Have to disagree, planet_healh:
The latest science around muscle building appears to be converging on mechnotransduction as the stimulation for hypertrophy and strength adaptations: put muscle under tension, force it to work hard, that triggers a hormonal signaling cascade that produces gains. I have never seen any evidence that the tension has to come from gravity based resistance.
Jones invented a whole bunch of machines that applied variable, rather than constant resistance, and they seemed to work OK.
Many training methods employ discontinuous application of resistance: just think about deadlifts, dead stop presses off of pins, box squats. All allow the muscle to unload between reps, and those methods of training clearly work.
I’ll point out that Dr Darden indicated that he has a Bowflex and uses it daily. This, despite the fact that it has no weight stack. It only provides elastic resistance, yet he finds value in it.
Anecdotally, I’ve heard stories of experienced trainees making additional gains by using volitional contraction against motorized resistance.
Many roads lead to Rome, pick the route you enjoy…