Edinburgh Napier University

  To examine the effects of short messaging service (SMS) frequency and timing on the 13 efficacy of an SMS-intervention for Hong Kong Chinese adolescents. Sixty nine students aged 14 between 12 and 16 (mean age 13.75±.90) were recruited from five schools in Hong Kong. 15 Participants were randomly assigned into one of five groups: high-frequency+self-selected timing 16 (HST), low-frequency+self-selected timing (LST), high-frequency+assigned timing (HAT), 17 low-frequency+assigned timing (LAT) and the control group. The total duration of the intervention 18 was four weeks. No significant intervention effects were detected in adolescent's PA among the 19 five groups (F = 1.14, p = 0.346). No significant differences were observed in the stage movement 20 among the five groups (χ2 = 6.18, p = 0.627). No significant differences appeared in the exercise 21 benefits, barriers and benefits/barriers differential scores. However, a growth trend in the exercise 22 benefits score in the LST and HAT groups was found in contrast to the downswing in the control 23 group. The exercise barriers score in the HST group showed the largest reduction after 24 intervention. The benefits/barriers differential score in all the intervention groups increased, 25 whereas it decreased in the control group. Although an increase is demonstrated in the high dosage 26 SMS frequency and timing, no significant intervention effects were found among the five groups in 27 PA behavior, stage of change and exercise benefits and barriers among Hong Kong Chinese 28 adolescents. 29