Hypothesis
The harder your muscles work, the sooner they become fatigued
Equipment
Stopwatch
Method
Sitting at a desk, resting your elbow on the desk, clench and unclench your fist once every 3 seconds. Record how long it takes before it becomes too difficult to keep doing this - this is the point where your muscles are fatigued. Rest for a short while and repeat this process, each time increasing the rate at which you're clenching and unclenching your fist.
Variables
Independent - Rate of fist clenching/unclenching
Dependent - Time for muscles to become fatigued
Control variables - Arm/hand used for fist clenching, rest time in between tests
Risk Assessment
This is a very safe, low risk experiment but might be painful. Make sure you rest your arm in between tests.