How much can one guy take?

As I was reading the article "Decoding the DNA of the Toyota Production System" by Steven Spear and H. Kent Bowan, I started to think about the role of robotics in manufacturing. I guess what bothered me about the article was the constraints placed on Toyota's employees who are on the assembly line. There was nothing in the article to say that the workers were mistreated or unhappy in their work, but I can't help but feel that there may be a point in lean manufacturing or assembly line production where you just have to have a robot. Let me explain...

Operators at Toyota plants, in this case a plant in Georgetown, Kentucky, follow a very well-defined sequence of steps when undertaking a specific task. For example, when they install a car seat, they have six steps to follow, with each step controlling for quality. My concern involves the time constraints placed on employees. Let's say, as in the case, that an employee must complete one of the steps in 50 seconds, and if they don't, then management is alerted that something is wrong. At what point does a monotonous task (with a time limit) have a detrimental effect on human psychology and, ultimately, performance?

I understand that adding jobs builds value to the community and economy...but, at what point is a task so lean and specific that it should be carried out by a robot for health and safety concerns? Have any universities or consulting agencies published any studies about the long-term effects of lean labor practices on assembly line workers? Even at the height of Henry Ford's time (when I'm assuming workers' rights were not of the highest priority) production couldn't match the speed and efficiency of today. So, when does a manufacturer have to stop and say, "I'm not just using robots because it's economical, I'm using them because the task is unbearable."?

If management science has taught me one thing, it's that 100% utilization of employees ends in undesirable results. Lean manufacturing, as it seems to me, certainly comes close to 100%.