Let’s say you are a very fit person, someone who has exercised pretty much your whole life, and definitely for the past 10 years or so.  You do resistance exercise, aerobic or cardio as well, and you stretch religiously.  I will say that you need isometrics!
 
And, on the other hand, let’s say you are a elderly person who has never exercised much, and definitely not in the past 10 years.  You really need isometrics!
 
How can each of these diametrically opposed situations have the same solution?  The answer is simple- isometrics will enhance your fitness, dramatically, in a very large way, no matter what your fitness level.
 
Very dramatic strength increases can be realized, very quickly, and with very little risk or danger, simply by doing a simple series of isometrics regularly!  No special equipment is needed, or even a change of clothing, and it can be done anywhere, anytime, in very little time. 
 
Isometrics have been shown to dramatically lower blood pressure, with no other changes in lifestyle.  Also, the strength increases also are beneficial for joints, strengthening the tendons and ligaments along with the muscles themselves.  Weight lifting pretty much bypasses the tendons and ligaments, concentrating on the bellies of the muscles first, which makes you more prone to injury.
 
Often, it is said that doing isometrics at a single angle only increases strength at that one angle.   Although technical true, if you simply do the same exercise at three angles- the start, the mid-point, and the near end-point of the motion, you effectively strengthen the entire strength curve!  I have found that a demanding, very effective isometric protocol takes about 10 minutes, total, to cover the whole body!
 
Here’s how you do it:  You will need a web cable, like a moving strap, at least 15 feet long.  About 2” wide is ideal; I use two moving straps I had that are each about 12 feet long.  I lay them on the floor, overlapping about a foot atop one another, and I step on the overlap, which locks them together.  Then, I grasp each strap, and run them overhead on each side, to where my hands are just above my shoulders, as if I am doing a military press with two dumbbells.  Then, I push upwards at about 80% or so of my power, for about 5 seconds.  Rest.  Repeat, for 6 seconds. Rest.  Repeat again for 7 seconds.  This is the protocol: 5,6, and then 7 seconds for each exercise in three positions. 
 
Next, I just lower my arms to my sides, still holding the straps, and do the same thing with curls: bottom, middle, and top extension, 5,6,7 seconds, at each position.  After that, I do bent over rows by bending over with the straps still held in each hand, and do all three positions from there, 5,6,7.