Originally Posted by
Michael Wolf
I think this is the simplest correct answer - more muscle mass is used when the body is required to balance and stabilize itself.
But even more fundamentally, we might say a prerequisite before we even get to the four criteria, is that the lifts themselves need to be able to be compared apples to applies to know for certain whether progress is being made. Two machines that work the same muscles or do the same movement may not be designed in exactly the same manner. Their weight stack is different, their cables that attach to the weight stack are different, even something as simple as how well and often they're maintained - they have a lot of moving parts - is different. Even the same machine will perform differently over time, depending on age and maintenance. They're also one-size-fits-all and unable to accommodate the movement pattern for different anthropometries. Cables can do that but you can't do any heavy main lifts on cable. etc etc etc...lots of very fundamental reasons why even if some of those things are being used as assistance work for a sufficiently advanced lifter, they do not represent a viable option for the serious strength trainee for the main lifts.