Bolhuis turned her exercises into a company, Gymtrix, that offers a library of videos starting with training for babies as young as six months. There is no lying in the cot playing with toes.
Infant athletes, accompanied by doting parents on the videos, do a lot of jumping, kicking and, in one exercise, something that looks like baseball batting practice.
"We hear all the time from families that have been with us: 'Our kids are superstars when they're in middle school and they get into sports,'" Bolhuis said.
The growing competition in marketing baby sports DVDs includes companies with names like athleticBaby and Baby Goes Pro. Even experts in youth sports seem startled that the age of entry has dipped so low. Children, it appears, are being groomed as athletes before they can walk
Dr Lyle Micheli, an orthopaedic surgeon and founder of the first paediatric sports medicine clinic in the US, at the Children's Hospital in Boston, said he did not see any great advantages in exposing babies to sports. "I don't know of any evidence that training at this infancy stage accelerates co-ordination," he said. One of his concerns is "the potential for even younger ages of overuse injury".
Bob Bigelow, a former National Basketball Association player and a critic of competitive sports for young children, said: "This is Baby Mozart stuff; you play Mozart for the baby in utero and it comes out some sort of fine arts major. There are millions of American parents worried to death that their children might fall behind somebody else's kid. So the emphasis in youth sports has become more, more, more, younger, younger, younger."
The Little Gym, based in Scottsdale, Arizona, begins classes for children at four months. Chief executive Bob Bingham said that about 20,000 youngsters under two - about one-quarter of the total enrolment - were signed up for classes in the US, Canada and Puerto Rico. That is a sizeable increase from last year, he said. The company, which has gyms in 20 countries, plans to open in 100 locations in China over the next five years.
My Gym, based in Sherman Oaks, California, said 55 per cent of those who attend classes at its 200 locations - 157 in the US - were 2? or younger.
The entrepreneurs behind these businesses - gym teachers, accountants and former professional athletes among them - make no claims about turning today's babies into tomorrow's Super Bowl stars. In the past, the marketing of products geared toward babies has been challenged. Disney, which owned the Baby Einstein brand, dropped the term "educational" after a children's-rights group objected to claims that babies who watched "Baby Einstein" were learning.
Most sports-video entrepreneurs promote their products as early intervention for combating obesity. Others claim they provide time that parents and children can spend together. "We're not suggesting your kid will turn pro; we have to be careful about that," said Gigi Fernandez, a former professional tennis player, who is one of the founders of Baby Goes Pro.
Fernandez and a partner started the company this year. She said that when she plays the Baby Goes Pro's "Discover Sports" video for her two toddlers, she lays sports equipment, like rackets and softballs, around the room. When she comes back, she finds them swinging away.
Someday, Fernandez predicted, this could give them an edge — a small one, perhaps. "The first time they go to a baseball field or tennis court, they'll have a clue," she said.
One mother who has enrolled her child is Lisa Mullen, though she says she is not concerned about whether her toddler develops into a sports star. She visited several children's gyms in the Baltimore area to find "a physical outlet for our high-energy son". The one she settled on for 16-month-old Michael was appealing for its low-key atmosphere and varied activities. During the first class, Michael and his two classmates swung from a bar, walked on a low balance beam and banged drumsticks.
Other programmes, however, appear more serious. At Lil' Kickers, a soccer academy in 28 states, parents can enrol their children at 18 months; about 55 per cent of the 100,000 children signed up this year are three or younger. In beginner classes, toddlers run and kick. Lil' Kickers also hands out improbably small soccer jerseys. But the program, created by child-development experts, is relaxed, insists chief executive Don Crowe. "Our emphasis is on the child, not trying to turn them into the next Pel," he said.