Conventional observation-based methods of ergonomic risk-assessment are not properly suited for the assessment of manual forestry work. The work activities and working conditions in the forest are not standardized and therefore difficult to assess. This paper describes a reference concept to investigate the influence of working conditions on physical workload based on biomechanical parameters during manual forestry work. For this purpose, a reference scenario consisting of the work activity manual skidding and the working conditions working posture, ground conditions and soil slope was selected. Furthermore, the biomechanical model of Jäger et al. [1] as well as a linear force sensor and inertial measurement units were selected for this reference concept based on a utility value analysis. A software-solution to calculate the compression and shear force on the L5/S1 intervertebral disc for the selected scenario was implemented. Evaluations show that the devised concept can assess risks more accurately compared to using an observation-based method and that some working conditions contribute towards higher physical workload on its own. This concept as well as the procedure to derive it can be used as a reference for similar investigations.