Here is my take on this...

There are two factors in muscle contraction, and both revolve around chemistry. First, neurological stimulation comes from the nervous system to the motor unit. The cascading wave of ions from the nervous tissue to the muscle tissue prompts a chemical change inside the muscle cells and encourages them to contract. This process is active and under a load, in most cases.

It's been shown that isometric does very little for strength in any joint angle other than the one the joint is at during the isometic force application. It has also been shown that very little, if any, hypertrophy occurs as a result of isometric training.

An external electrical stimulator may provide isometric contraction but it is obviously passive and doesn't include the complex neurological activity that occurs in the brain. In addition, a question to be raised is does the machine stimulate the muscle tissue in the EXACT same way as the neurons do? What kind of damage occurs? Wha are the consequences of any damage that might occur?

One last piece of information I was able to find. My wife is a Physical Therapist and she uses the TENS device almost daily. For those patients with a joint replacement, the TENS device is a common therapeutic tool. She told me that while patients do exercises to both sides of the body, the TENS device is usually only applied to the affected or injured side. She states that there is no unexpected strength or size increases on the affected side do to the application of the TENS device. For those scientists out there, I realize that does not qualify as a controlled study, however the general concept is solid. More investigation is required, but a hypothesis would be that there is no major muscular growth or increase in strength with external electrical muscule stimulation.

If it were me, those devices are like snake oil and should be left in the hands of trained individuals. In paramedic class, we had to cardiopace ourselves, but our calf muscles in stead of our hearts. In cardio pacing, you connect two leads to the muscle and start applying electricity. This is very painful and definitely causes traumatic damage to heart tissue when a patient is being paced. Many of us had strange sensations in our calf muscles for a few hours. One student reported pain that took DAYS to resolve. Probably something you wouldn't want to mess with, especially when the currently available data seems to point to the contrary.

That's my take on the whole thing. If I missed something or are unclear or flat out wrong, I take all the credit