Fitness peaks in mid-30s, then declines regardless of earlier training
22 Dec 2025

A long-term Swedish study has revealed that physical fitness, strength, and endurance begin to decline much earlier than previously thought.

The research, published in the Journal of Cachexia, Sarcopenia, and Muscle, shows that while fitness improves during youth and early adulthood, it starts deteriorating after peaking around the mid-30s.

This decline occurs regardless of earlier training levels or activity habits.

Study tracks fitness evolution over decades
Research methodology

The study tracked the same individuals for nearly five decades, observing how fitness develops from adolescence to adulthood.

This long-term approach allowed scientists to see real changes within individuals over time, challenging common beliefs about peak fitness.

The research provides important insights for public health and long-term well-being by mapping the evolution of strength, endurance, and muscle performance over time.

Peak physical ability and decline rates
Fitness peak

The study found that physical ability peaks at around 35 years of age, after which a steady decline begins.

This drop was observed in strength, cardiovascular endurance, and muscle power once the peak was crossed.

The rate of decline increased with age, showing that fitness levels gradually decrease after reaching their highest point in mid-adulthood.

Swedish Physical Activity and Fitness (SPAF) study
Research details

The findings are based on the Swedish Physical Activity and Fitness (SPAF) study conducted by Karolinska Institutet.

Over 400 randomly selected men and women across Sweden were followed from their teenage years into their 60s.

Participants underwent repeated physical testing over 47 years, with muscle strength, power, aerobic capacity assessed through various exercises such as bench press tests and vertical jump assessments.

Fitness levels and activity habits influence long-term outcomes
Impact of lifestyle

Along with performance data, participants' physical activity levels were recorded to understand how lifestyle habits influenced long-term outcomes.

The results showed a steady improvement in fitness during teenage years and early adulthood.

However, once peak performance was reached, fitness levels gradually declined by some 30-48% by the early 60s compared to peak levels.

Gender differences in fitness decline
Decline patterns

While the overall trend was similar for both sexes, some differences emerged.

Aerobic endurance began declining around the mid-40s for both men and women.

However, muscle power started decreasing earlier in women (early 30s) than men (a few years later).

Despite these differences, the long-term rate of decline was largely comparable between both groups.