🟠 Moderate Evidence
A 47-year longitudinal study tracking the same individuals across decades reveals that physical capacity peaks much earlier than commonly believed—typically between the late teens and mid-30s—after which aerobic fitness, strength, and muscle power decline at an accelerating rate, particularly after age 40, according to research published in the Journal of Cachexia, Sarcopenia and Muscle (2025).
Key takeaways
- Peak physical capacity is reached in late teens to mid-30s, not middle age
- Even late-starters who began exercising later in life improved physical capacity by 5–10%
- Early fitness creates a “physiological reserve” that determines functional health decades later
Study at a Glance
| Source | Journal of Cachexia, Sarcopenia and Muscle |
| Study type | Prospective longitudinal cohort |
| Duration | 47 years follow-up |
| Outcome measures | Aerobic fitness, muscle strength, muscle power, physical capacity |
| Country | Sweden |
Physical capacity across the lifespan
Peak fitness attained in late teens to mid-30s; accelerated decline after age 40
Source: Westerståhl et al., Journal of Cachexia, Sarcopenia and Muscle, 2025 | Georgian Medical Journal News
The aging curve begins earlier than expected
The research conducted by Dr. Filip Westerståhl and colleagues at the Karolinska Institute tracked participants over nearly five decades, documenting the trajectory of physical fitness from youth into older age. The findings challenge the common assumption that peak fitness remains stable into the 40s or 50s.
According to the study published in the Journal of Cachexia, Sarcopenia and Muscle, the decline in physical capacity accelerates significantly after approximately age 40, with rates of loss in aerobic fitness and muscle strength increasing markedly in subsequent decades. This trajectory is consistent across aerobic capacity, grip strength, and leg power measurements.
Building physiological reserve in youth protects health in aging
The key finding from this research is the concept of “physiological reserve”—the difference between an individual’s maximal functional capacity and the minimum level required for daily activities. People who achieved higher fitness levels during their youth and early adulthood entered the aging process with a larger reserve cushion, according to Westerståhl’s analysis in the Journal of Cachexia, Sarcopenia and Muscle.
This reserve determines whether someone can remain functionally independent in older age or faces limitations in mobility, self-care, and daily living. A person who begins life with high physical capacity can decline substantially and still retain adequate functional ability; conversely, someone with low peak fitness may cross into functional disability relatively early in the aging process.
People who were fitter early in life entered aging with a larger “physiological reserve,” but even those who started exercising later improved physical capacity by approximately 5–10%, demonstrating that activity meaningfully reshapes the aging curve at any age.
— Dr. Filip Westerståhl, Karolinska Institute, Sweden (Journal of Cachexia, Sarcopenia and Muscle, 2025)
It’s never too late, but earlier is always better
While the research underscores the value of establishing fitness early in life, the good news is that exercise intervention at any age produces measurable improvements. The study found that individuals who took up exercise later in life still achieved approximately 5–10% gains in physical capacity, suggesting that the aging curve can be reshaped even after decades of sedentary behavior.
The mechanisms underlying this benefit include improvements in muscle protein synthesis, cardiovascular adaptation, and neuromuscular coordination—processes that remain responsive to activity stimulus across the lifespan. However, the magnitude of benefit is smaller than for those who maintained consistent activity from youth, reinforcing the preventive advantage of early fitness establishment. See our Clinical Updates section for evidence-based guidance on exercise prescriptions across age groups.
What this means
Frequently asked questions
Can I regain fitness I lost in middle age?
Yes, but with lower absolute gains than if you had maintained activity. According to the 47-year longitudinal data, people who resumed exercise later achieved 5–10% improvements in physical capacity. The mechanism is that muscle and cardiovascular tissue remain responsive to activity stimulus, but the decline in capacity is not fully reversible—hence the importance of consistent early activity.
What type of exercise is most important for building physiological reserve?
The research measured aerobic fitness, muscle strength, and muscle power—all improved with structured physical activity. A combination of aerobic work (walking, running, cycling) and resistance training (weights, bodyweight) appears most effective. Consistency and duration matter more than intensity for most people.
At what age should I be most concerned about starting exercise?
The study suggests that peak fitness years are late teens through mid-30s. However, there is no age cutoff for benefit. Even people who exercise for the first time at 60+ show improvements. Starting early maximizes your physiological reserve, but starting late is infinitely better than never starting.
The implications of this 47-year longitudinal study are clear: the foundations of healthy aging are laid decades in advance. While individual genetics influence the absolute peak and rate of decline, the modifiable factors—establishing high fitness early and maintaining activity throughout life—remain within personal control. Healthcare systems and public health policy should reflect this reality by prioritizing physical activity promotion from childhood onwards, while reassuring adults at any age that exercise engagement produces meaningful physiological benefits. For more on aging and prevention, explore our Health Policy coverage and SheniEkimi’s wellness explainers.
Source: Westerståhl et al., Journal of Cachexia, Sarcopenia and Muscle, 2025
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Disclaimer. This article is health journalism intended for general information and education. It is not medical advice and is not a substitute for professional diagnosis or treatment. Always consult a qualified healthcare provider about your individual circumstances. Full disclaimer →
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Medically reviewed by Prof. Giorgi Pkhakadze, MD, MPH, PhD. Spotted an error? Contact the editorial team.
