There was a theory in process improvement called “loose ropes”. It means that all the different entities of your systems are “connected”. You can work on one element for some improvement, but eventually one of the other aspects of will drag on the progress and need to be addressed.
For example, you may embark on an inventory reduction program in your factory, but find that after some improvements, your material handling system is preventing you from making further reductions, so you logically start addressing your material handling. You then find that even with improved material handling, machine breakdown is forcing you to carry a larger safety stock, so you then start working on total preventive maintenance.
Bottom line, everything is connected and if you pull on one area to improve, eventually the rope tightens so that you will need to address another area to continue improvement. If you keep pulling on element, eventually all the elements will get pulled in as the connecting ropes tighten.
Ergonomics and Safety will eventually need to be addressed when you embark on any improvement initiative because they are connected to every part of your manufacturing. Employee fatigue, injuries, soreness and absenteeism due to work issues will negatively affect on-time-delivery, quality improvement, cycle time reduction and fast response scheduling. So to answer the question posed in the title of this article: At some point you will have to address safety and ergonomics to effectively implement lean manufacturing.