There's no single age when your body peaks. In a Swedish cohort tested from 16 to 63, aerobic fitness, muscular endurance and jump height each peaked at a different age, between 19 and 36 depending on the measure and sex. After the peak, performance slid slowly at first, then faster.
The headlines say strength starts declining around 35. That's half right. The study tracked endurance and power proxies, not maximal strength or muscle mass. So the useful question isn't "when do I peak?" It's "what keeps the slope shallow?"
What the evidence supports: strength train at least twice a week, stay aerobically active, and hit a protein target you can measure. Here's the system, and where the evidence stops.
What the 47-year study measured (and what it didn't)
The Swedish Physical Activity and Fitness study (SPAF) followed 427 people born in 1958 (222 men, 205 women), first tested at 16 and last at 63. It was published in the Journal of Cachexia, Sarcopenia and Muscle in 2025 and covered by ScienceDaily in October 2026.
The researchers repeated three kinds of test:
Aerobic capacity, estimated from a run at 16 and a submaximal cycle test later. Estimated, not lab-measured VO2max.
Bench-press endurance: reps at a fixed light load (20 kg for men, 12 kg for women). That's muscular endurance, not a one-rep max.
Vertical jump height: a proxy for leg power relative to body weight.
What it did not track: maximal lifting strength or muscle mass across the lifespan. Grip strength and chair stands were added only at 63. Anyone saying this study proves "strength peaks at 35" is reading past the data.
It's also observational. People dropped out over five decades, tests happened at only a handful of ages, and the peaks are modeled, not observed year by year.
Every capacity peaked at a different age
These are the study's modeled values (Table 2).
Measure | Peak age, men | Peak age, women | Drop from peak to 63 (men / women) |
|---|---|---|---|
Aerobic capacity (L/min) | 35 | 36 | 33% / 30% |
Aerobic capacity per kg of body weight | 26 | 31 | 40% / 37% |
Bench-press endurance | 36 | 34 | 35% / 32% |
Jump height | 27 | 19 | 41% / 48% |
Two patterns stand out. The decline started small and accelerated with age. And power (jump height) fell furthest for both sexes. It also peaked earliest in women, at 19.
That 30–48% range is where the headline comes from. It's the spread of average declines across these measures, not a forecast for you, and not a measured loss of muscle. One wrinkle: the paper's results text lists the aerobic-capacity peak ages the other way round (women 35, men 36). The table is used here.
Getting active later still showed up in the numbers
Participants who went from inactive to active as adults scored higher: about 7% on aerobic capacity, 6% on aerobic capacity per kg, 11% on bench-press endurance and 4% on jump height (SPAF). ScienceDaily rounds this to 5–10%.
Read the fine print. "Active" was a self-reported yes/no, and it counted things like walking and gardening. There was no dose and no lifting program. It's an association, too: people whose health improved may simply have become more active.
So it tells you starting later isn't pointless. It doesn't tell you which program to run.
Strength can fall faster than muscle, later in life
To compare strength with muscle, you need a different study. The Health ABC study tracked 1,880 older adults for three years. Leg strength fell about 2.6–4.1% a year, depending on sex and race group. Leg lean mass fell about 1% a year, so strength dropped roughly three times faster than mass.
Two limits. These were adults in their 70s, and the paper is from 2006. Don't apply those rates to a trained 40-year-old.
The part that transfers: muscle size isn't the whole picture. That's why the system below trains strength directly instead of assuming muscle will carry it.
The system for holding strength past 35
The floor comes from public-health guidance. The CDC's adult guidelines call for at least 150 minutes of moderate activity a week, plus muscle-strengthening on 2 or more days that works all major muscle groups. The NSCA's 2019 position statement backs resistance training for older adults.
Component | Weekly minimum | How to run it |
|---|---|---|
Strength sessions | 2+ days | Full body: legs, hips, back, abdomen, chest, shoulders, arms |
Aerobic work | 150 min moderate | For example 30 minutes, 5 days: walk, cycle, row |
Protein | about 1.6 g/kg/day | About 0.4 g/kg per meal, over 4 meals (next section) |
Tracking | Every session | Same lifts, logged loads and reps; watch the trend over months |
Set it up in five steps:
Book two strength sessions as fixed calendar appointments. Two is the floor, not the ceiling.
Cover every major muscle group each week. A squat or leg press, a hinge, a push, a pull and a plank or carry covers most of it.
Log loads and reps. Without a log, you can't see a slow slide.
Fill the aerobic minutes around the lifting. Brisk walking counts.
Set protein per meal, using the table below.
What this doesn't settle: exact sets per muscle, whether one session a week holds your gains, or how much fast, power-focused lifting to add. The research behind those numbers wasn't verified for this piece, so we're not prescribing them.
Protein: a review-derived target, not an age-40 switch
A 2018 review by Schoenfeld and Aragon proposed about 0.4 g/kg per meal across at least four meals, for a daily minimum of 1.6 g/kg. At the upper end, up to 0.55 g/kg per meal gets you to 2.2 g/kg a day.
Treat that as a practical target from a narrative review. It isn't a requirement proven by long-term trials, and nothing in it says your needs jump at 40.
Body weight | Per meal (0.4 g/kg) | Per day (4 meals) |
|---|---|---|
70 kg | 28 g | 112 g |
80 kg | 32 g | 128 g |
90 kg | 36 g | 144 g |
Bottom line
There's no single peak. In the SPAF cohort, different capacities topped out between 19 and 36 and declined afterwards, faster with age. The study didn't measure maximal strength or muscle, so treat 35 as a nudge, not a deadline.
The system: 2+ full-body strength days, 150 minutes of aerobic work, about 0.4 g/kg of protein per meal, and a training log so you can see the trend.
Sources
The article links each of these inline. Add this list under Bottom line if you want a sources block on the post.
Westerståhl M, et al. Rise and Fall of Physical Capacity in a General Population: A 47-Year Longitudinal Study. Journal of Cachexia, Sarcopenia and Muscle, 2025. DOI 10.1002/jcsm.70134.
ScienceDaily. A 47-year study reveals the age when your body starts losing strength. October 14, 2026.
Goodpaster BH, et al. The Loss of Skeletal Muscle Strength, Mass, and Quality in Older Adults: The Health, Aging and Body Composition Study. Journals of Gerontology, Series A, October 2006.
Fragala MS, et al. Resistance Training for Older Adults: Position Statement From the National Strength and Conditioning Association. Journal of Strength and Conditioning Research, August 2019.
Schoenfeld BJ, Aragon AA. How much protein can the body use in a single meal for muscle-building? Implications for daily protein distribution. Journal of the International Society of Sports Nutrition, 2018.