musculature is actually very simple. It dosent run down the entire length of the arm eather -or else youde get your muscle bump from contraction in the middle of your elbow- its just one muscle to do one way, and an antagonistic muscle to go the other, connected up to the joints. Muscles used for movement anyway, the ones for facial expression get a little more complicated, but thats not really relavent at this point.
Your method would be interesting, but flawed, since nature already does this sort of movement.
http://www.straightdope.com/columns/read/2180/do-spiders-have-hydraulic-legshttp://www.asknature.org/strategy/d33d113f9f62c63f8f6bc6a607b03a4chttp://www.ncbi.nlm.nih.gov/pubmed/22279064The flaw comes in many points:
1)You still need two sets of hydrolics, muscles or a mixture of both to gain movement both ways. The cost of creating hydralics is the amount of room they take up compared to a muscle system which can be compacted into a smaller space.
2) without specalised backup systems, if the hydrolic tube is cut or leaks, the animal is buggerd. Its inherently less robust than normal muscle systems and just straight up more likely to fail if damaged.
3)To maintain hydraulic pressure would mean sacrificing the ability to develop finer organs notably Better brains and any form of lungs, since this pressure would have to be generated in the entire body -if not confined to the leg, which if it is, makes the leg even less robust in events of injury- This is due to the fact those systems use very thin and complex structures that would be damaged by high pressure. Or in the case of the lungs end up forcing it to bleed internaly and suffocate the animal. -Less robust in terms of injury would be the healing and repressurization time, were as a muscle just needs to patch up some muscle protein fibres and its ready to go again-
4) Hydraulic systems have less bend available to there respected joints, they are just chunkyer, as such, dexterity comparativly suffers.
5)The smaller cell bundle in use means it will tire out faster.
However there are still some Pros:
1)The abaility to quickly use large amouts of force -proporionaly- in a small space of time. Usefull for jumping or running away.
2)less material requirement and maintenance due to the fact the majority of the leg would be fluid filled tubes and not cells.
3)The reset time for the system is substantially smaller than muscles, ie the rate at which it can contract, relax and contract again. However, since muscles are still so fast, this only matters in such a great extent on a purely microscopic to small scale. Once you start getting bigger than your average spider the efficiency starts to drop faster.
-Efficiency drops due to how muscles are wired, the more muscle is used the more is activated for more use in a cascade effect, as such, your introducing more new muscle bundles per contraction, which in a prolonged encounter will give you the edge to just keep going for longer, were as the cells in the hydraulic system has eather to use the same cells harder everytime, or design a larger system that copys normal muscle cascade effects in a less space efficient area.
As such: if your part of a fast living species robustness isent much of a concern as youll probably die before the system fails, but for larger longer lived life forms, muscle wins out for utility.