Companies adopting Lean increase their productivity, quality of product and delivery in the short term. But in the medium term, these performance criteria are mitigated by events such as absenteeism, sickness leaves, turnover, etc. To explain this, Occupational Health scientists identify contradictions between the components of Lean and human functioning. One of these contradictions relates to the regulation mechanism of human work activity, specific to human functioning, which would be inhibited in a Lean working environment. In this article, we focus on the integration of this mechanism in the Lean performance approach and, to illustrate our approach, we choose the example of semi-finished stock lying between each workstation: the ‘work in progress’ inventory. We propose to use a performance indicator that measures the need of ‘work in progress’ inventory taking into account the regulation mechanism of the work activity. We call this indicator “number of regulating ‘work in progress’ inventory”.