Necessary sampling rate
The sampling frequency you are referring to (twice the highest frequency) is for capturing waves, but is not really a concern for the sensors you are using and the types of data they are used to measure.

If you wanted to analyse the magnitude of a 40kHz vibration in a structure, you should worry about sampling rates on your accelerometer; however, if you want to measure the accelerometers current angle to the ground (assuming its not accelerating), you only need to take one sample.

With the ECG, there will be a signal among all the analog voltages you read of a heartbeat (I'm assuming its hooked up to a person). This signal will never get much over 200 beats per minute, or 3.3 Hz. To capture a 3.3Hz signal you need a minimum sampling rate of 6.6Hz, which as still incredibly slow. The same goes for capturing a gait with your accelerometer. 

If you capture the analog voltage of your sensors output with analogRead 100 times a second, you should have a very clear and well defined view of the very low frequency signals you are trying to track, and should not have any issues with transferring that much information over serial. 

5, 16 bit values can theoretically be sent at 120Hz over 9600 baud, and its very easy to use faster speeds then that if you want more information.