In 427 Swedish adults followed from age 16 to 63, fitness and strength started falling around age 35

What the research found

Study (longitudinal cohort). Maria Westerståhl and colleagues at Karolinska Institutet report in the Journal of Cachexia, Sarcopenia and Muscle (2025; 16(6); DOI 10.1002/jcsm.70134). ScienceDaily recirculated the paper on 14 Oct 2026. The Swedish Physical Activity and Fitness study followed 427 men and women born in 1958, with tests from age 16 through 63. Aerobic fitness and muscular endurance were highest between ages 26 and 36. Performance began to decline around the mid-30s, and explosive muscle power peaked earlier. By age 63, average performance was about 30 to 48 percent below peak. The pattern was similar in men and women. People who became active in adulthood gained about 5 to 10 percent in physical capacity. Activity slowed the drop and did not stop it. A 2026 follow-up in Scientific Reports (DOI 10.1038/s41598-026-52389-8) found that better aerobic fitness at ages 34 and 52 lined up with less arterial stiffness at 63, after accounting for obesity, smoking, and blood pressure. That link is not proof that fitness prevented stiff arteries. Not medical advice.

Evidence level: Human study. A study in people, but an association isn't proof of cause.

Read the original at Karolinska Institutet / Journal of Cachexia, Sarcopenia and Muscle