Exercise/training can be a literal and figurative pain in the ass. But it exists due to two things/phenomenon:
*Gravity
*Joints

Gravity is why any work on earth feels like work. Carrying a bag is harder than not carrying a bag since gravity bestows weight to the bag. Gravity is also why muscle exists. Astronauts quickly lose as much as 20 percent of their muscle mass and bone density in space as there is no gravity to work against. They can lose 25 percent of their strength. NASA make astronauts strength train for 2.5 hours a day to maintain muscle! They put time and millions of dollars to develop a machine to load squats, deadlifts and calf raises in space. 

Now why does this matter to non-astronauts like us? What zero-gravity space is for astronauts, age is to us earth-dwelling gravity bound humans. We lose muscle when we stop moving our joints. This costs us quality of life. Every joint in the body has muscles that attach to move it through an extensive arc across various planes. Here are just a few examples:

* Our knees and elbows can bend and straighten
* Our shoulders can rotate and reach for the sky in many planes
* Our wrist and ankles can rotate and bend
* Our hips can rotate, bend and straighten 
* Our spine can rotate, bend and straighten

We have 370 joints linking 206 adult human bones to produce movement that can range from neurosurgery to breaking rocks. In a future post, we are going to dwell on the complexity of how the heart can be subject to a variety of stresses and our skeletal muscle changes in very different ways based on the type of activity we perform. 


TAKEAWAY:

* Our body needs a large dose of varied movement to utilise all the muscles and joints we have evolved to carry over millions of years
* Comparing running to lifting weights or walking to jump rope or climbing to swimming is really really asinine and unnecessary. Every activity produces very distinct adaptations. 
* There is a subjective component to what we deem useful adaptations. But we can all agree that a useful joint is one with functional muscle that can move it with stability, control and strength as and when needed across the entire arc the joint was built to move through.