When you work out whether that’s lifting weights, going for a run, or doing a fitness class your muscles are doing more than just moving. Inside, a whole series of chemical reactions are happening that help your body use fuel, build strength, and get fitter over time.

In this post I’ll attempt to break it down in simple terms so you can understand what is happening. Don’t worry about the fancy scientific terms! I’m sure they were just made up to sound good!

Getting Energy In: How Muscles Take Up Sugar Without Insulin

One of the most important things your muscles need during exercise is energy, and they get a lot of that from glucose (sugar) in your blood.

Normally, your body uses insulin to move glucose into your cells. However, during exercise, your muscles don’t need insulin to do this. The muscle contractions themselves trigger special sensors that open the doors (called GLUT4 transporters) to let glucose in.

This process is controlled by molecules like AMPK, which acts a bit like a fuel gauge—when your energy levels drop, AMPK switches on and tells your body to pull in more glucose and start burning fat for fuel (Richter & Hargreaves, 2013; Hardie et al., 2012).

This is one reason why exercise is such a powerful tool for people with insulin resistance or type 2 diabetes—it helps manage blood sugar even when insulin isn’t working properly.

Building Muscle: How Lifting Weights Helps You Grow.

When you lift weights or do resistance training, you’re putting stress on your muscles. This tension switches on a key growth signal called mTOR. Think of mTOR like a “go” button for muscle building—it tells your body to start using amino acids (the building blocks of protein) to repair and grow muscle fibres.

This signal is stronger when you also eat protein especially leucine-rich foods like chicken, eggs, or Greek yogurt within a few hours of training (Phillips et al., 2009; Bodine et al., 2001).

That’s why the combo of strength training and a protein-rich meal or shake is perfect for muscle growth and recovery.

Endurance Training and Mitochondria: Boosting Your Body’s Energy Factories.

If you’re doing cardio or endurance workouts (like running, cycling, or swimming), your muscles respond differently. They don’t grow bigger in the same way but they get better at using oxygen and making energy.

This is thanks to a molecule called PGC-1α, which gets activated during long bouts of exercise. It helps your body build more mitochondria tiny energy-producing structures in your cells (Popov et al., 2015).

 More mitochondria = more energy = better stamina.

A Little Bit of Inflammation Helps You Get Stronger.

Exercise especially the kind that includes lowering weights slowly (eccentric movement) can cause tiny bits of muscle damage. That sounds bad, but it’s actually a good thing. It triggers a repair process that helps your muscles grow back stronger.

One of the molecules involved here is IL-6, which your muscles release when they contract. IL-6 also helps with fat burning and sugar uptake during exercise (Pedersen & Febbraio, 2008).

So a little muscle soreness is your body’s way of saying, “I’m adapting.”

In a Nutshell

When you move your body, your muscles are doing a lot more than you think:

They pull in sugar from your blood without needing insulin.

They use amino acids to repair and build new muscle.

They build more energy factories to improve stamina

They spark mild inflammation to kickstart recovery and growth

Whether you’re working out for health, strength, or performance, these internal changes are why consistency matters. Your body adapts every time you train on the outside and inside.

References

Bodine, S.C. et al. (2001). Akt/mTOR pathway is a crucial regulator of skeletal muscle hypertrophy. Nature Cell Biology, 3(11), 1014–1019. Hardie, D.G., Ross, F.A. & Hawley, S.A. (2012). AMPK: a nutrient and energy sensor that maintains energy homeostasis. Nature Reviews Molecular Cell Biology, 13(4), 251–262. Pedersen, B.K. & Febbraio, M.A. (2008). Muscle as an endocrine organ. Physiological Reviews, 88(4), 1379–1406. Phillips, S.M. et al. (2009). Resistance training increases muscle protein synthesis in humans. Journal of Applied Physiology, 106(1), 202–211. Popov, D.V. et al. (2015). PGC-1α: role in skeletal muscle and metabolic disease. Biochemistry (Moscow), 80(5), 617–628. Richter, E.A. & Hargreaves, M. (2013). Exercise, GLUT4 and glucose uptake. Physiological Reviews, 93(3), 993–1017.