Why do our legs sometimes shake uncontrollably during a leg press or leg curl?
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Think of skeletal muscles as being built up of a bunch of threads that have wires that tell them to get shorter. When lots of them shorten together, that moves one end of a muscle closer to the other. Usually, when doing routine things, the wires send signals to move that cooperate smoothly and not continuously, and then reset. But, with a heavy, sustained effort, those signals have to keep coming in order for the muscle to “know” to keep up the effort. Eventually a maximum rate is reached; lots goes into that like energy stores and use and other things, but particularly important here is limitations on how often the wire can send signals, and how well it can do them together- in the case you mentioned, everything is at “full blast” which means that although there is still large-scale cooperation (luckily) the individual parts that do it are less able to “smooth it out” and time things identically. This is called tetanic contraction. That word might sound familiar- the disease tetanus does exactly that, but in bad ways, like bending the entire body terribly or making a scary-looking smile or making the jaw unable to open because it’s always biting.
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