1 . The rate of childhood obesity in the U.S. has tripled over the past 50 years. But what this trend means for children’s long-term health, and what to do about it (if anything), is not so clear.

The American Academy of Pediatrics (AAP) made waves this year by recommending that doctors put obese kids as young as two years old on intensive, family-oriented lifestyle and behavior plans.     1     This advice marks a shift from the organization’s previous stance of “watch and wait,” and it reflects the AAP’s belief that obesity is a disease and the group’s adoption of a more proactive position on childhood obesity.

Yet the lifestyle programs the AAP recommends are expensive, inaccessible to most children and hard to maintain — and the guidelines acknowledge these barriers. Few weight-loss drugs have been approved for older children, although many are used off-label.     2     And surgery, while becoming more common, has inherent risks and few long-term safety data — it could, for instance, cause nutritional deficits in growing children. Furthermore, it’s not clear whether interventions in youngsters help to improve health or merely add to the stigma overweight kids face from a fat-phobic society. This stigma can lead to mental health problems and eating disorders.

Rather than fixating on numbers on a scale, the U.S. and countries with similar trends should focus on an underlying truth: we need to invest in more and safer places for children to play where they can move and run around, climb and jump, ride and skate.

    3    In 2020 the Centers for Disease Control and Prevention found, unsurprisingly, that kids’ sports participation increases with their parents’ incomes: about 70 percent of kids whose families earn more than $105,000 a year participate in sports, but only 51 percent of middle-class kids and 31 percent of children at or below the poverty line do. This disparity hurts people of color the most. More than 60 percent of white children, for instance, participate in athletics, but only 42 percent of Black children and 47 percent of Hispanic children do. Experts blame these problems on the privatization of sports — as public investment in school-based athletics dwindles, expensive private leagues have grown, leaving many kids out.

According to the U.S. Department of Health and Human Services’ Physical Activity Guidelines for Americans, children between ages six and 17 should get at least an hour of moderate to intense physical activity every day. Yet only 21 to 28 percent of U.S. kids meet this target, two government-sponsored surveys found. The nonprofit Active Healthy Kids Global Alliance evaluates physical activity in American children, and in 2022 the group gave the U.S. a grade of D–.

Why is it so hard to get kids moving? In addition to fewer opportunities at school, researchers cite increased screen time, changing norms around letting kids play outdoors unsupervised, and a lack of safe places for them to play outside the home.

New York City, for example, had 2,067 public playgrounds as of 2019 — a “meager” amount for its large population, according to a report from the city comptroller — and inspectors found hazardous equipment at one quarter of them. In Los Angeles in 2015, only 33 percent of youths lived within walking distance of a park, according to the L.A. Neighborhood Land Trust. Lower-income neighborhoods tend to have the fewest public play spaces, despite often having a high population density.     4    

Kids everywhere need more places to play: trails, skate parks and climbing walls, gardens and ball fields, bike paths and basketball courts. Vigorous public funding to build and keep up these areas is crucial, but other options such as shared-use agreements can make unused spaces available to the public.

2 . Bouncier running shoes may be bad

Despite regular changes in running shoes over the years, it is estimated that every year at least a third of runners get muscle or joint injuries caused by repeated striking of the ground. Many sports shoe-makers have begun adding extra material to running shoes, to try to soften the impact on the legs — so-called maximalist footwear.     1    

A new study suggests this is because the extra cushioning alters the spring-like mechanics of the legs of a runner in a way that means their legs experience a greater impact with every step.

Juha-Pekka Kulmala at the University of Helsinki in Finland and his colleagues studied the biomechanics of 12 healthy men aged between 22 and 32 as they ran in two shoes types.

The first wore regular running shoes with 33 millimetres of cushioning under the heel and 22 millimetres under the forefoot, and then highly-cushioned shoes with a heel 43 millimetres thick and a forefoot of 37 millimetres.

The participants ran at two set speeds — 10 and 15 kilometres per hour — along a 30-metre platform that measured how hard their feet hit the ground. They also wore reflective stickers that allowed video cameras to capture their motion for analysis.

    2     The peak impact force was 6 percent higher on average at the slower running speed and 11 percent higher at the faster speed.

Normally when we run, our legs act like springs, with the ankle and knee joints bending, so that the leg as a whole compresses (压缩) as the foot lands, says Kulmala. But because highly cushioned shoes already compress under the feet, our bodies subconsciously respond by no longer bending the leg joints as much. In other words, our legs become stiffer (更僵硬的).     3    

Kade Paterson at the University of Melbourne in Australia says the findings make sense from a biomechanical point of view.     4     I’ve seen patients who have reported improvements with maximalist running shoes and others who have got injured in them, so there probably isn’t a one-size-fits-all approach,” He says.