Activity theory is aimed at understanding the mental capabilities of a single human being. However, it rejects the isolated human being as an adequate unit of analysis, focusing instead on cultural and technical mediation of human activity

Roots in Vygotsky, developed by Leont'ev
- wikipedia

Yrjö Engeström, Professor: Professor of Adult Education and Director of the Center for Activity Theory and Developmental Work Research at University of Helsinki
Activity Theory and Individual and Social Transformation by Yrjö Engeström
Activity theory as a framework for analysing and redesigning work
Learning by Expanding: An Activity - Theoretical Approach to Developmental Research