What on earth is a motor engram? Could you answer this question without looking it up?
The definition of a motor engram comes from the field of neurophysiology:
1) An interneuronal circuit involving specific neurons and muscle fibers that can be coordinated to perform specific motor activity patterns
2) A permanent trace left by a stimulus in nerve tissue
Motor engrams help us perform coordinated movements ranging from simple to complex tasks, and usually require thousands of repetitions to become established.
Do you notice how easily you perform the movements needed to bush your teeth? A strong motor engram has been established. I'm always amazed that regardless of how many years it has been, I can easily perform complex volleyball movement patterns without even thinking, yet a novel task (like a new lifting style) can make me feel like a child again!
The term "muscle memory" isn't really accurate. The actin and myosin fibers aren't responsible for "muscle memory", it is actually "memory" (or stored sequencing patterns) in our nervous system. It's never to late to establish new motor engrams, or to learn new motor skills!