Studies have shown Rebar Tie Machines significantly reduce adverse effects on the spine caused by tying steel manually. This is due to the reduced impact on posture when compared to bending over a bar and simultaneously mobilising your upper body to tie wire manually. Furthermore you can perform ties up to five times faster with a Tie Machine which simply means you are bending over for less time therefore reducing overall impact.
There are also significant health advantages with the use of a trigger mechanism as opposed to manually twisting and tying wire which puts a great deal of strain on the wrist. Manual methods have often led to Carpal Tunnel Syndrome (CTS) a painful disorder in the hands and wrist as a result of repetitive movement. The use of a trigger mechanism nearly eliminates the risk of CTS.
Finally the Rebar Tie machine significantly reduces exposure to sharp objects. With the use of the Tie Machine you are no longer handling tie wire or reinforcing during the steel fixing process.
For further reading: http://www.nexgenergo.com/ergocenter/trends/Intervention_Study_HFES_Paper_2.pdf