# From Lean Production to Lean 4.0: A Systematic Literature Review with a Historical Perspective
## 4 Conclusions  
This article explores the origins and diversification of the term \"lean\" as a management concept, in both the manufacturing and service sectors. It takes a historical perspective in answering three research questions. To achieve this, 4.962 indexed records and 20 seminal books were analyzed by following a systematic literature review methodology.  
Our research questions can be answered as follows:  
About the historical origin of the term \"lean\": it was created in 1988 as \"lean production system\", a generic denomination for the Toyota production system. The best-selling book, _The Machine that Changed the World_ (1990), populated the term \"lean production\" by absorbing other alternative expressions that existed at that time.  
The previously used terms (which had similar intentions of denominating the Toyota production system without naming Toyota) were: _Japanese manufacturing techniques, stockless production, JIT production, value-added production, continuous improvement manufacturing, non-stock production_, and _fragile production_.  
Since 1990, the term lean has evolved over time. Its evolution and diversification can be explained through four mechanisms (combined over time): expansion, transfer, targeting, and combination. This resulted in the creation of a confusing puzzle of lean specifier.  
This paper has outlined the paths of evolution by using the most cited specifiers in the academic literature:  
* Between 1990 and 2000, the term lean remained mainly in its original field of operations management, with the following specifiers: lean production, lean manufacturing, lean logistics, lean supply chain, lean product, lean construction, and lean and green. The first attempt to upgrade the concept to a more conceptual level was greeted with initially limited academic interest: lean management, lean enterprise, and lean thinking.  
Figure 19: Papers about relationships between lean and Industry 4.0.  
Figure 20: Papers linking lean and Industry 4.0 by corresponding author’s country (2015-2020).  
In 2000, the combined term \"lean six sigma\" emerged and up to the present has received much attention in both the manufacturing and service sectors.
* Since 2006, the term \"lean\" was progressively applied in the service field with new specifiers: lean service, lean hospital, lean healthcare, lean office, lean startup.
* The last specifier, lean 4.0., was created in 2017 as a synergetic combination between lean manufacturing (or lean production) and the Industry 4.0 paradigm. At the moment, it focuses only on the manufacturing field.  
The term \"lean\", as a management concept that allows organizations to remain competitive by removing waste from their processes, has been fully adopted by management researchers. Based on a bibliometric analysis of published papers-per-year, we can say that research interest in this topic has grown exponentially since 1988.  
This paper reveals some implications for future research: The use of lean perspective can be further extended beyond its current development, adapting its principles and tools to different sectors or applications. The diversification mechanisms described above can open new research areas in a fast-changing, complex and competitive world. The lean approach combined with the new emerging disruptive technologies (so-called Industry 4.0) open new avenues for future research as intelligent construction, sustainability, smart cities, environmental improvement or public governance.  
Conceptualization, F.G.-V.; methodology, F.G.-V. and A.S.; formal analysis, J.A.Y.-F.; investigation, F.G.-V.; data curation, F.G.-V.; writing--original draft preparation, F.G.-V.; writing--review and editing, A.S.; supervision, J.A.Y.-F. All authors have read and agreed to the published version of the manuscript.  
This paper was funded by the DGA-FSE (Diputacion General de Aragon--Fondo Social Europe) project T56_20R: Grupo de Ingenieria de Fabricacion y Metrologia Avanzada.  
Not applicable.  
The authors declare no conflict of interest.