This circuit uses a flex sensor, positioned on the spine, and two FSR sensors, positioned on the shoulders, to tell a user when the backpack they are wearing is too heavy by changing an LED on the wearers wrist from green to flashing red.

Using the above mentioned sensors, create voltage dividers with 10K resistors. This code, available here, and at github, will make the circuit functional. Some tweaking/calibration may need to be done to get appropriate value ranges for the sensors.