The Institutional Review Board at the University of Michigan approved all methods and procedures for this study. Seventy-two children with autism between the ages of 9-18 years (n = 72, male = 55, female = 17), who were recruited as a part of an adapted physical activity intervention study (teaching children how to ride a two-wheel bicycle), met the requirements to be included in this study. All participants were unable to ride a two-wheel bicycle prior to the study. An adapted two-wheel bicycle was used to teach the participants how to ride a conventional two-wheel bicycle. Parental consent and participant assent was obtained for all participants. To be included in this study all participants met autism cut-off criteria based on the mild-moderate and severe codes of the Social Responsiveness Scale (SRS), a valid assessment reliable in measuring autism severity [14, 15]. Additionally all children met physical activity monitoring guidelines based on best practice recommendations (outlined below) [11, 16].
Physical Activity Measurement
Physical activity was measured using the Actical® accelerometer (Mini Mitter/Respironics, Inc., Bend, OR) over a seven-day period during a typical week and prior to the adapted physical activity intervention. Data was collected during the spring months while the participants were still in school. The Actical® accelerometer is one of the smallest accelerometers available (28 × 27 × 10 millimeters and 17 grams) and uses an omni-directional sensor with a 0.5-3 Hz range capable of detecting movements in all planes to create a composite measure of movement. The voltage generated by the sensor is amplified and filtered via analog circuitry and then passed into an analog to a digital converter, and the process is repeated 32 times each second (32 Hz). The resulting 1-second value is divided by four and then added to an accumulated activity value for the duration of the specified 15-second epoch . For this study, a 15-second epoch was selected based on literature related to the erratic and sudden bursts of activity common to youth [17, 18].
Participants wore the monitor for all waking hours of the day on the right ankle using an elastic belt. The monitor was to be worn for all activities except swimming, showering/bathing and sleeping. Parents/guardians of the participants were provided with a log to record any times when the monitor was not worn (i.e. forgetting to put it on in the morning, taking it off for comfort or any other reasons for which it may have been removed). Monitors were returned after a seven-day period via priority mail and were downloaded using an Actical Reader interface unit and associated software.
Physical activity data reduction
Participants were included in this study if their physical activity data met the following criteria: the accelerometer was worn for at least four days (inclusive of at least one weekend day) for a minimum of 10 hours each day. These criteria have been previously established in the literature as suggested guidelines for obtaining valid and reliable accelerometry data [11, 16]. Based on a 15-second epoch the data were then reduced and assigned to one of the following categories: sedentary activity (counts of <25), moderate physical activity (counts of 376-1625) or vigorous activity (counts > 1626). Data counts assigned to physical activity categories are related to energy expenditure validated in typically developing children [16, 17].
Based on the time of day, physical activity data was partitioned by time spent in school, after school and evening hours. Time in school was between the hours 8:00 am - 3:00 pm, after school hours was between the hours 3:00 - 5:00 pm and evening hours were between the hours 5:00pm - 12:00am.
The Weshsler Abbreviated Scale of Intelligence (WASI) is a standardized measure of intelligence for individuals aged 6-89 years . Two subtests were administered (the vocabulary and matrix reasoning subtests), to assess verbal and non-verbal intelligence and generate a standardized full-scale IQ. The WASI was administered to the participants by a clinician or graduate student with experience in cognitive assessment.
The SRS is a 65-item questionnaire, completed by the parent or guardian, and is valid and reliable in measuring autistic traits . The SRS measures five areas of social development, as indicated by the parent: social awareness, social information processing, capacity for reciprocal social communication, social anxiety or avoidance and autistic traits. Furthermore the SRS provides a standardized score that qualitatively identifies the severity of autism ranging from mild to severe. Standard scores measured at 76 or higher represent a diagnosis in the severe range, standard scores measured between 60-75 resulted in a mild to moderate diagnosis.
Height and weight were measured without shoes. Height was measured in centimeters to the nearest tenth of a millimeter with a portable stadiometer (SECA S-214 portable stadiometer). Two measurement trials were administered and the average of the trials was recorded. Weight was measured in kilograms to the nearest gram (Health O Meter H-349KL digital scale). Two measurement trials were administered and the average of the trials was recorded. Body mass index (BMI) was calculated using the standard formula: body mass (kg.) divided by height (m2). Percentage of body fat was calculated using a gender-specific regression equation for children with triceps and calf skinfolds . A physician experienced in measuring skinfolds, using Lange skinfold calipers, took two skinfold thicknesses at each site (triceps and calf) on the right side of the body. Measurements were taken twice at each site and rounded to the nearest tenth of a millimeter. The average at each site was used in the analysis.
All analyses were conducted in PASW version 18.0. Participants were initially divided into three age groups (9-11 years, 12-13 years & 14-18 years), however no significant differences were found between the 12-13 year age group and the 14-18 year age group, these age groups were combined for further analysis. Physical activity patterns were examined for each group in the sedentary and moderate to vigorous categories. Moderate and vigorous physical activity was combined for analysis based on established norms and recommendations in physical activity literature as the target intensity for receiving health benefits .