Validity of an Accelerometer Used to Measure Step Count in Children With Idiopathic Toe Walking : Pediatric Physical Therapy

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Validity of an Accelerometer Used to Measure Step Count in Children With Idiopathic Toe Walking

Christensen, Catie PT, DPT; Haddad, Amanda PT, DPT; Maus, Elizabeth PT, DPT, PCS

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Pediatric Physical Therapy 29(2):p 153-157, April 2017. | DOI: 10.1097/PEP.0000000000000364
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To assess the validity of accelerometer use in children with idiopathic toe walking (ITW).


Seventy-five children, 2 to 13 years old, with ITW were videotaped ambulating 50 ft wearing a NL-1000 accelerometer. Because of concerns for accelerometer accuracy in young children, 2 groups were created: 5 years or younger (n = 45) and 6 years or older (n = 30). Step counts recorded via accelerometer and video were compared and correlated.


There was a significant difference in accelerometer and video step counts for 2- to 5-year-olds (W = 72.00, P < .001). No significant difference was found for 6- to 13-year-olds (W = 65.00, P = .24). Spearman ρ values were 0.78 and 0.92 for the 5 years or younger and 6 years or older groups, respectively.


Accelerometers accurately count steps in 6- to 13-year-olds with ITW but are inaccurate in 2- to 5-year-olds.


Idiopathic toe walking (ITW) is described as a preference for toe walking without neurologic involvement.1 It was first described by Hall et al1 in 1967 and affects 2% of children who are otherwise developing typically.2 Conditions such as a leg length discrepancy, autism spectrum disorder, cerebral palsy (CP), muscular dystrophy, or other neurologic disorders must be ruled out before making the diagnosis of ITW.3

Pedometers and accelerometers record the number of steps taken by individuals. However, the counting mechanism varies between the 2 devices. Pedometers use a mechanical arm that swings in response to vertical forces over 0.35 g.4 Accelerometers use more sophisticated strain gauges to monitor stepping, which allows for a graded response based on the intensity of the activity.4 Accelerometers are found to be more accurate than pedometers in measuring step count in children.4,5 However, they are more expensive and require more complex equipment.4

There has been no investigation into the accuracy of pedometers or accelerometers as a measure of step count in children with ITW. A heel accelerometer, designed to use an algorithm to differentiate a toe-walking step from a heel-to-toe step in children with ITW, has been investigated.6 Although it was 98.5% accurate when used on a treadmill, specialized equipment that is not commercially available is required to use this technique.6 In addition, it was assessed only on a treadmill and not over ground.6

In children developing typically and those with physical disabilities including CP, pedometers and accelerometers are accurate at a self-selected walking speed.7–9 However, accuracy may be reduced in children younger than 6 years who are developing typically.10

Participants with ITW, CP, or a typical heel-to-toe gait pattern share common gait characteristics, but gait patterns also vary between and within each diagnosis.11 For example, children with ITW present with increased knee extension and ankle plantar flexion in stance phase, increased ankle plantar flexion in swing phase, and increased lower extremity external rotation when compared to children with a typical heel-to-toe gait pattern.12,13 When comparing the gait patterns of children with CP and ITW, increased cadence is found in CP but not ITW.11 In addition, significantly longer double limb support times occur in children with CP when compared to those with ITW.11

Kinematic deviations unique to CP include increased pelvic range of motion in the sagittal plane, increased hip adduction, and increased knee flexion in stance phase.11,12 An internal foot progression angle is commonly present in children with CP,11 whereas an external foot progression angle is more prevalent in ITW.12 In both CP and ITW, the degree of ankle plantar flexion in stance phase of gait is increased; however, children with ITW have more plantar flexion present.12,14 The mechanisms behind contacting the ground in plantar flexion are different. In children with CP, initial contact on the toe is the result of decreased knee extension and residual ankle plantar flexion.11,12 Among children with ITW, it is due to active plantar flexion that occurs at the end of swing phase.11,15 Children with CP also display unique deviations in the timing of their muscle contractions during gait. Only children with CP demonstrate delayed peak knee flexion and extension in stance phase and delayed gastrocnemius contraction during swing phase.11,14,15

Because of differences in gait characteristics, pedometer and accelerometer accuracy cannot be assumed in individuals with ITW despite promising results for children with typical gait patterns and those with CP. Determining the accuracy of pedometer or accelerometer use in children with ITW is a step toward the creation of a clinically feasible outcome to assess the percentage of toe walking. This outcome measure will be described in detail in a future study. It was important to choose a tool that is clinically feasible and accurate. Thus, an accelerometer with a piezoelectric strain gauge sensor was chosen. A piezoelectric strain gauge accelerometer combines the benefits of an accelerometer and pedometer.4 This style of accelerometer has a graded response to vertical movement, is cost-effective, easy to use, and commercially available, making it feasible for use by clinicians and researchers.4

The purpose of this study was to evaluate the validity of accelerometer use for children with ITW. Given the findings of lower accuracy in younger children,10 validity was determined for 2 age groups: 2 to 5 years of age and 6 to 13 years of age. Concurrent validity was assessed by correlating step count obtained using an accelerometer to the count recorded through videotaped observation, which was considered the gold standard.



Seventy-five children, 2 to 13 years of age, with a diagnosis of ITW were consecutively recruited from referrals to a physical therapy department. Nationwide Children's Hospital Institutional Review Board approved this study. Recruitment occurred between December 2014 and February 2016. Figure 1 presents a flow chart of the recruiting process. Informed consent was obtained from a legal guardian for each child 2 to 8 years of age. Both informed assent from the child and informed consent from a legal guardian were obtained for children 9 years and older.

Fig. 1.:
Flow chart of participant recruitment.

The inclusion criteria included children 2 to 18 years of age with a diagnosis of ITW. However, no one was referred to physical therapy over the age of 13 years. Thus, only children 2 to 13 years of age were studied. Individuals with diagnoses of autism spectrum disorder, CP, muscular dystrophy, or global developmental delay were excluded. Participants were also screened for a nonidiopathic cause of toe walking using the Toe Walking Tool (TWT).16 The TWT uses demographic, subjective, and objective information to evaluate for potential neuromuscular, neurogenic, or traumatic causes of toe walking.16 Content validity was established through the use of an expert panel; the TWT has good interrater reliability (κ = 0.90).16 If red flags were identified via the TWT, the child was excluded from the study. Because of concerns for pedometer and accelerometer accuracy in young children,10 2 groups were created: 5 years or younger (n = 45) and 6 years and older (n = 30). Table 1 provides demographic information on the study participants.

TABLE 1 - Demographics for Study Participants
Characteristic 2- to 5-y-olds (N = 45) n (%) 6- to 13-y-olds (N = 30) n (%)
Male 18 (40) 19 (63)
Female 27 (60) 11 (37)
Caucasian 34 (76) 21 (70)
African American 5 (11) 3 (10)
Bi-Racial 1 (2) 4 (13)
Somali 2 (4) 2 (7)
Other 3 (7) 0 (0)
Median (Q1b, Q3c) Median (Q1b, Q3c)
Child's age, y 4.08 (3.17, 5.08) 7.67 (6.46, 9.58)
Age at onset of toe walking, moa 12 (10, 13) 12 (11, 13)
Percentage toe walkinga 75 (50, 85) 72.5 (50, 90)
aPer parent report.
bThe 25th percentile or the first quartile.
cThe 75th percentile or the third quartile.


A New-Lifestyles NL-1000 accelerometer (New Lifestyles Inc, Lees Summit, Missouri) was used to record the number of steps taken by each child. This type of accelerometer was chosen because it is reliable (intraclass correlation coefficient [ICC] = 0.88-0.99) and valid (ICC = 0.78-0.95) as compared with video observation when used with children with CP.9 In addition, it combines the benefits of an accelerometer and pedometer by using a piezoelectric strain gauge.4 It has a graded response to vertical movement, is cost-effective, easy to use, and commercially available, making it feasible for use by clinicians and researchers.4

A 2.5″ × 48″ Nylatex wrap (DJO Global Inc, Chattanooga, Tennessee) was used to secure the accelerometer to the child's waist because use of a firm elastic belt improves stability and reduces undercounting.17 Furthermore, use of the wrap allowed for consistency between participants. Some participants arrived with attire that would not allow for a pedometer to be donned at their waist without use of the wrap. If needed, Goody Ouchless Elastic Hair Ties (Goody Products, Inc, A Newell Brands Company, Atlanta, Georgia) were used to secure the child's shirt so that it did not interfere with the accelerometer. Duct tape was used to mark off a 50-ft distance for ambulation, and each child was videotaped using an iPad (Apple Inc, Cupertino, California).


The child's shoes and socks were removed, and they were barefoot throughout the testing. The Nylatex wrap was placed with the top portion at the level of the umbilicus. The NL-1000 accelerometer was donned posteriorly in midline on the Nylatex wrap. If needed, the participant's shirt was secured with a hair tie. The child's parent or a clinician stood at the end of the 50-ft walk to assist him/her in stopping, so the accelerometer could be read. The distance of 50 ft was chosen in preparation for the accelerometer's use in a potential clinical outcome measure to assess the amount of toe walking a child is performing. Fifty feet was thought to be a distance available in most settings, making the outcome measure feasible for a variety of clinicians, researchers, and children.

Once the child was at the starting point, the accelerometer was set to 0. The child then ambulated 50 ft while being videotaped from at least 6 ft behind. Videotaping was completed from behind to allow the child to walk at a self-selected pace. Because children with ITW are able to change their gait from a toe-to-toe gait pattern to a heel-to-toe gait pattern,12,18 a cognitive task was used as distraction. The goal of the distraction task was to help the child achieve the gait pattern they unconsciously self-select. Examples of cognitive tasks used included spelling, performing math problems, singing a song, counting by 7s, or discussing a favorite activity. If the original cognitive task did not provide enough distraction to encourage their self-selected gait pattern, the test was repeated using a different cognitive task.

The researcher determined the need for the test to be repeated based on observation of gait during periods when the child was unaware and based on parent report. For example, gait was observed by the researcher during times when the focus was not on the child. This included when he or she walked from the lobby to the therapy room or during play while the parent and clinician were engaging in discussion. This allowed the researcher to gain an understanding of the child's self-selected gait pattern. Parents were asked whether the gait pattern observed during testing was consistent with the child's typical pattern. If the researcher and parent felt that the initial trial was consistent with the child's typical pattern, only 1 trial was performed. If it was inconsistent, an additional trial was completed. A maximum of 2 trials were performed.

The accelerometer step count was recorded at the end of 50 ft by the researcher present with the child during the testing. A second researcher, masked to the accelerometer results, viewed the video on Windows Media Player (Microsoft, Redmond, Washington) in slow motion (half the real time speed) and recorded the child's step count. The accelerometer and video step counts were then compared.

Data Analysis

The data were not normally distributed; therefore, nonparametric statistics were used. There were no missing data. The data were assessed in 2 groups (2- to 5-year-olds and 6- to 13-year-olds) because of the concern that accelerometers may have reduced accuracy in children younger than 6 years.10 A 2-tailed Wilcoxon signed rank test was used to compare the number of steps counted by the accelerometer to the number of steps counted via video observation, which was considered the gold standard. A Spearman ρ was used to determine the correlation between the 2 methods of obtaining step counts. Descriptive statistics were calculated.


Figure 2 displays descriptive information as boxplots. For children 2 to 5 years of age, the median difference in step count between the accelerometer and the video was 2, with a range of 0 to 20 steps. For children 6 to 13 years of age, the median difference in step count was 1, with a range of 0 to 5 steps. The 2- to 5-year-old age group had a number of outliers. Nonparametric statistics were used, which are not influenced by outliers. However, calculations were performed with and without the outliers when they were identified.

Fig. 2.:
Comparison of step count: accelerometer versus video.

There was a significant difference between the accelerometer and video step counts for children 2 to 5 years old (W = 72.00, P < .001). When the outliers were removed, there continued to be a significant difference between the step counts (W = 496.00, P < .001). There was no significant difference in the step counts of children 6 years and older (W = 65.00, P = .24). Spearman ρ values of 0.78 and 0.92 were identified for children 2 to 5 years of age and 6 to 13 years of age, respectively. When the outliers were removed, the Spearman ρ value was 0.94.


The accelerometer was found to be accurate for children 6 to 13 years of age but not for children 5 years and younger. Among children 6 to 13 years of age, no significant difference was found between the accelerometer and the video step counts, and the accelerometer was valid.

These results are consistent with the findings of prior studies evaluating the accuracy of pedometers or accelerometers in children with a heel-to-toe gait pattern or those with developmental disabilities including CP.7–9 Beets et al7 investigated the use of pedometers with children 5 to 11 years of age who were developing typically. As in our study, they evaluated the accuracy of pedometers at a self-paced walking speed over ground although it was evaluated over the longer distance of 400 m.7 Similarly, they demonstrated a high level of agreement between the pedometer and the actual step counts at a self-selected walking speed, with ICC values ranging from 0.985 to 0.997.7

A study by Maher et al9 evaluated the accuracy of the same accelerometer used in the current study (NL-1000) with individuals with CP, 7 to 17 years of age, with a Gross Motor Function Classification System level of I or II.19,20 Each participant ambulated 3 minutes at a self-selected speed.9 Consistent with the results of our study, no significant difference was found between the accelerometer and the video step counts and the ICC value was high at 0.88 to 0.99.9 A review study also confirmed these results by evaluating current literature on the validity of pedometer use in adults and children with neurologic or physical disabilities.8 The study concluded that pedometers are valid in research and clinical practice for individuals with physical disabilities.8

Our study demonstrated that for children 5 years and younger with ITW, the NL-1000 accelerometer did not accurately count steps over the 50-ft walking distance. This result is consistent with a study by Oliver et al,10 which reported that pedometers did not accurately count the number of steps taken by preschoolers 3 to 5 years of age who were developing typically.10

Our study had a number of limitations. The assessment of the NL-1000 accelerometer was evaluated over the very short distance of 50 ft. Thus, the results cannot be extrapolated to further distances. This limits the understanding of the accuracy of the accelerometer in assessing physical activity levels in this population. However, as noted earlier, this distance was chosen in preparation for the potential use of the accelerometer in a clinical outcome measure to assess a child's percentage of toe walking. A second limitation is that children with ITW are able to modify their typical toe-to-toe gait pattern to a heel-to-toe pattern.12,18 A cognitive task was used as distraction to encourage the children to use their typical gait pattern. However, at times, it was difficult to elicit their usual self-selected toe-to-toe gait pattern, and the effect of the use of a cognitive task on gait speed and parameters is unknown. In addition, during testing, the children were barefoot. Because gait performance may be altered by shoe wear,21,22 the accuracy of accelerometer use cannot be generalized to children with ITW who are wearing shoes. Furthermore, the younger age group had a larger proportion of females and the older age group had more males. It is unclear how the difference in gender distribution may have impacted the results. Finally, the reliability of the accelerometer was not assessed and the reliability of the rater who determined step count from the video was not evaluated.


The NL-1000 accelerometer accurately measures step count for children 6 to 13 years of age with ITW when ambulating 50 ft at a self-selected walking speed. It is not accurate in children with ITW who are between 2 and 5 years of age. The accuracy of an accelerometer in children with ITW has an impact on the future development of a clinically feasible outcome measure to assess the percentage of toe walking.


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ankle/physiopathology; equinus gait; outcome measure; pediatrics; walking

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