I've wondered the same thing....

but my shifus arms (bones and muscles) are like iron....so it obviously works for him! people who do weight training getting strong bones from the tendons place stress on the bone when the weight is lifted....possibly something similar places on conditoning when it is muscle to muscle.

interestingly we do have top of foot to top of foot conditioning (OUCH!) and heel to heel. Is just that the arms seem to be limited to muscle for now (been training 4.5 years in this style so far).

He also tells us to practise our arm conditioning on trees if we dont have a partner.