At baseline, subjects were randomly assigned to either a back-exercise (BE) or a control (C) group. Thirty-four women
(average age 55.3 years)
had been instructed to perform a progressive,
resistive weight-lifting exercise program for the back extensor muscles.
Using a backpack that contained weights equivalent to 30% of the maximal isometric back extensor strength (BES),
each subject had been instructed to lift the backpack ten times in the prone position (Figure 1).
As their back strength increased, the amount of weight lifted was increased.
However, the maximal weight of the backpack was limited to 22.7 kg (50 lbs.).
The exercises were performed at home once a day, 5 days a week.
Thirty-one women served as controls (average age 55.9 years).
Subjects in both groups were evaluated every 4 weeks for the duration of 2 years,
and proper lifting principles and good posture were reviewed.
Muscle strength and physical activity level were recorded at each 4 week visit for both groups.