The main purpose of back muscles are to hold the body upright. Humans create inordinate stress at the lower back even when bending over to pick up small objects such as a piece of paper. Poor body mechanics during repetitive bending likely contribute much to the wear and tear on the lower back and the all too frequent problems of lumbar strain. In Figure 3 the person attempting to transfer is leaning towards the individual being transferred. Head and shoulders are together in approximately the same vertical plain.
Legs are bent and prepared to straighten while guiding the individual up out of the chair.
The arch of the lower back is really very important. The legs then must bend in order to have a firm hold upon the waist of the individual being assisted in transfer. This will force the much stronger muscles of the legs to guide the transfer. Whenever one attempts to apply some general principle to a particular situation, there is risk of some mismatch or failure. For example, while aspirin is a marvelous compound with abilities to diminish risk of heart attack and stroke, it can be lethal to someone who is allergic to it and it has been responsible for many severe bleeding stomach ulcers in certain individuals.