It's true that nurses are at increased risk for back injury from patient handling. Sudden, unexpected body movements can occur when a patient loses his or her balance or faints.
We haven't found any studies comparing male to female nurses and lifting. There is a recent study from Denmark that might offer some helpful information. Researchers used 10 well-trained judo fighters to "catch" falling patients. The patient was a 33-year old healthy male who weighed 220 pounds.
Pressure inside the abdomen and force of abdominal muscle contraction were measured for each catch made. These two factors are known to increase spinal stability. They do this by increasing the "stiffness" of the spine when it's suddenly exposed to a heavy or shifting load.
In this study no one used the full intra-abdominal pressure (IAP) possible, but women used a higher IAP for the same load as men. Other studies show women have equal or greater endurance in the back muscles compared to men when lifting the same amount of weight or load. Men are usually stronger than women and can lift heavier loads.