Say i have a leg. And say it has muscles. Now, when i want to lift my leg up, my muscles on the thigh(so i assume) contract and boom - leg is up. Do the muscles also contract when an external source, like my hand or a robotic kangoroo tries to lift my leg? I mean, a train is used to work its engies to get from one place to another. If optimus prime picks up the train and flies to the trains destination, the train driver would sit back and not activate the eniges as the train is already getting to its destination, though decpticon missiles flying around him would make the drive quite unconfortable(though what is a little inconvenience for not burning fossil fuels and making the world a better place?)
TL:DR : Do muscles contract even when theyre not needed to(the limb being moved by an external source)?
Yes, they contract and elongate (if you fold your arm, the biceps contracts, the triceps elongates). If they didn’t, the bone would go through the muscles: ouch, to put it mildly.
But it’s a passive contraction/elongation, the energy for it comes from outside and not from inside; for someone who can’t move their muscles, having a PT or other caretaker do it is a routine part of daily treatment, but for people who can move by themselves, the energy expended and the health benefits will be much greater if they move the muscles themselves than if they have someone else or a machine do it for them.
Muscles can’t contract with outside power alone. The myosin heads won’t separate from the actin filaments without the breakdown of ATP into ADP. If there is no ATP present to break the connection, the muscle stays rigid until it decays or gets more ATP. This is the cause of (some) muscle cramps on the mild side to rigor mortis on the extreme side.*
When you have your arm passively folded up, the muscle just changes shape. It doesn’t actually “contract”. It can “hang over” the humerus or fold up in a way, but the actual muscle fibers don’t get shorter.
OP, to see the difference, curl your arm up and grab the wrist with your other hand, like a football referee signals holding. Push one way on the wrist but resist movement. Then switch directions. Watch how the bicep and tricep change shape and size without the arm moving at all. That’s the difference between passive lifting and actual muscle contraction.
There used to be a gym in my area (late 80s, early 90s, maybe?) that specialized in “assisted exercise”… My mother signed up for a membership briefly and described the electronic/robotic exercise machines as ones that did about 90% of the work for you.
She said that she did feel a little bit tired after a “workout” there, and the next morning her joints and muscles hurt as though she’d gone for a long walk.