There is a saying in tai chi that when one part of the body moves the rest of the body should be moving too. If you were on a tug of war team and everyone on your team was pulling, you would all need to do less work for the same result. However, if only one or two people were pulling, they would need to pull a lot harder to make up for those who were not pulling. And they will get tired more easily. Your body is like this. Simply relying on your shoulders, your quads, your calves, your arms and eventually they will get overused . This is repetitive strain injury, not just from repetitive movement but overuse from poor quality movement. Instead, earn to harness the whole power of your body, and you will be stronger. It's simple.
This is in contrast to the bodybuilding culture that has pervaded our modern fitness world. You do not want to isolate. You cannot isolate even if you wanted to.
Form should follow function but so many people have gotten it all mixed up then they end up like the tin man or a Frankenstein. A stiff, awkward collection of body parts.
Chest out belly in is the modern, militaristic notion of strong posture when in fact it is weak and has no stability. It is an inverted pyramid and cannot be maintained. Hence, there is a drifting from hypertension to collapse. This is not sustainable.
Rather one should move from the hara and not the head or the chest. However, the body must also be connected so that when one part moves, all parts move!
When you really connect with your body you will come to realise that rather than the solid and permanent 'thing' we like to think of, most of it is fluids and space. Space is what allows movement. Space is what then allows connection.
Connection is a skill. It is about movement and the nervous system, not simply muscles.
We are more than skin and bones, but we are also more than just our muscles, which is why the masters emphasised mental training as well. Strength requires focus, determination, self knowledge and a strong mind body connection.
Learn to use your body as one and you will be stronger without needing to use as much muscular force. Like a network of vines woven together, the body is strongest when its parts are connected. When muscles are fluid like a cat's rather than hard and tense like steel. Power is increased exponentially rather than linearly.
Many in the modern world have lost this but ancient movement and energy traditions knew this. The Russians, the Chinese, Japanese and many others. Many so called 'more primitive' cultures still know this today.
Learn to discover a strength you never knew you had, a strength that goes beyond muscles, age or even styles of movement or practice