Try developing your leg muscles to give you explosive strength (like Gordon D'Arcy) because the 12 channel tends to be pretty full of bodies and it'll help you break tackles.
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Excellent advice there- and also for defence, as you`ll be very busy making hits in the no12 channel. Remember that most of your power in making the tackle comes from firstly timing, and secondly leg drive, much more than from pure upper body strength.
Also get out the age-old skipping rope, as it is a real proven winner in helping to get the feet going, always important to be quick on your feet in order to step past defenders, and if necessary to step just before contact, in order to get the front-foot ball going for your cleaners to run onto.
Also, try to learn reading the game, this is the most essential ingredient to playing 12 IMO, as you often have more time to see things happening on the field than the 10, since his eyes will be on the ball coming from the scrumhalf. You`ll need to be a good communicator and decision-maker, as you`ll be the main controller of the defensive line, and the 2nd most important decider of where the ball will go when in possession.
While I was still playing, I concntrated on shoulder presses, bench press and chin-ups for upper body strength, squats, straight-leg deadlifts, leg curls, leg extensions and leg presses for leg strength and explosive power, loads of sit-ups for core strength and of course the old skipping rope for quick feet. Hope this helps.