My initial workouts were heavily focused on exercises that would isolate the muscles I wanted to grow. Looking back, I now believe these moves actually held back my progress even once I focused on compound movements.
The tricky thing about muscle isolation is the fact that the body likes to use muscles in synergy. So The abs like to work along with the hip flexors. The biceps love to work with the shoulder muscles and so on
Because of the synergistic tendencies, muscle isolation can sometimes really be a challenge. How does someone ensure the biceps are working with as little involvement from the shoulders or back? How can you use the ass-jacking glute machine without the quads or hamstrings doing some of the work? After all, if the hip flexors are working during sit ups, then how can the abs get the workload to change?
That’s what I used to believe and I spent a great deal of effort trying to isolate muscles as much as possible. Now I believe that such an approach can be a step back. If you want to make a muscle work really hard, it’s to your advantage to involve the other muscles as much as possible.
The reason is because any given muscle can only work as hard as the amount of stress placed upon it. If you want to place as much stress on a muscle, taking the other muscles out of the picture means you can’t use as much resistance. This then carries over to less work over all going to the target muscles. Examples include the amount of resistance you can pull during a pull up vs a curl or how hard the quads can work during a squat as opposed to a leg extension.
Sometimes it might feel as if the other muscles are getting more work instead of the target muscle. A common example would be someone saying their hip flexors are working more than their abs during leg raises. In this case the hip flexors may very well be working harder, but that’s not a bad thing. It simply means the hip flexors are the weakest link in the chain. Once they get stronger the amount of tension they can withstand will grow and that tension can be carried through the abs as well.
In the end, the strength and potential work load of any given muscle is related the other muscles surrounding it. The stronger everything else becomes the stronger the more developed your target muscle groups can become.