In a fluorescent-lit room decked with posters, people slouch in their seats, shoulders rounded and spines curved. A few bent forms prop their heads on elbows, seemingly unable to sit up without support; others appear ready to slip from their seats.

These slumped bodies are not residents in a nursing home lounge; they’re students in school. A frightening number of otherwise healthy young people look as if they had spinal osteoporosis.

This classroom scene, while sobering, should not surprise us. Children spend six to seven hours a day sitting in school (not counting transport time), often with only one or two 15-minute breaks for free play outside.

Hour after hour, they hunch over laptops, worksheets and tests – spines rounded and shoulders rolled forward. Outside school, they often remain bent over screens doing homework, playing video games and web-surfing.

Sedentary indoor routines contribute to obesity, Type 2 diabetes and attention deficit/hyperactivity disorder – challenges that have gotten widespread attention. But lack of outdoor play harms children in other ways, increasing clumsiness, reducing stamina, decreasing creativity, weakening immune systems and heightening reactivity and aggression.

Schools that have to contend with these effects unwittingly aggravate them by cutting back recess time. Under pressure to increase test scores, they pack in more academic drills at the expense of physical movement.

Playground equipment that gave earlier generations a chance to stretch and build muscles has been replaced or shrunk due to concerns about safety and litigation. Meanwhile, the list of recess rules grows: don’t jump from or spin on the swings, don’t hang upside down and don’t cross the top of the monkey bars.

Older generations recall leisurely days spent in outdoor play, a primary focus of life outside school. That landscape of childhood has eroded, leaving overscheduled families with few unbooked hours each week. Parents are often stressed, and some remain anxious about having children unattended outdoors due to stranger danger (although research indicates those risks are greatly exaggerated).

Children do get outside in organized sports, but these team experiences don’t necessarily foster physical development or social skills as effectively as unstructured free play.

“Play allows children opportunities to get creative, to practice regulating emotions, to enhance social development,” writes pediatric occupational therapist Angela J. Hanscom, author of the new book “Balanced and Barefoot.” “When children play outdoors, they are naturally motivated to move – strengthening their muscles with … each step and every encounter with nature.”

In her professional practice, Hanscom has seen more and more children referred to occupational therapy for help with coordination and sensory processing challenges, such as an inability to listen or hypersensitivity to touch. “The cold hard truth,” she writes, “is that … they just can’t keep up.”

Young people today lack many of the capacities once gained effortlessly through kids’ play – swinging, climbing, clambering, digging, running and lifting. In some cases, children’s core strength and gross motor coordination decline to the point where they fall out of chairs or need to lean on desks for support while seated.

Hanscom has seen many of these problems resolve themselves as kids participate in a nature-based program where they play for hours outdoors – climbing on logs, wading in mud puddles and building stick and rock structures.

Time in nature, it turns out, is a remarkably effective prescription for many of the ailments afflicting today’s seat-bound children. Lifestyle diseases are not easy to reverse, but improving postural alignment, core strength and coordination may require little more than free play outdoors.

When the novelty of holiday toys begins to fade, offer children a more enduring gift – time to play outside. Explore local parks and preserves, giving kids freedom to set the agenda. Let them roam without too much cautionary guidance, and accept that there will be scratches, bruises and bug bites (do nightly checks for deer ticks).

Encourage kids to explore within their own yards so that outdoor time can be integrated into daily routines. Consider ways to make your yard more appealing, adding features such as log rounds, a sand pit, climbing nets or a rope swing.

Time outdoors will do far more for the children in your life than strengthen muscles and straighten spines. It will fortify their spirits. If they’re lucky, they’ll cultivate what Rachel Carson called “a sense of wonder so indestructible that it … (will) last throughout life.”

Few gifts we offer children could measure up to that.

Marina Schauffler is a writer whose work is online at naturalchoices.com.