Muscle fatigue can be measured by a dynamometer or a surface EMG or the combination of both. Moreover, for estimate central fatigue, a digital inclinometer can be used to measure the pre motor time.
A inclinometer is an instrument for measuring angles of slope (or tilt), elevation or depression of an object with respect to gravity. I don't know how central fatigue assessed by inclinometer. if you have any reference paper, please send me.
For mental cognitive testing, you may want to look at the Uchida-Kraepelin test. The Wisconsin Card Sorting Test and the Tower of London are also commonly used to assess flexibility and planning. Good luck!