In a study done over 80 years ago the relationship between pressure and suction alone established the need for hand expression to completely empty the breast. Suction alone is not sufficient to assure that a high percentage of fat will be extracted from the breast. Pressure such as hand expression is needed.
The normal action of a baby feeding upon a breast can be analysed into two factors: pressure and suction; and it would appear from a study of the preceding tables that the pressure is the important factor in the production of a high percentage of fat. The greater the pressure apparently the steeper will be the rise in the fat percentage. Suction employed alone, on the other hand, tends to lower the percentage of fat. The larger, therefore, the element of suction in the act of extraction, the smaller apparently will be the ultimate rise in the fat percentage.
Download the entire study here. Widdows The influence of Method of extration of Breastmilk on the fat percentage 1933