Step 1: Where these techniques come from.
F.W. Taylor introduced Scientific Management to replace guesswork on the shop floor with tested, measurable methods, and fatigue study and motion study are two of his best known tools for this.
Step 2: Fatigue study.
Working continuously without breaks tires a worker out and output drops. Fatigue study looks scientifically at how many rest breaks a worker needs in a day, how long each break should be, and when it should be given, so productivity stays high through the whole shift.
Step 3: Motion study.
Motion study observes and records every single movement a worker makes while doing a task, and then removes the wasteful or unnecessary movements, keeping only the ones that actually add value. This leaves the worker with a shorter, more efficient way of doing the same job.
Step 4: Why both matter together.
By cutting out wasted movement and by protecting the worker from fatigue, both techniques raise output without simply demanding harder physical work from employees.