There is no clear evidence that doing resistance training on unstable surfaces works more muscle at this time but I contend that it is important to do at least some exercises on the apparatuses that make you work harder to develop stability. I also contend that it isn’t the ‘movement muscles’ that you aim at working with this type of exercise anyway, but the ‘postural muscles’ that provide stability…even to using exercises where you need to balance on one limb will do this…so you don’t need to use these unstable surfaces if you don’t have them.
If you do a basic Bench Press on a Swiss Ball you will add an element to the exercise by working your Core muscles as compared with a Bench Press on a stable bench. Of course you will only be able to use lighter resistances this way but I think that people will gain far more functional strength on such an unstable surface…and ‘usable strength’ should be your main priority for most people doing resistance training, for what is the use of excessive strength if you can’t use it? If you don’t even have a Swiss Ball, you can do an Upper Body movement such as a Push Up Position Row (see below) or Pressing movements while standing on one leg.
With the Lower Body movements you can do mostly the exercises that utilize one limb at a time such as any variations of a One Leg Squat, Lunges or Step Ups and Step Downs to provide a certain amount of instability. But I place this type of exercise in my client’s programs to provide movements that even up any discrepancy between the left and right side of the body. Of course if you are not cognitive of concentrating on technique you won’t get this evening of strength discrepancy! It is also worth using the various unstable surfaces when doing exercises if you have them at your disposal.
My short answer to this question of using unstable surfaces or not is using both types of surface but ensure that you are doing the exercise with correct technique. Best to do any exercise while someone is watching so that you can correct any errors in technique…but failing that, do the exercises in front of a full length mirror. (I have stated before that it is amazing what contortions the body will go through to work around certain weaknesses you may have without you being aware of it!)
Lastly, you may have seen some people doing exercises on unstable surfaces that come into the realm of a circus performer? This level of exercise is not necessary for 99.9% of us…only the circus performers?