Young adults who were born prematurely may have weaker muscles than their peers born at full term, a Finnish study suggests.

These young adults born preterm also considered themselves less physically fit, even though the study didn't find their cardiorespiratory fitness levels to be much different than people who weren't born early.

Pregnancy normally lasts about 40 weeks, and babies born after 37 weeks are considered full term. In the weeks immediately after birth, preemies often have difficulty breathing and digesting food. Some premature infants also encounter longer-term challenges such as impaired vision, hearing, and cognitive skills as well as social and behavioral problems.

Previous research has also found that the tiniest and most immature preemies may have poor muscular fitness. But the current study is important because it suggests that this problem may extend to all pre-term babies, even those who are only slightly early or a little bit underweight, said lead author Dr. Marjaana Tikanmaki of the National Institute for Health and Welfare and University of Oulu in Finland.

"The differences in muscular fitness of young adults born preterm were detected in our study across the full range of preterm birth, but not for cardiovascular fitness," Tikanmaki said by email.

But, Tikanmaki added, "The test we used to measure cardiovascular fitness may not be sensitive enough to detect small differences between those born preterm and those born at term."

To see how the timing of birth might impact fitness later in life, the researchers studied 139 young adults born before 34 weeks gestation, which is considered early preterm, as well as 247 people born from 34 to 36 weeks, or late preterm. They compared these individuals to a control group of 352 full term individuals.

On average, the participants were around 23 years old.

Researchers assessed muscular fitness based on the number of modified push-ups performed in 40 seconds, a test that measures short-term endurance capacity of the upper body and the ability to stabilize the trunk.

On average, women did about 10 push-ups and men managed about 14. But the people who were born preterm typically did about one less push-up than their full-term peers.

In another assessment of muscular fitness, researchers also examined grip strength based on how hard people could squeeze a force-measuring device with their dominant hand.

With this test, people born early preterm didn't do as well as those born late preterm or full term.

To check cardiorespiratory fitness, researchers asked participants to step on and off a bench at a set pace for four minutes.

By the end of this test, the average heart rate for women was 160 beats a minute, while it was 153 beats a minute for the men. There wasn't a difference based on the timing of birth.

Researchers also asked participants to rate their own fitness, from 1 to 5, with higher scores for better abilities in this area. Average scores were 2.3 for women and 2.6 for men.

The early preterm people typically scored themselves about 0.2 points lower than full-term participants, and higher scores were linked to better measurements of physical fitness, the researchers reported in the journal Pediatrics.

One limitation of the study is that the cardiorespiratory fitness test didn't necessarily push people to their maximum ability, a type of exercise that often takes longer and may gradually increase in difficulty until participants can no longer continue.

It's also possible that these relatively young participants may have been too fit relative to older adults to detect large differences in their cardiovascular health, Tikanmaki noted.

Even so, the results highlight the importance of focusing on heart health early in life to ward off potential problems down the line, noted Dr. Ravi Shah, a researcher at Harvard Medical School in Boston who wasn't involved in the study.

"These findings support efforts to reduce obesity and improve physical fitness throughout early life, though any specific recommendations regarding exercise should be individualized and undertaken only after counseling from a physician," Shah said by email.

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