The Goldilocks dose
Too little movement harms us, but so does too much. Where is the line?
By Professor Alister Hart
Most of my patients come to me with a joint or implant that’s worn out, and a clear conviction about the cause. They tell me about the marathon they ran decades ago, the school rugby they played, the half-remembered ankle sprain from 1987 — as if every worn joint must have a crime scene.
But when I look more closely, only a handful ever had a serious sports injury. Many describe long stretches of relative inactivity. Some barely exercised for years. The more stories I hear, the more convinced I become that the relationship between activity and joint wear is neither linear nor predictable. People often want movement to carry the blame — it offers a neat explanation for a complicated problem: ‘I ran, therefore my knees wore out’.
And yet just as often, I hear the opposite fear — not “Did I overdo it?” but “Have I done enough?” People who walk, swim or stretch worry they should be doing more: running marathons or lifting weights. They’ve heard that exercise prevents arthritis and slows ageing — and wonder if they’re missing out by not doing more.
So the real question — the one I hear from marathoners, footballers, office workers, ballet dancers, ultracyclists and even fellow surgeons — is this: “How much exercise is actually healthy?”
It’s a question I’ve heard from professional footballers, ultramarathoners, new runners, ballet dancers, office workers, and fellow surgeons. Everyone senses the same tension:
Too little movement harms us. Too much movement harms us. So where is the line?
It turns out this isn’t just a training question — it’s a biological one. And for the musculoskeletal system, this Goldilocks zone matters more than people realise.
The dangers of too little movement
When we under-load the body, predictable “inactivity diseases” start to appear — especially in the tissues I treat most:
Osteoporosis — weak, brittle bone that fractures under minor force
Sarcopenia — thinning, weakening muscles that slow us down
Low-back pain — often the result of weak glutes, weak core, and stiff movement patterns
These aren’t minor issues. They’re among the biggest causes of long-term pain, falls, frailty, and loss of independence in later life. And all three improve with the same medicine: regular, moderate loading. But there’s a flip side.
The dangers of too much movement
Push the body past its ability to adapt and you see the classic over-training injuries: stress fractures, tendinopathies, chronic fatigue and loss of muscle power. None of these are failures of ‘toughness’, but of energy balance and tissue recovery.
RED-S — when the body runs out of fuel
At the extreme end is RED-S — Relative Energy Deficiency in Sport. This occurs when training demand chronically exceeds fuel supply, leaving too little energy for normal physiology.
Originally described in female athletes as the “Female Athlete Triad”, RED-S is now recognised in men just as often — especially in endurance runners, cyclists, rowers, climbers and ballet dancers. Instead of menstrual changes, men typically present with low libido, irritability, mood change, persistent fatigue and unexplained performance decline.
Across all groups, RED-S disrupts bone turnover (reducing bone formation and increasing bone breakdown), weakens tendons and muscles, and leaves athletes more vulnerable to injury.
Stress fractures — when bone can’t keep up
Stress fractures are perhaps the clearest sign of mechanical overload. They are tiny, incomplete cracks in bone caused not by a single traumatic event but by repetitive sub-maximal forces that exceed the bone’s ability to repair. Because bone is a living tissue constantly being broken down and rebuilt, a stress fracture represents an imbalance: osteoclasts breaking down bone faster than osteoblasts can rebuild it. This can happen in healthy bone exposed to excessive load, or in weak osteoporotic bone under normal load.
This raises a revealing paradox: too little exercise leads to osteoporosis — which increases stress-fracture risk — but too much exercise can also cause stress fractures. The reason we don’t all break is simple: somewhere in between lies the “Goldilocks” dose — the level of impact that keeps bone cells in equilibrium.
Horses for courses
I hear the same refrain from patients, colleagues and friends: “A marathon? That must be too much for the knees.” Yet when you press that logic further — is 10K too much? 5K? A brisk walk? — the certainty disappears. We can agree that running an ultramarathon is demanding. But very few people will confidently declare that walking 5 kilometres a week is “too far”.
The same applies beyond running. In one of our MRI studies of cyclists, it was remarkably easy to recruit mid-life amateurs riding more than 8,000km a year — typically one 100km ride every weekend plus shorter rides in the week. For some cyclists that is extreme; for others, it is half their normal training volume.
“Too much” turns out to be a moving target — different for everybody and every life.
As close as it gets to an optimal dose of exercise?
If too little damages the system, and too much does the same, how do we find just right?
When I ask patients how much they move, the answers fall into two categories: “not enough, and I know it,” or “I probably overdid it.” Almost no one feels they’re in the sweet spot.
That alone tells us something. We don’t just lack the right dose — we lack a shared definition of one. The science now suggests it exists. We just haven’t taught people how to find it.
Across all health research — cardiovascular, metabolic, cognitive and musculoskeletal — the same pattern appears: the greatest benefit comes from modest, regular, sustainable movement. Not marathons. Not ultracycling. Not heavy gym work seven days a week. Just… enough.
Two of the strongest clues come from:
1. Mortality and cardiovascular data
Running 5–10 minutes a day, or jogging 5–10 km per week, reduces all-cause mortality by 30–40%.
More running did not improve outcomes in the Copenhagen City Heart Study (2015): the lowest mortality was seen in those running 1–2.4 hours per week at moderate pace. Heavy, frequent running lost this advantage.
This is why global guidelines settle on 150 minutes of moderate exercise per week: not because it’s perfect, but because it’s enough to drive measurable health benefit
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2. Cartilage MRI studies
In my own MRI research on runners’ joints, the sweet spot for cartilage health — thickness, hydration, and signal quality — appears at around 5–20 km per week. Below that, joints behave like under-loaded tissues. Above that, some runners adapt well, some don’t — and the difference is usually biomechanics and strength, not mileage alone.
It suggests something important: your joints like being used — but they prefer consistency to heroics.
Why Parkrun might be the Goldilocks dose
If any single event embodies the idea of “just enough exercise,” it’s the 5K Parkrun — the largest weekly participation event on the planet. The formula is simple: a free, inclusive, 5-kilometre walk/jog/run across 2,000+ locations.
Parkrun is a distance long enough to strengthen bone, muscle and cartilage. But it’s also short enough to recover easily from, frequent enough to build habit, and enjoyable enough to want to repeat. People keep coming back because it feels good.
If you include all global “parallel events” (same brand, multiple locations), Parkrun likely has the highest participation of any recurring exercise event worldwide. The only true rival is the Wings for Life World Run — 265,818 participants in 2024 running simultaneously worldwide until a “catcher car” overtakes them.
When something attracts millions of ordinary people worldwide, week after week, without breaking them, it’s worth paying attention.
So, is 5K the answer?
Not exactly. The right dose depends on your injury history, biomechanics, strength, energy availability and age — and your goals. For some, walking is the Goldilocks dose. For others, it’s a half-marathon. For many, it’s the quiet rhythm of a weekly 5K.
But across all groups, the principles never change:
· Joints like load, but not shocks.
· Tendons like progression, but not surprises.
· Bones like impact, but not exhaustion.
· Muscles like challenge, but not starvation.
The right dose is the one you can repeat — not endure.
The ongoing puzzle
I’ve spent 25 years scanning joints, treating stress fractures, advising athletes and walking patients through injury and recovery. And the conclusion I’ve come to is surprisingly simple:
Your body thrives in the middle. Not too little. Not too much. Enough to stimulate adaptation — not enough to cause breakdown.
The true optimum is different for everyone. But the principle is universal: movement heals — when the dose is right.
