In the present study, a biomechanical in-vitro experimental design for assessing lumbar instability following discectomy had been complemented by computer simulation of the muscular influence. The most impressive changes of the instability parameters chosen, i.e., mobility (flexibility) and neutral zone, occurred in respect of the study movement directions Ty, Tz and Mx (lumbar disc height, anterior/posterior dislocation, and flexion/extension). The influence of the multifidus muscle had enabled the flexion/extension neutral zone changes to be offset. Flexibility (+/- Rx) was found to have been influenced to a minor degree.