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How much can you safely lift? The wrong answer to this question has caused a lot of back pain.
Under ideal conditions, a male electrician in good physical condition can safely lift a box weighing half his lean body weight from ground to chest. Two problems with this concept are determining your lean body weight and almost never having ideal conditions.
What’s your lean body weight? The body fat level of the average 30-year old American male is about 25%. If you’re 30 and tip the scales at 185, you probably carry just over 45 pounds of fat. Note that this is a “typical” number for use in an example. You personally could have a far higher or far lower percentage.
Subtract the fat and you have 140 lbs of lean tissue. Half of that is 70 lbs. That would be your maximum safe lifting weight in this example.
You could be far from ideal conditions, for example the box is an awkward size or shape. Just a few factors like that, and your maximum safe lifting weight could be only 20 lbs unless you have a helper and/or lifting equipment.
You don’t need to be a physics professor to figure out what conditional factors make a lift unsafe. Examine the situation and use your judgment. You even can try a partial lift to judge the conditions. Unless you’re certain you can lift it safely, err on the side of caution and get help.