EMG: Muscle Activity
EMG (electromyography) is a technique for sensing the electrical activity of muscles. You can build your own sensors, but there are fairly low-cost readymade products that you can use also. The kind of sensors you would normally use outside medical facilities are surface electrodes that are attached on the surface of your skin. The raw signal is very noisy and requires amplifying and filtering. These low-cost sensors are not incredibly accurate, but can be used for many applications that do not require high levels of accuracy like you would for actual medical sensors.
The MyoWare muscle sensor is an easy-to-use EMG sensor with built-in filtering and rectifying for the signal. The signal can be read with an Arduino or any other microcontroller.
The sensor pads are one time use so prepare to have a lot of them stocked up, if you plan on using this sensor.
I recommend going through both of the tutorials and the official guide from MyoWare.
Please make sure you take care of safety precautions when working with sensors that connect directly to your body! Use battery power or isolator circuits to be safe, connecting the sensor to your computer through a USB powered microcontroller could potentially expose you to dangerous spikes and surges of currents.
EEG: Brain Activity
ECG or EKG: Heart Rate Activity
ECG or EKG (electrocardiography) is used to measure the activity of the heart. EKG for accurate medical purposes requires 12- electrodes and specialised hardware, but low-cost solutions for DIY purposes exist. Here are some easily available options:
Other Pulse Sensors
There are also other pulse sensors available that are not as accurate as real ECG or EKG sensors, but they can be used to get a rough reading of your pulse. Usually these sensors are attached to your finger or earlobe.