Anna Kelsey-Sugg and Amanda Smith
If you can play Chopsticks on the piano, dance the Nutbush or ride a bike, chances are you've experienced muscle memory.
The human function allows us to perform actions without conscious effort — but is that 'memory' in the muscles, or the mind?
It might seem uncontroversial, but there's considerable disagreement about where muscle memory resides — and even what it should be called.
That's because there are actually two distinct systems of memory at play: one about muscles, the other about coordinated movements.
"I don't actually like the term 'muscle memory'," says neuroscientist Alan Pierce.
He says a better term is 'motor memory', which emphasises a neurological role.
"It is about the central nervous system, which is the brain and spinal cord, retaining motor skills and being able to memorise motor skills," Dr Pierce says.
That could mean memorising forehand in tennis, he says, or even playing a piano.
The neurological component of muscle memory isn't always a positive — something anyone who regularly mistypes the same few letters might attest to.
Dr Pierce says someone first learning to type is "very conscious" about their movements, and "putting in a lot of thought and energy into that".
Slowly, however, he says "other parts of the brain can take over" so that typing patterns are reinforced while becoming less of a conscious act.
There's hope for change, he says — but it'll take some work.
"In trying to unlearn [the incorrect typing] and relearn a new pattern, you'd need to employ the same processes again — and that requires more effort," he says.
"As you start to improve, providing you're getting some feedback you actually aren't doing the wrong thing, you'll start to relearn and rewire, which is the basis of neuroplasticity."
Putting muscle 'in the bank'
Exercise physiologist Craig Goodman, on the other hand, is quite happy with the term 'muscle memory'.
He says there's "some inherent memory" in muscles.
"That [memory] accelerates the regaining of muscle mass," he says.
Dr Goodman, working from a 'muscle' perspective, uses the example of bodybuilders and athletes to describe the "phenomena" that is muscle memory.
When they stop training, they lose muscle mass.
But the muscle memory kicks in when they start to train again, and they regain muscle a lot faster than the first time around.
According to one current theory, Dr Goodman says, it all comes down to the muscle cells' nuclei. The nucleus is the part of a cell that contains its genes and controls its growth and reproduction.
Muscle cells are the largest cells in the human body, and, unlike most other cells in the body, they can contain multiple nuclei.
The more strength training you do, the more nuclei in your muscles.
"One of the current theories, and this is based largely on animal studies, is that with something like strength training or what we call 'mechanical overload', there's an increase in the number of nuclei in the muscle fibres," Dr Goodman says.
"Muscle fibres have multiple nuclei, and those numbers of nuclei can increase and it seems to be when you undergo a period of de-training or you stop training, those nuclei stay there even if the muscles atrophy [or] get smaller."
While Dr Goodman says other research contradicts this hypothesis, the current theory is that "somehow having that extra number of nuclei helps you regain your muscle size when you re-engage training".
And there could be an evolutionary explanation for why.
"Imagine if you're in, say, the summer months, doing tasks that require a lot of strength and muscle size, then you went into a period of relative inactivity during the winter," Dr Goodman says.
"It would make sense that, when you come back to the warmer months and you need to get going again, you regain your strength and muscle size relatively quickly. You don't have to spend the rest of the summer trying to increase your strength.
"It is certainly an evolutionary advantage, at least theoretically."
But there might be other advantages too.
Dr Goodman says some research suggests the earlier someone does strength training to get 'memory' into the muscle, the more deeply embedded that memory is.
If people in their teens and 20s, for example, undertake strength training, then undergo a long period of inactivity, they maintain a physical advantage over those who never trained.
"When they're older and they suddenly want to become more active and undertake strength training as an older adult, then the increases in strength and size they get will be amplified," he says.
That suggests that even if you let your earlier strength dissipate, you can still benefit from it later.
"In the muscle case, specifically, at least according to that theory, you're putting, in a sense, these extra nuclei in the bank," Dr Goodman says.
Different muscle memory theories abound, and it will take more work for researchers to arrive at a consensus on just what muscle memory is all about.
"It's a kind of watch this space," Dr Goodman says.
"Duelling teams are going to be working this out in the future."