Can you tell me about hydraulic machines and there place in the exercise arena? How are they different from free weights or the typical machine found in gyms. Besides the fact that there is resistance in both directions what other differences are there? Does it break the muscle down differently... or not??
I'm going to assume you are referring to hydraulic cylinder machines. Hydraulics are simply a different medium to provide resistance with, which as you mentioned have a dual agonist/antagonist resistance feature. This feature may have added benefits of increased calorie expenditure, as well as increased volume in a shorter period of time (with a possible goal of increased calorie expenditure).
This water based resistance also alters the effects of INERTIA (Newton's laws of motion), since a gravity based resistance it is NOT!
Another point to consider is the effect hydraulics play on the RESISTANCE PROFILE of the machine VS. THE STRENGTH PROFILE of the body. Ideally, it is desirable for these two parallel as closely as possible. (EXAMPLE: The resistance profile of a machine chest press "should" be arranged so that at the endpoint of the concentric phase, the moment arm of the machine/cam/etc. is the longest, making the resistance the "heaviest," matching the strength curve of the body in this position.)
As far what effect this will have on the muscles, I a have an acronym for you that you may have heard before: S.A.I.D.S. (Specific Adaptations to Imposed Demands.) This basically is stating that the body specifically "ADAPTS" to the demands "IMPOSED" upon it over a given period of time. I.E. Apply load to a muscle consistently over time and you will achieve strength and hypertrophy gains (relatively speaking). This is all especially true when the STRENGTH TRAINING PRINCIPALS OF OVERLOAD, VARIATION, SPECIFICITY, INDIVIDUALIZATION, AND PROGRESSION are followed, as well as the ACUTE TRAINING VARIABLES OF PROPER SETS, REPS, INTENSITY, VOLUME, SPEED, AND REST INTERVALS,,, ,,, and lets not forget adequate NUTRITION! So, to clarify, as long as you are adhering to the above principals and variables in terms of the load you're using, you may achieve the desired effect.
That being said, let's not forget that a MACHINE IS STILL A MACHINE, regardless of the resistance medium! (I.E. FIXED, UNIPLANER, & ARTIFICIALLY STABILIZED) This means little if any CORE RECRUITMENT or NEUROMUSCULAR INTEGRATION and over time with chronic regularity can produce "dumb" muscles due to a confused nervous system that no longer can efficiently communicate with the muscles due to muscular imbalances, poor functional flexibility, excessive reciprocal inhibition, and ultimately a weak CORE. (Please read Mike Clark's PTontheNET article series: Essentials of Integrated Training).
We as fitness/performance professionals must be very careful not to get caught up in "fads," and "the next big thing." The more we all UP-EDUCATE, and learn our sciences (biomechanics, physiology, nutrition, etc.) the LESS we have to guess about a new trend or product, because the ability to scientifically analyze the question or item at hand will be there.