Why do muscles get bigger after exercise?

Chris Smith asked sport scientist Dan Gordon...

Dan - The key to this is that not all exercise does this, in many ways. It’s very much about the load that you apply through the muscle. We talk about stress, and we have to apply stress and it’s a biological stress that you apply to the muscle. When we think about the muscle adaptation which, in essence, is the idea of hypertrophy which is that the muscle in itself bulking up. What’s actually happening is that it’s the muscle, in essence, resisting the load that’s being applied to the muscle. If I apply a greater load to the muscle then I’m going to create a greater stress. If I create a bigger stress, I produce more fatigue. If I produce more fatigue, I produce more biological adaptation.

The trick with it is that if I’m only applying a light resistance so, for example, I go to the gym and I lift a very light load, I can do lots and lots of reps, and actually what I train there is more the endurance of the muscle. If I want to make the muscle bigger, what I’ve got to do is apply a heavier load but I can only do less reps, and what starts to happen is, and one of the misconceptions is that we increase the number of muscle fibres. It doesn’t happen in the human. What we get is a hypertrophy, we get an increase in the size of the fibre.

The thing that fascinates me about this is that if you take somebody who’s never done any kind of strength training before, they will report quite quickly that they feel like they’re getting stronger. And, in fact, the load that they lift increases quite dramatically but the muscles haven’t got bigger at all. The reason for this is what we start to see in the first 8 to 12 weeks is you get a heightened neural recruitment of the muscle. So we actually recruit more motor units and, over time, as the muscle starts to adapt we get an increase in the cross sectional areas of the muscle. The neural recruitment starts to decrease. It’ll be heightened compared to where we were at baseline but it starts to decrease, and that’s because we’ve engaged more of the muscle fibres and the fibres are actually larger in size.

Chris - When you say the neural recruitment, say I want to lift something, message comes out of my spinal cord and activates the right muscle groups to do the lifting. But as I train that response, at least in the early stages, rather than say 10 different groups of muscle fibres turning on, I might be able to train myself to turn on 20 all at the same time and I’ll  get more force out even though I haven’t made the muscle physically stronger?

Dan - Correct. The reason for that is partly because we don’t know what the movement is so the movement is actually alien to us.

Chris - Is that my nervous system rewiring itself to make that movement occur more otimally then?

Dan - In essence that’s exactly what’s happening. People talk about muscle memory, which I don’t think is a great phrase. But, if we think about it in that concept, the first time you do an action - you go and see somebody who’s doing olympic lifting. They’re lifting a bar and never done it before. The bar is moving all over the place, there’s poor coordination so what we do is we recruit what are called the synergists, which are the support muscles. But over time the prime movers that are involved in the actions start to become dominant and the synergists starts to decrease so that will explain why the neural recruitment goes down because we’re not recruiting these additional support fibres.

Chris - So if we want to be able to bulk up a muscle, to get the best of both worlds we need to train not just the reps - how long we can go for and get good blood supply into the muscle and so on, but you’ve got to stress the muscle in terms of lifting very heavy loads beyond what you would be comfortable lifting in order to stimulate it to grow more?

Dan - Correct. And the key to any adaption, it doesn’t matter whether it’s strength or endurance is you have to move beyond the biological habitual level, and the greater the amount of fatigue you produce, the bigger, potentially, the biological adaptation you’re going to produce. So, if I want to get a hypertrophy of the muscle, I’ve got to lift around about 85 to 90% of my maximum effort.

Actually, strength training’s really easy. It’s just two numbers, it’s threes and fives. If I want to get hypertrophy, then all I’ve got to do…

Chris - This is muscle getting bigger?

Dan - This is muscle getting bigger. All I’ve got to do basically is three sets of five repetitions. I can hear gym coaches now going no! If I just want to increase the pliability of the muscles, I want muscles that are stronger but not necessarily bigger, I just do it the other way round. I do it fives of three. And so if I’m doing threes of five, that’s three sets of five, I’m doing more reps, so I can afford to do a lighter load. If I’m doing it the other way round, I’ll do a higher load. And that changes the biological adaptation.