How much should I bend my knees while lifting objects?

There are two possibilities: a semi-squat with knees slightly bent, or a full squat using a deep knee bend. The semi-squat position is advised over the full squat.

In the semi-squat position, the center of the body is higher and the knees are straighter. This requires less work to lift as measured in laboratory studies. Using a semi-squat stance also reduces the pressure on the knees and allows for a faster shifting of the center upper-body. This means less chance of losing your balance and falling.

The semi-squat position does increase the load on the low back compared to the full squat. However, this doesn't appear to be a problem or cause injury to the back, especially when safe lifting habits are used. On the other hand, using a full squat may cause knee injuries if the weight of the load is greater than the strength of the muscles around the knee.