In a blog back in October 2014 I talked about the different forms of muscle contraction:
The one we're most familiar with is concentric contraction - when the muscle becomes shorter (and bulkier), e.g. during a bicep curl.
But have you ever thought about the process that goes on inside the muscle to allow it to contract like that?
It's common to hear the analogies of the muscle like a stretched out rubber band or spring, waiting to contract - but think about it, that can't be right, because if it was, then it would mean your arms and legs would snap "shut" when you weren't concentrating... you'd be constantly using energy to stay out of the foetal position. That's not happening, so the analogy can't be an accurate one.
The fact is that we don't actually know the tiny details of how muscle contraction works.
Until 1954 there were several competing theories. Then, in '54 one theory emerged which has since become the dominant theory: the sliding filament theory of muscle contraction.
To understand the sliding filament theory of muscle contraction, you need to know a little about the structure of muscle, in particular that individual muscle fibres are bundled together in segments/chunks called sarcomeres, that run the length of the muscle.
You also need to remember that each bundle of muscle fibres has a binary setting only: it can either be contracted or not. So to lift a small weight, like a salt pot from the table, only very few muscle fibre bundles are instructed to contract. To lift a weight in the gym, on the other hand, loads of muscle fibre bundles are told to contract.
Each individual muscle fibre within the bundle is made up of a "layer cake" of 2 types of protein filaments: actin and myosin.
So each muscle fibre is made of layers of thick (actin) filaments and thin (myosin) filaments.
When the muscle contracts, the layers of filaments slide past one another. The actin filament has globular components that will slide over the myosin filament as the fibre contracts (gets shorter). The clever bit is that those globular components are a bit like barbs facing the wrong way... they'll allow the muscle to shorten, but "dig in" if the muscle tries to lengthen again; this is what ensures that the muscle will continue to contract under load.
Once the signal to relax is given, the "barbs" are withdrawn and the filament layers can slide back to their resting position.
Now it's fair to say, that that's my explanation of the sliding filament theory - what I've referred to as "digging in" and "barbs" is more correctly referred to as cross-bridges, and if you want to check out more detail on the web, that's the phrase to search for.
The sliding filament theory of muscle contraction is just that - a theory. That said, it's the best we have. It's really hard to prove because in order to see what's happening down at that microscopic level you have to look at the muscle of a cadaver... and dead people can't really do muscle contraction like living ones!
So this theory is pretty much universally accepted as the way in which standard concentric isometric contraction works.
But it doesn't explain how the opposite eccentric contraction works (e.g. lowering the weight down slowly after the bicep curl). In fact, the sliding filament theory suggests that eccentric contraction can't possibly work... but it does.
That's one of the reasons that the theory remains just that. Eccentric contraction can't be explained by it.
Actually, there isn't a single universally accepted theory of eccentric contraction.
Funny isn't it, that we can put a robot on Mars, but we still don't know how our own bodies work.