Understanding the stress/strain relationship between exercise and bone is critical to understanding the potential benefit of exercise in preventing postmenopausal bone loss. This study examined the effect of a 2-year exercise intervention and calcium supplementation (600 mg) on bone mineral density (BMD) in 126 postmenopausal women (mean age, 60 ؎ 5 years). Assignment was by block randomization to one of three groups: strength (S), fitness (F), or nonexercise control (C). The two exercise groups completed three sets of the same nine exercises, three times a week. The S group increased the loading, while the F group had additional stationary bicycle riding with minimal increase in loading. Retention at 2 years was 71% (59% in the S group, 69% in the F group, and 83% in the C group), while the exercise compliance did not differ between the exercise groups (S group, 74 ؎ 13%; F group, 77 ؎ 14%). BMD was measured at the hip, lumbar spine, and forearm sites every 6 months using a Hologic 4500. Whole body BMD also was measured every 6 months on a Hologic 2000. There was no difference between the groups at the forearm, lumbar spine, or whole body sites. There was a significant effect of the strength program at the total (0.9 ؎ 2.6%; p < 0.05) and intertrochanter hip site (1.1 ؎ 3.0%; p < 0.01). There was a significant time and group interaction (p < 0.05) at the intertrochanter site by repeated measures. This study shows the effectiveness of a progressive strength program in increasing bone density at the clinically important hip site. We concluded that a strength program could be recommended as an adjunct lifestyle approach to osteoporosis treatment or used in combination with other therapies. (J Bone Miner Res 2001;16:175-181)

Computers are now widely used by children. Tablet computers are becoming widely available and promoted for use by school children. The primary objective of this study was to compare the posture and muscle activity of children using a tablet computer to the posture and muscle activity of children using a desktop computer and paper technology. Eighteen children (mean age 5.6 years) performed a colouring-in task in tablet, desktop and paper conditions. 3-D posture and muscle activity around the neck and shoulder was assessed. Tablet computer use was similar to paper use, with less neutral spinal posture, more elevated scapular posture and greater upper trapezius and cervical erector spinae activity. This was offset by greater variability of posture and muscle activity. Tablet computer use clearly results in different musculoskeletal stresses than desktop computer use. Computer use guidelines need to be appropriate to traditional and emerging technologies. Tablet computers are being promoted for use by adults and children. However, the physical impact of using this type of technology is not known. The findings of this study provide the first tablet-specific evidence to inform guidelines on wise use of tablet computers by children.

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Computer use by children at home and school is now common in many countries. Child computer exposure varies with the type of computer technology available and the child's age, gender and social group. This paper reviews the current exposure data and the evidence for positive and negative effects of computer use by children. Potential positive effects of computer use by children include enhanced cognitive development and school achievement, reduced barriers to social interaction, enhanced fine motor skills and visual processing and effective rehabilitation. Potential negative effects include threats to child safety, inappropriate content, exposure to violence, bullying, Internet 'addiction', displacement of moderate/vigorous physical activity, exposure to junk food advertising, sleep displacement, vision problems and musculoskeletal problems. The case for child specific evidence-based guidelines for wise use of computers is presented based on children using computers differently to adults, being physically, cognitively and socially different to adults, being in a state of change and development and the potential to impact on later adult risk. Progress towards child-specific guidelines is reported. Finally, a set of guideline principles is presented as the basis for more detailed guidelines on the physical, cognitive and social impact of computer use by children. The principles cover computer literacy, technology safety, child safety and privacy and appropriate social, cognitive and physical development. The majority of children in affluent communities now have substantial exposure to computers. This is likely to have significant effects on child physical, cognitive and social development. Ergonomics can provide and promote guidelines for wise use of computers by children and by doing so promote the positive effects and reduce the negative effects of computer-child, and subsequent computer-adult, interaction.

Purpose We studied individual variability in exercise responses in twins. We hypothesized that 1) endurance (END) training would reduce fat mass whereas resistance (RES) training would increase lean mass, 2) individuals who did not respond to one modality would respond to the other, and 3) cross-sectional heritability estimates would be higher than estimates based on training responses. Methods DXA was undertaken in 84 same-sex untrained twins (30 monozygotic [MZ], 12 dizygotic [DZ]). Participants underwent 3 months of END and RES training, separated by 3 months washout. Twins trained in pairs. Results RES (P < 0.001) and END (P = 0.002) increased lean mass, with a greater change in RES (P < 0.001). Similarly, RES (P = 0.04) and END (P = 0.006) decreased fat mass. Eighty-four percent of subjects responded positively to RES for lean mass and 58% to END (P < 0.001). For fat mass, RES and END induced 56% and 66% responder rates, respectively (P = 0.28). Cross-sectional intraclass correlations, used to assess the similarity in twin responses, were higher for MZ than DZ pairs for all variables. Following training, only MZ pairs were significantly correlated (P < 0.001) for change in lean mass to RES. Conclusion To our knowledge, this study is the first to report individual responsiveness in body composition to both RES and END in the same subjects. Although RES and END induced favorable changes in fat mass, RES was superior for lean mass. The frequency of lean mass responders to RES exceeded that for END, whereas response rates for fat mass were similar. Cross-sectional heritability estimates were higher than training response estimates, and shared environment had the largest influence on changes in body composition. This study suggests that exercise professionals should consider modality and environmental factors when optimizing exercise interventions.