Objective: To estimate national and international temporal trends in handgrip strength for children and adolescents, and to examine relationships between trends in handgrip strength and trends in health-related and sociodemographic indicators. Methods: Data were obtained through a systematic search of studies reporting temporal trends in the handgrip strength for apparently healthy 9-17 year-olds, and by examining large national fitness datasets. Temporal trends at the country-sex-age level were estimated by sampleweighted regression models relating the year of testing to mean handgrip strength. International and national trends were estimated by a post-stratified population-weighting procedure. Pearson's correlations quantified relationships between trends in handgrip strength and trends in health-related/sociodemographic indicators. Results: 2,216,320 children and adolescents from 13 high-, 5 upper-middle-, and 1 low-income countries/special administrative regions between 1967 and 2017 collectively showed a moderate improvement of 19.4% (95%CI: 18.4 to 20.4) or 3.8% per decade (95%CI: 3.6 to 4.0). The international rate of improvement progressively increased over time, with more recent values (post-2000) close to two times larger than those from the 1960s/1970s. Improvements were larger for children (9-12 years) compared to adolescents (13-17 years), and similar for boys and girls. Trends differed between countries, with relationships between trends in handgrip strength and trends in health-related/sociodemographic indicators negligible-to-weak and not statistically significant. Conclusions: There has been a substantial improvement in absolute handgrip strength for children and adolescents since 1967. There is a need for improved international surveillance of handgrip strength, especially in low-and middle-income countries, to more confidently determine true international trends. PROSPERO registration number: CRD42013003657. Temporal Trends in Children's Handgrip Strength 3 Key points There has been a moderate international improvement in handgrip strength for children and adolescents since 1967, with the rate of improvement progressively increasing over time and more recent values (post-2000) close to two times larger than those from the 1960s and 1970s Internationally, improvements in handgrip strength were nearly twice as large for children compared to adolescents, yet similar for boys and girls. Nationally, trends varied in magnitude and direction Collectively, the relationships between trends in handgrip strength and trends in healthrelated/sociodemographic indicators were negligible-to-weak and not statistically significant
Objectives Increasing the reinforcing value of a stimulus occurs after repeated exposures to the reinforcer via neuroadaptations that increase the incentive salience of the stimulus. Exercise is a reinforcer and increasing exercise reinforcement (RRVex) may be dependent on simultaneously increasing tolerance for exercise intensity. Positive outcome expectancy (POE) of participating in an intervention can be an important determinant of treatment efficacy, such as when attempting to increase tolerance for exercise intensity or RRVex. We hypothesized that (1) high-intensity interval training (HIIT) that produces great discomfort would increase tolerance for exercise intensity, (2) adding a positive outcome expectancy (POE) component to HIIT would further increase tolerance for exercise intensity and, (3) increases in tolerance for exercise discomfort would mediate increases in RRVex. Methods A randomized controlled trial with a factorial design included HIIT + POE (n = 33 adults, n = 19 women) and HIIT-only (n = 33, n = 19 women) groups. Both groups participated in HIIT 3 d/wk for 6 wks. HIIT + POE received POE treatment each exercise session. Outcomes were measured at baseline, after 6 weeks of HIIT, and 4 weeks post-HIIT (10 wk). Changes in the RRVex were assessed by a progressive ratio schedule of reinforcement task. Other outcomes were outcome expectations, tolerance for exercise intensity, and behavior regulations of exercise. Results Outcome expectancy did not change in either group. Tolerance for exercise discomfort increased (P < .001) above baseline by 12% at 6 wk and 13% at 10 wk. Intrinsic, integrated, and identified behavior regulations of exercise were all increased (P < .01) at 6 wk and remained so at 10 wk. However, RRVex was not changed and change in RRVex was not correlated with change in tolerance for exercise intensity. Conclusions HIIT increases tolerance for exercise intensity and intrinsic, integrated, and identified behavior regulations of exercise. Funding Sources USDA-ARS.
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