It is illegal in most places and when I was a ref I always checked shins for injection marks.
Anyhow if there is no official eye watching these are long-acting novocaine-related local anestetics (novacaine even when mixed with adrenaline which makes it act longer has a local anesthetic effect of less than 1 hour).
Mepivacaine and bupivacaine are long-acting (6 to 8 hours which covers a whole bout lenght at ease) but take care they are toxic much more than novacaine, they should be administered by one who is used to handle them and can't be bought without medical prescription. The main side effect of all these substances is that they drop blood pressure so they may cause lightheadedness, dizzyness, impairement of balance and coordination, mepivacaine and bupivacaine have also central nervous system toxic effects. Anyhow, they can bring an advantage (an unfair one if you ask me). I use them when I develop a regular clot because of a vein and I want to break it beyond repair so that it wont make me problems - then I anesthesise the damn thing and kick a pole as hard as I can. Iron pumping of any kind tends to develop the venous system (including on the shins if one works tighs) so veins become larger, thinner and more apparent. This can cause bruises and the goal of shin conditioning is breaking the small vessels again and again until scar tissue wraps the shins and shearing forces are no longer able to cause subcutaneous clots.