Extended Reference Values for the Muscle Power Sprint Test in 6- to 18-Year-Old Children

Abstract

Purpose: 

The first aim was to extend the reference values and curves of the Muscle Power Sprint Test (MPST) in children with typical development. The second aim was to examine test/retest and intertester reliability of the MPST.

Methods: 

A total of 683 children aged 6 to 18 years (mean = 11.9 ± 3.8 years) were tested on the MPST. Test/retest reliability was examined in 71 children and intertester reliability in 77 children. MPST scores were merged with existing values and transformed into height-related normative reference curves.

Results: 

Sex-specific MPST curves were generated for children aged 6 to 18 years. Test/retest reliability was good (intraclass correlation coefficient = 0.90) and intertester reliability was excellent (intraclass correlation coefficient = 0.97).

Conclusions: 

New and more complete MPST normative reference values and curves for children from 6 to 18 years old are now available. The MPST is easy and reliably performed by clinicians.

In Brief

The study provides new and more complete MPST normative reference values and curves for children from 6 to 18 years old. The authors report the MPST was easy and reliably performed by all testers.

INTRODUCTION

Optimal anaerobic performance has shown to be important for children in their ability to participate in daily activities.¹ Children perform physical activities in short high-intensity bursts, whereas the activities of older children are usually more structured within a game or team sport.¹ In both cases, the ability to produce repeated sprint efforts is an important determinant of performance. Thus, children with a high anaerobic capacity are likely to be more active and have better physical health.²

Anaerobic means without air or oxygen. With the anaerobic system, energy can be produced without the use of oxygen. However, no activity will be completely “anaerobic,”³ as the aerobic system is activated within 5 to 8 seconds. Substrate-level phosphorylation features phosphocreatine breakdown and the breakdown of glycogen to lactate to produce adenosine triphosphate. At exercise onset the creatine phosphate system will be used for energy production, with anaerobic glycolysis beginning almost immediately and continuing approximately 30 seconds. From this point, the aerobic system plays an increasingly important role.⁴

To test the capacity of the anaerobic system, a short, high-intensity performance test is required. A previously valid and reliable anaerobic test is the Wingate Anaerobic Test (WAnT), a 30-second laboratory-based cycling test.⁴ The WAnT requires sophisticated, costly equipment and software, and reference values for children are not available. Therefore, it is not a very practical tool to use in pediatric physical therapy practice. Furthermore, on the basis of children's daily activities a running test would be more appropriate.¹ Accordingly, a running-based field test, the Muscle Power Sprint Test (MPST), has been developed to examine anaerobic performance. The MPST is known to be practical and reliable in children with cerebral palsy (CP) as well as in children who are healthy.^5,6 Construct validity is supported in children with CP⁷ and children who are healthy⁸ and confirmed on the basis of high correlations with WAnT performance.⁷ MPST reference values have been previously developed for 6- to 12-year-old children⁶ as well as for 6- to 18-year-old children with CP.⁹ Subjects with CP scored lower than children who are typically developing (TD), the difference seemed to be greater with increasing height, especially for the children with CP classified at Gross Motor Function Classification System level II.^10,11

To interpret and follow a child's anaerobic performance as they age, reference values of children who are TD are necessary.¹² Reference values of children who are TD will enable therapists to compare these children with those who have possible anaerobic deficits and will help pediatric physical therapists design child-specific interventions. This approach is also congruent with the demand for evidence-based practice among pediatric physical therapists.¹³

However, no normative reference values for anaerobic performance of children that are TD up to 18 years of age are available, and the lack of reference values for 13- to 18-year-old youth impedes the clinical usefulness of the MPST.⁶ Therefore, the first aim of this study was to extend the current sex-specific normative reference values and curves for anaerobic performance on the MPST to include 13- to 18-year-old children who are TD. The second aim was to determine the test/retest reliability and intertester reliability of the MPST in children who are TD.

METHODS

Data from 683 participants were included in this cross-sectional study, with 379 participants included in a previously published study that developed MPST reference values and curves in 6- to 12-year-old children.⁶ Additional data from 302 children who were TD and 13- to 18-years-old were added to this existing data set, which was collected over a 2-month period from October 2012 to November 2012. The Central Committee on Research Involving Human Subjects in the Netherlands approved all study procedures.

Participant Recruitment

Recruitment of the children between the ages of 6 and 12 years was described previously,⁶ whereas the 13- to 18-year-old children were recruited from schools from a rural area in the west of the Netherlands. Children with motor development impairment, those using medication affecting their exercise capacity, or those diagnosed with cardiovascular or pulmonary disease were excluded from the study. The children's parents and/or guardians provided written informed consent before their child's invitation to the study. A pretest questionnaire was administered to assess the current activity level of the child and his or her medical history.

A total of 843 children were eligible to participate, of which 93 declined participation and 67 were excluded because of a current sport injury or a preexisting medical condition. In total, 683 children completed the MPST and were included in the development of the reference values. The characteristics of the participants are presented in Table 1.

TABLE 1: Participant Characteristics

Procedure

All participants were tested during regular school physical education lessons. Each child had anthropometric measurements and a pretest questionnaire administered, and then individually underwent the MPST. Other participants were supervised by the teacher and asked to participate only in light activities until the MPST was performed. Participants were given 1 practice attempt; this allowed the physical therapist to provide extra instructions to the participants when necessary. All children were tested by the same physical therapist skilled in performing the MPST. The MPST was standardized as described by Verschuren et al.¹⁴

To examine the test/retest reliability, participants were requested to perform the MPST on 2 occasions separated by 1 week. Retesting was done at the same location and time of day for each subject. Data from the retest were not used to create normative values because this test score only provides information on within-subject variability, and thus would not be adding new data to the normative curve. To test the intertester reliability, 2 testers, who were both instructed to read the MPST descriptions and practice the test with 20 children, measured all participants on the last day of measurement.

All participants were tested in light exercise clothes without shoes, and weight was measured using electronic weight scales (Seca 813, Seca, Hamburg, Germany, with an accuracy of 0.1 kg) and height was measured using a stadiometer (Seca 206, Seca, Hamburg, Germany, with an accuracy of 0.5 cm). With this information, body mass index was calculated to define the weight status using 5 categories on the basis of age- and sex-specific body mass index cut-offs: extremely underweight, underweight, normal weight, overweight, and obese.^15,16

Muscle Power Sprint Test

A distance of 15 m separated 2 lines that were marked on the floor. Participants were instructed to run as fast as they could from line to line with a static start and a standardized rest. After crossing the line, they stopped running and turned around to get ready for the next sprint after a 10-second rest. Instructions were given to make sure the participants started at the right moment. For the first run the countdown was “Ready? Three, 2, 1, go!” For the other 5 sprints, a countdown from 6 to 1 preceded the start signal “go”; in this way, the child knew how much longer he or she had until the next start and could get ready on time. In the case of a false start or if the participant fell during 1 of the 6 runs, he or she was retested after a 5-minute break. Participants were verbally encouraged to go as fast as possible during each run. All participants performed a total of six 15-m maximum-paced runs. The time to complete each 15-m run was recorded to the hundredth of a second.

Studied Parameters

Anaerobic performance was defined as mean power (watts), which is the average power output of all 6 sprints found by calculating the velocity (m/s = distance/time), acceleration (m/s² = velocity/time), force (kg/s^f = body mass × acceleration), and power (watts = force × velocity) of every sprint. The mean power was found to be the most important parameter for the MPST.^9,14 Mean power gave an indication of the ability to sustain anaerobic performance throughout each of the 6 runs. The following equation was used:

Statistical Analyses

Statistical analyses were performed using the Statistical Package for Social Sciences version 20.0 (SPSS Inc, Chicago, IL) and MATLAB (R2012a) (MathWorks, Inc, Natick, MA). All data are expressed as mean ± standard deviation and range. To examine the mean anaerobic performance of an individual in the context of population-based norm values, reference centiles were derived using Growth Analyzer 3.5 (Growth Analyzer BV, Rotterdam, the Netherlands). Independent sample t tests were completed to determine whether significant differences existed between boys and girls or between children with a normal weight versus children who were under- or overweight. Intraclass correlation coefficients (ICCs; 2-way mixed) were computed for reliability measurements. Intraclass coefficients more than 0.80 were considered acceptable; more than 0.90 were considered good; and more than 0.95 were considered excellent.¹⁷ Limits of agreement of the test/retest reliability and intertester reliability were also calculated according to the procedure described by Bland and Altman.¹⁸ A Bland-Altman plot is a graphic representation of the individual subject differences against the respective individual means. Using this method, a rough indication of systematic bias and random error is provided by examining the direction and magnitude of the scatter around the zero line. Bland-Altman analysis describes the level of agreement between 2 measurements. In this analysis, the “precision” indicates how well the methods agree for an individual. By multiplying the precision by 1.96, the “limits of agreement” are calculated. This calculation represents the 95% likelihood range and indicator of absolute reliability. Statistically significant differences were inferred from P values < .05.

RESULTS

MPST Reference Values

The collected data provided reference values for 6- to 18-year-old children who are TD. Table 2 presents the characteristics of the MPST performance of the subjects as well as the sport participation of the children.

TABLE 2: MPST Results of the Total Sample of Children 6 to 18 Years of Age That are Typically Developing

Sex-specific MPST performance was presented in height-related centile curves, with the 3rd, 25th, 50th, 75th, and 97th percentile values of the mean power score on the MPST in Figures 1 and 2. In the weight categories, the mean power output of the children with normal weight was significantly higher than that of children who were under- or overweight (t = 1.8; P ≤ .05).

Fig. 1: Muscle Power Sprint Test centile curve for boys aged 6 to 18 years.

Fig. 2: Muscle Power Sprint Test centile curve for girls aged 6 to 18 years.

Year-by-year statistics for adolescent boys and girls are presented in Table 3, and showed that boys scored significantly higher from 12 years of age onwards for both mean and peak power.

TABLE 3: Mean and Peak Power for Adolescent Boys and Girls by Age Group^a

Test/Retest Reliability

Test/retest reliability was examined in 71 subjects. Because of holidays, school trips, and examinations, 228 children could not be retested 1 week after the first measurement. Five children reported sick on the day of the second measurement. Mean power output demonstrated an ICC (2-way mixed model) of 0.90 (95% confidence interval = 0.85-0.99; Table 4) for test/retest reliability. The Bland-Altman plot revealed no significant learning effect between the first and second tests (Figure 3).

Fig. 3: Bland-Altman plot test-/retest reliability.

TABLE 4: MPST Mean Power Output Reliability (ICC)

Intertester Reliability

Intertester reliability was examined on the last day of the measurements, when 77 subjects participated. Mean power output demonstrated an ICC (2-way mixed model) of 0.97 (95% confidence interval = 0.93-0.99; Table 4) for intertester reliability. The Bland-Altman plot revealed no significant differences between the outcomes of the 2 testers (Figure 4).

Fig. 4: Bland-Altman plot intertester reliability.

DISCUSSION

The first aim of this study was to extend the current sex-specific normative reference curves and values for the MPST anaerobic performance assessment for 6- to 18-year-old children who are TD. The second aim was to determine the test/retest reliability and intertester reliability of the MPST in children who are TD. This study provides novel reference values for the MPST in 13- to 18-year-old boys and girls, and when all curves are plotted together (including existing data of 6- to 12-year-old children that are TD), it provides a complete overview of the development of the MPST performance throughout childhood. These curves can be used in clinical practice to assess and interpret the anaerobic performance of children.

As seen in previous studies, the test/retest reliability of the MPST was good⁶ and the intertester reliability was excellent. The Bland-Altman plot in this study shows a 95% reliability interval of −16.6 to +16.8 W (mean value calculated). These values can be used as the minimal detectible change required for the MPST performance. This indicates that a child should improve at least 16.8 W, considering this change to be beyond the measurement error. For intertester reliability, the 95% confidence interval was −11.0 to +13.4 W. In case a different observer tests a child, the intertester reliability values should be added to the test/retest values. As already mentioned by Hendershot and Heathcock,¹¹ to use the MPST as a fully validated instrument, future measurement sensitivity research should be done on the instrument's responsiveness to change and the minimal clinically important difference.¹¹

The MPST was developed by Verschuren et al¹⁴ to assess and interpret the anaerobic performance of children with CP. Previously, Douma et al⁶ has published height-related reference curves for children who are TD aged 6 to 12 years. With the addition of the current reference values, the test can now be used to interpret a child's anaerobic performance up until the age of 18 years. Height was used instead of age for the construction of the reference curves because of its high correlation with MPST performance.^6,14 Moreover, data points of 6- to 12-year-old children⁶ collected in a different area of the Netherlands “fit” the current studies data points collected within 13- to 18-year-old youth. This suggests that the created reference curves and values are generalizable in a wider group of 6- to 18-year-old youth throughout the Netherlands than just locally around the school area.

Consistent with previous research, differences were found between boys and girls and between children of normal weight and those who are over- and underweight.⁶ The goal in the development of the MPST was to provide a practical and valid tool for pediatric physical therapists to measure anaerobic performance in children. This initial development was in children with CP, so running distances were modified until mean total test time was around 30 seconds. For this reason, a 6-repetition 15-m running test was chosen. However, on the basis of the anaerobic energy system, a workout time between 20 and 30 seconds is suitable to measure anaerobic performance.^4,14 Adenosine triphosphate and creatine phosphate are the fuel source for the first 10 to 15 seconds of physical activity, followed by the glycogen system and then the aerobic system. In this study, the mean running time of all participants was 20.1 seconds (standard deviation = ±1.9 seconds), which requires nearly 100% anaerobic energy sources, making the MPST an ideal anaerobic test in children who are TD.

A limitation in participant sampling may affect the ability to generalize the currently reported reference values. Nearly all the included participants had a normal body mass index, with only 1 subject being extremely underweight and 4 were obese. This study did not measure body composition, making it more difficult to interpret the differences between the groups and whether results were due to body weight from adipose stores or lean muscle tissue. Another limitation is that all participants were from similar neighborhoods—rural and urban areas in the midwest of the Netherlands. Economic status of the parents was not collected, and only 9 participants did not have parents from a Dutch ethnic background. Evidence is inconsistent whether these factors affect the physical fitness of children and teenagers.^1,6 As commented previously, anaerobic testing has some methodological limitations.¹¹ The MPST depends on the individual's motivation. Currently, no objective physiological criteria are available that can be used to establish a “true” maximal anaerobic performance in children. So, a clinician has to rely on the willing cooperation of the individual. The current data suggest that when a child performs below the third percentile, they deviate from the norm and may benefit from an exercise-based program. With the ability to interpret the outcome of the MPST, an easy assessment method is available for clinicians that could be used by pediatric physical therapists to assess children's anaerobic performance.

In the future, it would be interesting to measure body composition in all participants to gain a better understanding of the differences between groups. A greater variety in ethnic background, area of residence, and parental economic and social status should be investigated to secure generalizability of the centile curve for all children aged 6 to 18 years. Also, longitudinal research should be done to see how children develop over the years. Future research should examine the validity of the test, especially in the group of older children, as total running time will be approximately 15 seconds instead of the initial 30 seconds of the WAnT. Future research should determine whether the MPST is a valid test for young athletes. When the MPST is not valid for the athletic subject, we recommend using the original Running-Based Anaerobic Sprint Test for adult athletes, which consists of 6 sprints of 35-m distance, with 10 seconds of rest between the sprints.¹⁹ Furthermore, the minimal clinically important difference should be investigated to help with a better and more useful interpretation of the test results.¹¹

CONCLUSIONS

This study demonstrates that the MPST has good test/retest reliability and excellent intertester reliability for the assessment of anaerobic performance of 6- to 18-year-old children in a pediatric physical therapy setting. The MPST was easy and reliably performed by all testers. These data have produced new and more complete MPST centile curves for children from 6 to 18 years, so that anaerobic performance can be more easily compared with the subject's peers over time. This study will allow pediatric physical therapists to more effectively use the MPST to monitor a child's anaerobic performance in an evidence-based context.