A device or a system for measuring muscle signals. A system may include at least one processor, memory including computer program code, the memory and the computer program code configured to, with the at least one processor, cause the system to perform at least the following: measure a signal from a muscle to obtain a muscle activation signal； carry out at least one transform on the muscle activation signal to obtain a muscle activation spectrum； and determine a muscle state indicator from the muscle activation spectrum by using signal characteristics from a first and a second band of the muscle activation spectrum.
