In orthodontic treatment, in order to avoid injuries to the
tissues, the forces applied generally should not exceed:
It has been stated by Oppenheim and Schwarz, a result of their experiments, that optimum orthodontic force would be equivalent to the capillary pulse pressure or 20–26 g/cm2 of root surface. 
With this light force, tooth movement would be by direct osteoclastic activity in the area of greatest pressure; undermining resorption would not occur. 
The optimum force should be one that moves the tooth most rapidly in the desired attitude and direction with the least tissue damage and the slightest amount of pain.
Capillary blood pressure
Diastolic arterial blood pressure
Masticatory forces
Systolic arterial blood pressure