Backgrond and aims: Due to the rapid development of agricultural mechanization operations and the use of tractor as one of the most important agricultural machinery, focus on the health, ergonomic and safety working conditions of the tractor operators is a MUST.

Methods: In this research the impact of BMI index and the leg angles of Iranian common tractors drivers on pain threshold reduction of Gastrocnemius muscle during clutching were investigated. Experiments were performed on two tractors including MF285 and MF399 models as two Iranian common tractors. The experiments were conducted by employing 30 drivers.

Results: The results of data analysis showed that in MF285 tractor, the drivers with higher BMI index (within the normal range) and lower leg and thigh angles had small decrease of pain threshold. Consequently, these tractor drivers feel low level of stress in their Gastrocnemius muscle. The BMI index had the highest regression coefficient for all time intervals, during and after clutching of MF399 tractors. The impact of this variable for all time intervals on the reduction of pain threshold was the highest. So that one unit increase in BMI index would decrease the pain threshold reduction by 1, 0.89 and 0.69 N, respectively. The results of genetic algorithm showed that when the BMI index, knee, thigh and ankle angles are 24.75 kgm², 101, 76 and 100 degrees, the decrease of pain threshold in Gastrocnemius muscle is reached to 0.016 Newton. For the MF399 tractor, when the BMI index, knee, thigh and ankle angles are 24.86, 130, 115 and 95 degrees, the decrease of pain threshold is reached to 0.316 Newton.

Conclusion: The effect of BMI index was negative on the decrease of pain threshold for all time intervals, during and after clutching for both of tractors. The Gastrocnemius muscle of the drivers with higher BMI index is under less stress during clutching.