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When I lift something is my brain continously sending signals to the muscle to contract or does it send one signal to contract and one to relax?
Continuous signal for continuous muscle work. The signal stops when you will your muscle to relax. The signal itself is in action potentials per second. The neurons firing action potentials on muscle cells telling them to contract can fire more rapidly or slowly, leading to different contractions.
There are more AP/s overall lifting something heavy but not for each muscle fiber as each muscle fiber works in an all or nothing fashion. As in if you’re using only your bicep to lift something then as soon as you begin lifting it your body will tell a certain amount of muscle fibers, a motor unit, in your bicep to contract and they will contract to their max potential.
When something is heavy to lift it means that the muscle fibers already contracting are unable to lift the object on their own, so the brain begins to recruit more muscle fibers within the muscle to contract, it may also recruit other muscle groups to aid with the lift. When it does this the overall AP/s increases simply due to the increased amount of muscle fibers in use rather than more AP/s being sent to tell a singular muscle fiber to contract stronger as it is already contracted to its limit