During a movement, specific muscles act as primary movers and others as stabilizers. (We also have antagonist muscles, but that's for another tip). Since machines have levers and arms that dictate exact movement patterns, our stabilizers don't have to assist our primary movers as they do in real-life movements.
The result: muscle imbalance and muscles that fail to work together.
Secondly, while seated, the glutes, lower back, and stomach muscles get lazy...no need to contract and hold our form. What happens when we need to move a couch, pick up a heavy box, or lunge forward for that tennis ball? Will our foundation hold strong? Perhaps not.
Finally, when we don't engage our stabilizers and ignore trunk strength, we signal to our central nervous system (CNS), our motor neurons, and our neuro-muscular connections to kick back and relax a bit. A lazy CNS is slow to fire nerve impulses, slow to recruit all muscle fibers, and slow to produce your best human movement.
I have a little more to say on this, so look forward to Part 3.
Make it a great day!