A huge study from Sweden followed the same people for almost five decades. It discovered that our bodies start losing fitness, strength, and stamina around the mid‑30s.
The research was carried out at Karolinska Institutet as part of the Swedish Physical Activity and Fitness (SPAF) project. Hundreds of men and women, first measured at 16 years old, were tested again and again until they were 63.
Most earlier studies only compared different age groups at one point in time. SPAF, however, kept checking the same individuals over many years, giving a clearer view of how we change as we grow older.
Fitness Begins to Slip Near 35
Results showed that even people who exercised before 35 started to see a gentle drop in physical ability after that age. The decline kept going and became steeper as participants grew older.
All three measures—overall fitness, muscle strength, and endurance—followed the same downward pattern.
Exercise Still Makes a Big Difference
Good news: staying active later in life still helps. Those who started regular workouts during adulthood improved their performance by about 5‑10 %.
The researchers will check the group again next year when they reach 68. They hope the continued data will show how habits, health, and the body’s biology shape physical ability throughout life.