Isometrics are old, but they work
Concentric contractions produce less force than isometric (for the same level of muscle activation).
Although a fad in the 60s, as Ken Barnes mentions below, isometrics remain an underused and valuable exercise regime. Better than the gym machines, that’s for sure.
I think that Isometrics are a great way for older people, like me, to build and maintain muscle strength and endurance, as I write about below. I also use them consistently to build leg and hip strength for running.
The old ways still work. I have many books from the old-time coaches and they speak less BS and make more sense than most trainers today.