A muscle is made up of a number of muscle fibers, each of which may vary in a variety of properties. However, each muscle fiber is itself an on-or-off process - it’s contracting or it’s not. The strength of a given muscle response is proportional to the number of fibers recruited to contract, and also the properties of each such fiber. When a muscle is regularly tense, this is due to a latent level of fiber contraction - a number of fibers being contracted in absence of the requirement for a muscle body response.
In other words - a tense muscle, although associated colloquially with strength, is actually much weaker than a relaxed muscle. Due to the various biophysics of the contractile process as well as the neurophysics of the innervating signal, a tense muscle is also slower to response than a relaxed one.
In other words - you don’t want your muscles to be tense all the time, if what you’re after is strength and speed.
Exercising doesn’t make individual muscles stronger by making them tenser - it makes them stronger by ‘teaching’ it optimum patterns of fiber recruitment, and by inducing metabolic changes which can cause individual fibers to thicken, which increases their contractile strength. Neither of these is associated with tension.
The act of exercising sends a wide variety of information to your body which is important in this process, beyond simply making your muscles tense.
In other words, simply causing your muscles to tense repeatedly will not have the desired effects you would get from having them ‘tense’ in addition to the other stimuli they recieve during ‘proper’ exercise.
Moreover, there’s a reasonable number of studies that suggest this kind of isolated tensing can have some undesirable side-effects such as raised blood presure. In addition, it fails to coordinate the movement of the muscle with the conditioning of the bones and ligaments, which is at least as important a process in ‘proper’ exercise as is the strict muscular processes.
Electronic stimulation is used most commonly in rehabilitation, where muscles are poorly or un- able to generate tension by themselves to induce the kinds of metabolic changes required to repair and grow. Thus, it can be a useful crutch. But it is not a pair of wings.
… or so it seems to me, I hope if I’m wrong someone will correct me.