Running | What Actually Happens to Your Body?
Your legs are moving — but what’s really happening inside you during a run?
From shifting energy systems to the silent role of electrolytes, here’s what your body’s doing behind the scenes every time you lace up. It’s more scientific — and more powerful — than you think.
You lace up. Step out. Start moving.
But under the surface, something complex — and pretty remarkable — is starting. Your body shifts into a different gear, systems kick in, and your internal chemistry changes to meet the demands you're placing on it.
Here’s a clear, no-nonsense breakdown of what’s happening inside you during a run — and why it matters if you want to improve.
1. Your Heart Rate Ramps Up (Fast)
Within seconds of starting, your sympathetic nervous system (aka your fight-or-flight response) kicks in. Your heart rate increases, pumping more oxygen-rich blood to your muscles.
Why it matters:
That faster heart rate is the delivery system. It brings oxygen and nutrients (like glucose and electrolytes) to where they’re needed most — fast-twitch and slow-twitch muscle fibres.
2. Your Breathing Gets Deeper and Faster
To support the increase in blood flow, your lungs work harder, bringing in more oxygen and removing excess carbon dioxide (a byproduct of effort).
Why it matters:
When oxygen can’t meet demand — like in hard intervals or hill repeats — your body shifts to anaerobic metabolism. This creates lactate, which builds up and contributes to muscle fatigue. More efficient breathing = more oxygen = better endurance.
3. Your Body Starts Burning Fuel
Your muscles run on ATP (adenosine triphosphate), your body’s energy currency. In the first few minutes, your body draws ATP from:
- Stored carbohydrates (glycogen)
- Blood glucose
- Fat stores (depending on pace and duration)
Why it matters:
Your ability to tap into these energy systems efficiently (especially fat and glycogen) determines how long and how hard you can run. Without proper fuelling before your run, you start depleted — and fatigue hits faster.
4. Electrolytes Keep Everything Firing
As you sweat, you lose sodium, potassium, calcium, and magnesium. These tiny minerals are essential for:
- Nerve signalling
- Muscle contractions
- Hydration balance
Why it matters:
Even small losses can affect coordination, focus, and pace. That’s why electrolyte support isn’t just for marathoners — it’s for anyone who wants to avoid fatigue, cramping, or that heavy-legged feeling halfway through a session.
5. Your Brain Starts to Adapt Too
Ever hit a rhythm in a run and feel your mind clear? That’s not a coincidence.
Your brain releases endorphins (natural mood boosters), dopamine (linked to motivation), and BDNF (brain-derived neurotrophic factor, which supports brain plasticity).
Why it matters:
Running isn’t just physical — it has a profound impact on focus, clarity, and mental health. That’s why so many people say their best ideas come mid-run.
So, Why Does This Matter for Performance?
Because if you understand what’s happening, you can fuel and train with intention:
- Hydrate early. Electrolytes aren’t just for hot days — they support every step.
- Eat something before harder or longer runs. Your body needs fuel ready to go.
- Take care of your recovery. What you do after a run sets up your next one.
And that’s exactly what we built Prerun for — to support the body systems already working hard on your behalf. From energy to focus to hydration, we made it simple to give your body what it actually needs, right when it needs it.
Small inputs. Better outputs. Every time you move.
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