase physical activity.15 However, steps are not applicable to consumers using manual wheelchairs or to researchers who wish to monitor physical activity in this population because typical daily activities are almost exclusively upper limb, primarily from self-propelled wheelchair pushing.16 Push counts – the number of times a wheelchair user moves their chair by applying a force to the rim of the wheel – may be a reasonable proxy for steps, but monitoring push counts is difficult because existing wearable activity devices, including those worn on the wrist, cannot differentiate wheelchair pushes from other upper limb movements. Wheelchair mounted devices such as odometers, accelerometers and wheel rotation calculators have been used previously to measure daily wheelchair-based activities. These devices are able to provide information regarding time and distance the wheelchair is moving, but they have not yet been validated for measuring free living wheelchair activities and they are unable to differentiate active propulsion of the wheelchair by the user from passive wheelchair propulsion such as when the wheelchair is coasting downhill or being pushed by someone else.17,18

In 2016, Apple® released software for the Apple Watch® (Apple Inc., Cupertino, California, USA), to enable monitoring of physical activity in manual wheelchair users by tracking wheelchair pushes. However, to our knowledge, its accuracy has not been independently evaluated. If the Apple Watch® is found to provide a valid estimate of pushing activity, it may be an effective physical activity self-monitoring tool for manual wheelchair users, and also provide the research community with an easily accessible device for evaluating exercise and rehabilitation interventions. Therefore, the aim of this study was to determine whether the Apple Watch®, provides a valid measure of wheelchair pushes during simulated everyday activities by wheelchair users when compared to direct observation.

Method

Study design

This study was undertaken as part of a larger body of research investigating the validity of multiple physical activity measures in wheelchair users, with data collected across two sites, one in Finland and one in Australia. Wheelchair pushes as determined by the Apple Watch® were compared to directly observed pushes. Ethical approval was granted by The Medical Research and Ethics Committee.

Participants

Community dwelling persons requiring the use of a manual wheelchair for everyday mobility were invited to participate in this study. The inclusion criteria for this study included participants who were: i) at least 18 years old and live independently in a community setting; ii) use a manual wheelchair as a primary means of mobility and were able to self-propel a wheelchair; iii) medically fit to perform at least moderate intensity physical activity as determined by the Adult Pre-Exercise Screening System (APSS)19 and iv) intellectually capable of understanding and following instructions to undertake the supervised activities. Participants were excluded if they had self-reported illness or injury that was affecting their ability to exercise. Potential participants were recruited by convenience sampling via public advertising, advertising through gatekeeper organizations and word-of-mouth in both Southern Finland and Southeast Queensland, Australia. Public advertisements included the use of social media, flyers, newspaper advertisements and email. The gatekeeper organizations included Sporting Wheelies and Disabled Association Queensland, Gold Coast Recreation and Sport Inc., the Finnish Association of People with Physical Disabilities, and the Finnish Sports Association of Persons with Disabilities. All gatekeepers were provided with details of the study, which they circulated, to eligible and interested persons. The participants who were interested in participating in the study contacted the principle investigator (KK) directly by email or phone call. Participants were then provided with the study material via email including: the participant information sheet, which detailed information regarding the study protocol and inclusion criteria in a language commensurate with the level of understanding of the participants; the APSS Health Screening Form; and the participant consent form. The consent form was based on the policies of the Medical Research and Ethics Committee of Queensland University. After being provided with opportunity to ask any questions regarding their participation, participants returned the signed consent form and APSS Health Screening Form and underwent a familiarization protocol in order to ensure their comfort with the testing equipment and protocol.

Procedures

Testing was undertaken in the participant’s own wheelchair in public sporting gymnasiums. Participants completed a standardized protocol comprising five categories with 21 tasks designed to simulate everyday wheelchair based activities20 whilst wearing the Apple Watch®. The tasks that the participants were asked to complete were not threatening or unfamiliar to the participants. These tasks included active and passive wheelchair pushing at different speeds, directions, distances and levels, maneuvering of the wheelchair and stationary activities of daily living such as using a tablet or folding laundry. Depending on the participant’s capacity, the time taken to complete the full protocol ranged from one to two hours. The overall protocol including categories, tasks and activities is presented in Table 1 and the full, detailed protocol is available as a Supplementary Digital Content file.

Table 1. The obstacle course test protocol comprised 21 tasks each of which was assigned to 1 of 5 Categories.

Categories (5) Tasks (21) Distance (m)
1. Wheelchair propulsion – linear discontinuous

  1. Push chair forward 20 m at self-selected COMFORTABLE pace with stop at 10 m

  2. Push chair forward 20 m at self-selected BRISK pace with stop at 10 m

  3. Push chair forward 20 m at self-selected FAST pace with stop at 10 m

  4. Reverse chair backward 20 m at self-selected pace with stop at 10 m

  5. Push chair forward 18 m at self-selected pace with stops at 6, 12 and 18 m

  6. Pushing the chair backward 18 m at self-selected pace with stops at 6, 12 and 18 m

 20 m

20 m
20 m 20 m
18 m

18 m
2. Wheelchair propulsion – continuous with turning in one direction

  1. Push chair forward with a 90° turn to the left every 10 m until two squares have been completed (80 m). Repeat to right (80 m).

  2. As per Activity #7 but wheelchair user is passive while their wheelchair is pushed by an assistant

  3. Continuous forward push around the perimeter of 4 rectangles of increasing size – the first 10 m × 1 m, the next 10 m × 3 m, then 10 m × 5 m and finally 10 m × 7 m. Total push distance of 125.6 m. The task was first completed with all turns to the left, then to the right.

 80 m (L)
80 m (R)
80 m (L) 80 m (R)
125.6 m (L) 125.6 m (R)
3. Wheelchair propulsion – with maneuvering*

  1. Five markers evenly spaced over 1.10 m with the wheelchair user beginning to the left of the first marker and passing through the next three markers in a slalom fashion, turning 180° at the last marker, returning to the start, turning 180° at the first marker and repeating the task completing 22.4 m a total push distance. Repeat to the right.

  2. Shopping aisle push - participants push a straight-line distance of 5 m with a stop every 1 m to, alternately, “take an item” from a bottom shelf on the left and then reach for an item above head height crossing to the right side completing 5 m total push distance. Repeat to right.

  3. Pushing 1 m flat surface, pushing up a 1.65 meter ramp, turning 180° left and pushing down a ramp and 1 m flat surface completing 7.3 m total push distance. Repeat to the right.

  4. Pushing chair over 5 thresholds 1cm high and 80cm wide, each 1 m apart for a total push distance of 5 m. Repeat.

  5. Pushing chair forward 2 meters to a 10cm curb, mount the curb and turning 180° left at the top of the curb, dismount the curb and push chair forward 2 meters completing a total push distance of 5 m. Repeat with 180° turn right.

 22.4 m (L)
22.4 m (R)



5 m (L)
5 m (R)


7.3 m (L)
7.3 m (R)



5 m (L)
5 m (R)
4. Confined space maneuvering

  1. Push chair forward with a four 90° turn to the left every 1.10 m completing a total push distance of 4.4 m. Repeat to right.

  2. Three markers evenly spaced over 1.10 m with the wheelchair user beginning to the left of the first marker and push the chair forward 1.10 m to a second marker. making a 360° spin to the left. Push the chair forward 1.10 m to the last marker and making a 360° spin to the left completing 6.55 m total push distance. Repeat to the right.

  3. Six 0.72 m wide parking spaces marked on the ground. Wheelchair user push chair forward 0.72 m and reversing the chair to the parking space and push forward 0.72 m and reverse to a next parking space completing 6 reverse and 20.7 m total push distance. Repeat to the right.

  4. Seven markers spaced on zigzag figure over 0.55 m. Wheelchair user beginning toes behind the first marker push the chair forward 0.55 m to a second marker. Reversing the chair 0.55 m with 45° left angle to the third marker. push the chair forward 0.55 m to the fourth marker. Reversing the chair 0.55 m with 45° left angle to the fifth marker. Push the chair forward 0.55 m to the sixth marker. Reversing the chair 0.55 m with 45° left angle to the seventh marker completing 6.6 m total push distance. Repeat to the right.

 4.4 m (L)
4.4 m (R)
6.55 m (L)
6.55 m (R)



20.7 m (L) 20.7 m (R)


6.6 m (L) 6.6 m (R)
5. Stationary activities

  1. Taking the water bottle from the back pack with left hand. opening it and drinking from it with left hand. Repeat with right hand.

  2. Taking an electronic tablet from backpack with left hand and using it for 2 min with left hand. Repeat with right hand.

  3. Folding laundry with alternating left and right side for 2 min

 Left. Right

Left. Right

Alternating left and right side
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Instrumentation

Apple Watch®

The Apple Watch® (Series 1, iOS version 10; Apple Inc., Cupertino, California, USA) was placed on the dorsal aspect of the participant’s dominant wrist, just proximal to the ulnar styloid process. The total number of pushes was recorded using the Apple Activity® Application (Apple Inc., Cupertino, California, U.S.A) by calculating the difference between the number of pushes at the beginning and the end of the protocol.

Direct observation

All testing sessions were filmed using either a handheld Panasonic HC-SD9® video recorder (Panasonic Corporation, Kadoma, Osaka, Japan) or an Apple iPad® device (Apple Inc., Cupertino, California, U.S.A). A member of the research team (AB) observed each participant’s video and manually recorded number of pushes in real time in a spreadsheet. Before the observation, four members of the research team (KK, SG, AB and ST) operationally defined ‘a push’. A wheelchair push was defined as any force applied to the rim of the wheel by the hand that resulted in movement of the manual wheelchair.

Data analysis

All statistical analyses were performed in STATA® (version 14.0; StataCorp LP, College Station, Texas, U.S.A). Normality of data was investigated by visual inspection of histograms, and the Shapiro-Wilk test. Two-way random intra-class correlation coefficients (ICC) with absolute agreement and Pearson correlations with 95% confidence intervals were used to summarize the relationship between manually counted push counts and Apple Watch® pushes. Correlations were defined as strong (r ≥ 0.7), moderate (r = 0.5–0.7) or weak (r < 0.5).21 A Bland-Altman plot with mean bias and 95% limits of agreement was used to investigate absolute agreement.22,23 The mean absolute percentage error (MAPE) was then calculated.24 A priori sample size estimates were based on previous research investigating the validity of Apple Watch® step counts which resulted in correlations of r = 0.7.25 With 80% power and an alpha level of 0.05, the minimum sample size required was n = 13.

Results

Twenty-six participants from Finland (n = 13) and Australia (n = 13) completed the protocol and characteristics are presented in Table 2. The majority of the participants had SCI (n = 15, 57%), with the remaining participants having a diagnosis spina bifida (n = 3, 11%), post-infectious autoimmune neuropathy (n = 2, 8%), epidural abscess (n = 1, 4%), transverse myelitis (n = 1, 4%), tumor (n = 1, 4%), cerebral palsy (n = 1, 4%), osteogenesis imperfecta (n = 1, 4%) and motor neuron disease (n = 1, 4%). Participants were predominately male (n = 20, 77%) and right hand dominant (n = 24, 92%). The results of 24 participants were included in the analysis with two participants results completely excluded because of technical difficulties with the Apple Watch’s synchronization process.

Table 2. Participant characteristics.

Characteristics Mean (SD)  
Age (years) n = 26 42 (13)  
Height (cm) 170.3 (17.9)  
Weight (kg) 80.0 (25.1)  
Time wheelchair use (years) 18.7 (12.5)  
Characteristics n %
Sex    
 Female 6 23
 Male 20 77
Diagnoses
 SCI 15 57
 Spina bifida 3 11
 Postinfectious    
 Autoimmune neuropathy 2 8
 Epidural abscess 1 4
 Transverse myelitis 1 4
 Tumor 1 4
 Cerebral Palsy 1 4
 Osteogenesis imperfecta 1 4
 Motor neuron disease 1 4
Hand Dominance
 Right 24 92
 Left 2 8
Country of testing    
 Finland* 13 50
 Australia 13 50
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Table 3 presents the average number of pushes performed by the participants across the data collection protocol as measured by direct observation (985 ± 300 pushes; range, 637–1978) and by the Apple Watch® (882 ± 239 pushes; range, 479–1392 pushes). The correlation between manually counted pushes and those measured by the Apple Watch® was strong, with a positive Intraclass correlation coefficient ICC = 0.77 (95% CI: 0.53–0.90) and with Pearson’s correlation coefficient of 0.84 (95% CI: 0.66–0.93) (P < 0.001) (Figs. 1 and 2). MAPE for the Apple Watch® was 13.5%. Bland-Altman analyses showed that the Apple Watch® underestimated pushes compared to manually counted pushes (Fig. 2; Table 3). The mean difference was 103 push counts (95% LoA = −423–217 push counts).

Table 3. Correlations and Bland-Altman findings for push counts recorded by the Apple Watch® compared to direct observation (n = 24).

Measurement
Method		 Mean Push Counts
SD (range of pushes)		 Pearson’s Correlation Coefficient
(95% CI)		 Push Counts Mean Difference
(SD)		 Upper LoA Lower LoA
Apple Watch® 882
± 239 (479–1392)		 .840*
(0.66–0.93)		 −103
(163)		 217 −423
Direct Observation 985
± 300 (637–1978)
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SD, standard deviation; CI, confidence interval; LoA, limits of agreement as determined by Bland-Altman Method.

*significant to P < 0.001.

Figure 1.

Figure 1

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Scatterplot of push counts recorded by Apple Watch® (x axis) vs. Direct observation (y axis); ρ = 0.84.

Figure 2.

Figure 2

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Bland-Altman plot for wheelchair pushes: the Apple Watch® compared with direct observation. Note: The solid line indicates the mean degree of underestimation by the Apple Watch® measured in push counts. The dashed lines represent the upper and lower limits of agreement (push counts). Figures have been rounded to the nearest whole push. LoA, limits of agreement; MD, mean difference.

Discussion

The Apple Watch® is the first commercially available wearable device able to monitor manual wheelchair pushes. By measuring pushes, the device has potential to track physical activity among people who use a manual wheelchair as their primary means of locomotion. Overall, it was found that, whilst the Apple Watch® pushes were strongly correlated with manually counted pushes, on average it underestimated pushes. In addition, the 95% limits of agreement were wide, indicating a large degree of individual prediction error.

The correlation between manually counted pushes and Apple Watch® pushes was similar in magnitude to that reported for other wrist-worn consumer devices in relation to steps. One study investigated the accuracy of the Apple Watch® and the Fitbit Charge HR® step count measures and reported similar correlations of r = 0.70–0.76 when compared to manually counted steps in adults who ambulate as their primary measure of locomotion.25 Another study investigated ten devices measuring step counts and reported correlations ranging from r = 0.7–1.0 in a treadmill based protocol and r = 0.3–1.0 in an over-ground protocol compared to direct observation.8,26 Hence the Apple Watch® measurement of pushes performs similarly to or better than devices measuring steps in the ambulant population.

The MAPE of the Apple Watch® push counts compared to direct observation was 13.5%. Previous studies that have evaluated the agreement between wearable step count devices and direct observation have reported MAPE ranging from 0.4% to 24%.27 While the magnitude of the MAPE is likely to limit the acceptability of the Apple Watch® for applications where precise push-counts for an individual is required, the push count estimate provided by the Apple Watch® is likely to be acceptable for monitoring of personal physical activity levels, the primary use of many commercially available wearable activity monitors.

In this study, there were 640 pushes between the upper limit of agreement (217 pushes) and the lower (−423 pushes). This degree of prediction error at the individual level of measurement is comparable to consumer-based step monitoring devices. In one study, under free-living conditions, the range of the 95% limits of agreement for the Fitbit Zip® and Misfit Shine was 861 and 1400 steps respectively.27 It is possible that prediction errors in wheelchair pushes may be explained, at least in part, by differences in the wheelchair propulsion styles of participants. Studies of wheelchair propulsion biomechanics have demonstrated significant inter-individual differences in pushing styles,28–30 which are attributable to differences in levels of experience pushing a manual wheelchair, the level of the person’s daily physical activity and the severity of their impairment.28–30 Previous studies have demonstrated that, compared with less experienced manual wheelchair users, users with greater than 4 years’ experience using manual wheelchairs used fewer pushes with lower muscle activity to maintain higher wheelchair velocity.28 Also, higher daily physical activity levels were associated with higher stroke frequency and reduced contact angle in wheelchair propulsion technique.29 The level of impairment may also influence the technique of the person pushing the chair, due to the extent of upper-limb and trunk involvement and/or asymmetrical impairment profiles.30

In this study, where all participants were highly experienced regular wheelchair users (mean years of wheelchair use 18.7 ± 12.5 years), it was noted that whilst some individuals utilized a pushing technique that resulted in a high number of short and fast pushes, other participants had a longer, smoother and slower push technique. Also, there were differences in participant’s wheelchair design including different wheel size, camber angle and chair width, all of which may influence the maneuverability of the chair. It is possible, that when maneuvering the wheelchair, different propulsion techniques such as short and fast pushes, do not allow the Apple Watch® to register the movement leading to underestimation. As a result, when using the device for self-monitoring of wheelchair pushes, the device may underestimate the number of pushes they performed.

The findings of this study have implications for the device’s utility as a self-monitoring device for manual wheelchair users. The strong correlation between Apple Watch® and directly observed push counts indicates that push count estimates provided by the Apple Watch® would be acceptable for tracking change in physical activity where high absolute accuracy is not required, as well as group level analyses where under and overestimations combine to provide an acceptable mean difference in push counts. However, for applications where a high degree of accuracy at the individual level of measurement is required – for example, some clinical applications – the validity of push counts estimated by the Apple Watch® is less acceptable.

There were a few limitations in this study. Firstly, there were occasional short periods of time where the participant was out of view of the recording, which prevented any pushes from being observed. This may have had an impact on the findings, but it was likely to be small, because it occurred in only three participants and during these cases unsighted time was <15 s of a 1.5 h protocol. Secondly, this study did not investigate the accuracy of the device across discrete tasks or speeds of mobilization and so it is not possible to determine whether the Apple Watch® has greater validity for specific tasks or types of tasks. For example, previous research has demonstrated that the validity of wearable step count monitors improves as movement speed increases.31 Nonetheless, the intended application of the Apple Watch® is most likely self-monitoring across a full day of incidental activity in order to provide meaningful feedback to community-dwelling wheelchair users. Hence, the intention of this study was to assess the validity of the device in a situation representative of its likely intended use.

The potential limitations of this study are off-set by a number of strengths. First, the data were collected from participants varying in age, nationality and experience with using a manual wheelchair. This improves the external validity of the study and the generalizability of the findings.32 Secondly, the protocol consisted of a range of tasks and short bouts of activity including mobilizing across a range of distances, directions, speeds and undertaking maneuvering tasks such as parking and traveling up ramps. These activities are representative of real-life situations for manual wheelchair users.20,33 In order to reflect the everyday life of people using manual wheelchairs, the protocols need to include a large variety of recreational and free-living activities.32 Therefore, the protocol in this study was more relevant to consumers’ daily life than results from studies reporting evaluations of accuracy performed in the laboratory.32 Thirdly, the protocol in this study also included a variety of upper limb tasks which were not “pushes” as defined in this study (e.g. reaching for and replacing a backpack and using a mobile). Incorporation of these activities enhances the external validity of the Apple Watch® push count estimate.

Future research may investigate whether the accuracy of Apple Watch® pushes varies by wheelchair speed.31 Accuracy will likely vary as a function of speed considering there are biomechanical changes in pushing patterns,34 which may be more easily recognized by the device. Similarly, future research may also assess differences in accuracy between participants with different diagnoses, wheelchair designs, daily physical activity levels and propulsion styles and techniques. We were unable to investigate this due to the small sample size and since the majority of the participants were people with spinal cord injury. Importantly, there is potential for this device to be used in large data studies. Using large data approaches as they become available, researchers can potentially determine thresholds for health benefits in the population of wheelchair users, and establish standards similar to those for ambulatory populations such as the 10,000 steps goal.35 Likewise, it may also be important to investigate whether there are any negative consequences associated with excessive daily pushes.

Conclusion

Push counts recorded by the Apple Watch® were strongly and positively correlated with manually counted wheelchair pushes. There was a general underestimation of push counts and large individual variation in accuracy, possibly due to differences between propulsion styles. For personal use as a self-monitoring device, individual estimates are acceptable, being commensurate with those provided by commercially available step counters. For research purposes, the device is likely to provide acceptable estimates of average push counts in large samples, which may be useful in determining the public health benefits of physical activity in manual wheelchair users. However, the magnitude of the individual level over-and under-estimates provided by the Apple Watch® in this study indicates that for individual level applications where a high degree of accuracy is required – the validity of push counts estimated by Apple Watch® is less acceptable.

Supplementary Material

Supplemental Material
Click here for additional data file. (24.2KB, zip)

Acknowledgments

We would like to acknowledge all the gatekeepers for their support and contribution to participant recruitment. Thank you to all participants who volunteered their time to the study.

Disclaimer statements

Contributors None.

Funding This work was supported by 1) The University of Queensland, Human movement and nutrition sciences, PhD candidature funding; 2) Motor Accident Insurance Commission Scholarship (Grant number 2159179); and 3) Suomen Kulttuurirahasto (FI) (Grant number 75162323).

Conflicts of interest Authors have no conflicts of interest to declare.

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