Twenty children (11 boys, nine girls) volunteered for annual treadmill testing over 5 years for assessment of submaximal and maximal measurements of respiratory exchange variables. Data from one girl who moved away were not available for the final testing session. The subjects were generally physically active. Seventeen were participants on community sports teams, but none was engaged in regular endurance training. With the exception of one child, all were white, and all subjects were in good health, taking no medications that would affect exercise testing results. Data from these subjects were included in a previous report of walking economy in children.
Average age at the start of the study was 9.2 years (range, 7.9 to 10.3 years). At the last testing session, three of the eight girls reported having experienced menarche, and 7 of the 11 boys had development of pubic hair, voice change, or facial hair (by parent-completed questionnaire), indicative of early puberty. Treadmill testing was performed yearly in an air-conditioned laboratory (20 to 22°C). Following determination of height and weight, subjects warmed up on the treadmill for 2 min, walking at 3.25 mph up a 6% grade. The test protocol involved an initial steady-state 4-min walk at 3.25 mph, 8% grade. Treadmill elevation was then increased 2% every minute until subject exhaustion, with speed individualized between 3.25 and 3.75 mph depending on subject size and fitness. Holding onto handrails was not permitted.
Gas exchange variables were determined with a computerized metabolic cart (Q-Plex Cardio-Pulmonary Exercise System; Quinton Instrument Co; Seattle) using standard open circuit techniques. Subjects breathed through a valve (Rudolph) (94 mL dead space), and a pneumotachometer was used for recording Vt and minute ventilation (Ve). Expired air traversed a combined tubing/mixing chamber volume of 6 L. Expired gas samples from the mixing chamber were analyzed for oxygen and carbon dioxide by zirconia oxide and infrared analyzers, respectively. Data were averaged every 15 s and used to calculate oxygen uptake (Vo2), expired ventilation (Ve), carbon dioxide output, and respiratory exchange ratio. The system was calibrated before each session with standard gases of known oxygen and carbon dioxide concentrations. Heart rate was monitored electrocardiographically.
Submaximal Ve, Vt, and breathing rate (fR) were determined as the mean of values recorded during the fourth minute of the steady-state walk. Maximal values were defined as the average of the two highest measurements during the final minute of exercise. Subjects were considered to have reached a true maximal effort if they demonstrated subjective evidence of exhaustion (unsteady gait, facial flushing, hyperpnea) and either peak heart rate >190 beats/min or maximal respiratory exchange ratio >1.00.
Informed permission was obtained from the parents, and each child provided assent for participation. This study was approved by the institutional review board of the Baystate Medical Center.
Changes in ventilatory and anthropometric variables were assessed in regard to time and gender by two-way analysis of variance. Post hoc paired comparisons were performed by the Neuman-Keuls test. Statistical significance was defined as p<0.05.