Muscles are more easily strained when fatigued. You’re more injury prone after long hours of heavy work, such as when performing a big job under pressing time constraints. It may help if the job can be arranged so that the heavier work takes place earlier in the shift.
Many back injuries are cumulative trauma disorders that do not come from a single lift, but occur from repetition of a particular irritating movement over time. In some cases the specific weight lifted at the time of injury may actually have little to do with the injury.
Smoking can also play a role in back problems according to American Academy of Orthopaedic Surgeons (AAOS) (see http://www.aaos.org/about/papers/position/1153.asp).
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