At 35, the body already starts losing physical performance — can it be prevented?

There’s a popular cliché that, repeated so often — at gyms, in doctors’ offices and over drinks at the bar — has become conventional wisdom about when physical decline begins in humans: “after 40, it’s all downhill.”

One of the longest studies on the subject followed 427 individuals for 47 years. Called the Swedish Physical Activity and Fitness Study (SPAF), it was designed by researchers at Karolinska Institutet to answer the question: When, exactly, does the human body reach its physical peak, at what point does it begin to decline — and how does that trajectory evolve over a lifetime?

The counterintuitive conclusion upended the common belief: decline doesn’t start at 40 — it starts earlier. Published in November 2025 in the Journal of Cachexia, Sarcopenia and Muscle, the study showed that estimated maximal aerobic capacity and muscular endurance, measured by bench press repetitions, peaked between ages 26 and 36 in both sexes, and declined gradually from then on.

What changes in the body around age 35?

“Thirty-five is the point where the curve turns,” sums up orthopedic surgeon and sports traumatologist Ana Paula Simões in an interview with Prime Health Report. According to the communications director of the Sociedade Brasileira de Medicina do Exercício e Esporte (SBMEE), the Brazilian Society of Exercise and Sports Medicine, the loss starts out almost imperceptibly — less than 1% per year — but accelerates to more than 2% per year by a person’s 60s. By 63, a person retains only about two-thirds of the capacity they had at their peak.

According to Simões, the response to training is usually the first practical sign of this change: the muscle starts responding less to the same stimulus, even when the person “still feels like they’re at their peak.” As she puts it: “That’s when the mind starts wanting and the body won’t.”

Behind this sign is a basic biological script that hasn’t changed. “The earliest trigger is metabolic: the mitochondria, which are the power plants of the muscle cell, already show signs of dysfunction in the early 30s,” explains the professor at the Faculdade de Ciências Médicas da Santa Casa de São Paulo.

As this metabolic problem sets in — the mitochondria failing — another change happens at the same time: the tissue silently enters a process of structural remodeling. The supporting structure and nerve connections change, with fewer motor units. “The muscle gradually loses part of its wiring,” sums up Dr. Ana Paula.

Only after these metabolic and structural changes begin to occur silently do the visible effects appear: the muscle shrinks (so-called atrophy), responds less to training and also suffers from inflammation that makes everything worse.

If you used to train and gain strength easily, after atrophy you do the same work but get smaller results. Known as “anabolic resistance,” this reduced muscle response to the normal stimuli that build lean mass (protein intake and exercise) means the body needs greater stimuli to produce the same result.

At first, the signs are subtle: “The person doesn’t feel weak; they feel they’re adapting less to training,” Simões describes. In her view, the study reinforces an important distinction: you don’t choose when you reach your peak — because that’s biological — but you can slow the decline with physical activity.

Why does the speed of decline matter more than when it starts?

For Andressa Dias, a geriatrician and graduate-program professor at Afya Educação Médica Curitiba, more important than knowing exactly when the decline begins is understanding how it unfolds over a lifetime. In geriatrics, we think long term: the plan is to maintain functional capacity and reduce mortality, she tells PHR.

The Swedish study itself reinforces this view by showing that the people who reached age 63 with the best physical performance were precisely those who had slowed this decline over the years.

Dias illustrates with a practical example: a person who shows a 20% drop in muscle functional capacity over five years deserves more attention than someone who, at 35, performs below average but remains stable. In other words, what’s concerning is how fast the body changes.

What really works to slow physical decline?

For Simões, the big change over the years shows up in the relationship between effort and recovery. “At 25, you overdo it, sleep poorly, train hard again the next day and the body delivers. At 40, that same overdoing it costs you: gains come more slowly and recovery stops being a detail and becomes part of training,” she describes.

In practice, this means understanding that the body keeps responding to exercise but requires more strategy: better dosing of volume, spacing out the most intense sessions and not giving up strength training — “because power and strength are exactly what’s lost most with age.”

One of the most important points Simões highlights debunks a common myth: that starting too late is pointless. The study showed that giving up a sedentary lifestyle at any stage of life brings measurable gains — with a 6% to 7% improvement in aerobic capacity and an 11% improvement in upper-body strength. “There’s no window that closes. People who start at 45 improve; people who start at 55 improve,” the orthopedic surgeon says.

Dias reaches a similar conclusion, this time looking at the body of accumulated evidence on healthy aging: the most consistent path to slowing performance loss combines strength training with progressive loading, less sedentary behavior and regularity — even more than maximum intensity or heavy loads on their own. “Training and then sitting for ten hours a day cancels out part of the gain,” the geriatrician warns.

In clinical practice, Dias recommends a simple but consistent combination: strength training two to three times a week, with gradual progression in load or volume, combined with aerobic exercise at the same frequency. She also recommends breaking up prolonged sitting with breaks of about five minutes every 45 to 60 minutes.

The most common mistakes she sees in her practice include ignoring sedentary time between workouts, abandoning the routine after a few months, always keeping the same load without progression and focusing on a single type of exercise — since that limits the ability to prevent sarcopenia.

Is there a warning sign that it’s time to reassess your training routine?

“The most honest sign is recovery,” says Simões. When the body takes longer than usual to recover, or when the same old workout starts taking a bigger toll, that doesn’t mean it’s time to stop — but rather to adjust the load. Fatigue that drags on, pain that doesn’t go away, sleep that doesn’t restore: to her, that’s the body asking for a review, not retirement.

Dias stresses that the logic remains the same at 35, 45 or 55: the body keeps responding to stimulus, but the strategy needs to keep pace with each stage of life. With aging, she says, it also becomes increasingly important to monitor factors such as diet and sleep quality alongside training.

Simões’s final message sums up the point where the two experts, despite starting from different perspectives, end up meeting: “We don’t choose the age of our peak, but three things are up to us — how much we improve before it, the level we reach and how fast we decline afterward.”

And in all of them, physical activity plays a decisive role.