If you observe the video below, you may see that density on the simulated scalp reduces as it is stretched. The more the scalp is stretched, the more the hair density is reduced.

This rule can be applied to scalp reduction procedures or strip hair transplant (FUT) harvesting method because density is calculated based on quantity of follicular units over total area under consideration. After a reduction, the crown hair growth direction tends to move lateral away from its natural forward growth angle. There is also a change in hair growth angles in the donor area following a strip harvest. Some "stretchback" may help increase density on the scalp to a small degree.

Tissue elasticity varies from patient to patient but reducing the scalp area inevitably reduces density. The total number of follicular units never increases after any reduction or harvesting and the overall appearance of the scalp and hair growth angles will be altered. Anytime you decrease the area of any donor area or recipient area, you will decrease the density of that area. Many patients have different densities from one specific area of the donor area to another. After a strip is harvested, the angles of the hair growth may even become skewed due to the varying densities above and below the strip incision. For the patient, the benefit is the reduction in expense for the procedure. The actual gain by the scalp reduction is usually partially or always lost after "stretchback" occurs. This means the net gain is minimal. For physicians, the only benefit of reducing the scalp is that the length of the surgical procedure is considerably reduced. For the patient, the benefit is minimal.

Density = the number of follicular units / surface area. Therefore, density decreases as surface area increases as a result of stretching forces.